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Chapter 9
LETG: Low Energy Transmission Grating

9.1  Instrument Description

The Low Energy Transmission Grating (LETG ) was developed under the direction of Dr. A.C. Brinkman in the Laboratory for Space Research (SRON) in Utrecht, the Netherlands, in collaboration with the MPE für Extraterrestrische Physik (MPE) in Garching, Germany. The grating was manufactured in collaboration with Heidenhaim GmbH.
The Low Energy Transmission Grating Spectrometer (LETGS) comprises the LETG, a focal plane imaging detector, and the High Resolution Mirror Assembly discussed in Chapter 4. The Chandra High Resolution Camera spectroscopic array (HRC-S) is the primary detector designed for use with the LETG. The spectroscopic array of the Chandra CCD Imaging Spectrometer (ACIS-S) can also be used, though with lower quantum efficiency below  ∼ 0.6 keV and a smaller detectable wavelength range than with the HRC-S. The High Energy Transmission Grating (HETG) used in combination with ACIS-S  offers superior energy resolution and quantum efficiency above 0.78 keV. The HRC is discussed in Chapter 7, the ACIS in Chapter 6, and the HETG in Chapter 8.
The LETGS provides high-resolution spectroscopy (E/∆E > 1000) between 80 and 175 Å (0.07-0.15 keV) and moderate resolving power at shorter wavelengths. The nominal LETGS wavelength range accessible with the HRC-S is 1.2-175 Å (0.07-10 keV); useful ACIS-S coverage is 1.2 to roughly 60 Å ( ∼ 0.20-10 keV).
A summary of LETGS characteristics is given in Table 9.1.
Table 9.1: LETGS Parameters
Wavelength range 1.2-175 Å (HRC-S)
1.2-60 Å (ACIS-S)
Energy range 70-10000 eV (HRC-S)
200-10000 eV (ACIS-S)
Resolution (∆λ, FWHM) 0.05 Å 
Resolving Power (E/∆E) ≥ 1000 (50-160 Å)
≈ 20×λ (3-50 Å)
Dispersion 1.148 Å/mm
Plate scale 48.80 μm/arcsecond
Effective area (1st order) 1-25 cm2 (with HRC-S)
4-200 cm2 (with ACIS-S)
Background (quiescent) 18-44 (at 50 and 175 Å) cts/0.07-Å/100-ksec (HRC-S after LETG/PI filtering)
<< 0.01 cts/pixel/100-ksec (ACIS-S, order sorted)
Detector angular size 3.37' × 101' (HRC-S)
8.3' × 50.6' (ACIS-S)
Pixel size 6.43 × 6.43 μm (HRC-S)
24.0 × 24.0 μm (ACIS-S)
Temporal resolution 16 μsec (HRC-S in Imaging Mode, center segment only)
 ∼ 10 msec (HRC-S in default mode)
2.85 msec-3.24 sec (ACIS-S, depending on mode)
Rowland diameter 8637 mm (effective value)
Grating material gold
Facet frame material stainless steel
Module material aluminum
LETG grating parameters
    Period 0.991216 ±0.000087 μm
    Thickness 0.474 ±0.0305 μm
    Width 0.516 ±0.0188 μm
    Bar Side Slope 83.8 ±2.27 degrees
Fine-support structure
    Period 25.4 μm
    Thickness 2.5 μm
    Obscuration < 10%
    Dispersion 29.4 Å/mm
    Material gold
Coarse-support structure
    Triangular height 2000 μm
    Width 68 μm
    Thickness < 30 μm
    Obscuration < 10%
    Dispersion 2320 Å/mm
    Material gold

9.1.1  Scientific Objectives

The LETGS provides the highest spectral resolving power ( > 1000) on Chandra at low (0.07-0.2 keV) energies. High-resolution X-ray spectra of optically thin plasmas with temperatures between 105 and 107 K, such as stellar coronae, reveal a wealth of emission lines that provide diagnostics of temperature, density, velocity, ionization state, and elemental abundances and allow precise studies of structure, energy balance, and heating rates. Absorption features provides similar information in cases where bright compact X-ray sources are embedded in cooler, extended gas clouds.
The high resolution (∆λ ≈ 0.05 Å) of LETGS  spectra at longer wavelengths (\mathrel > 100 Å) also permits detailed studies of spectral line profiles in the X-ray region. These studies may provide non-thermal velocities of stellar coronae, flow velocities along active-region loops, orbital velocities in X-ray binaries, and upflow velocities in stellar flares. The LETGS also allows time resolved spectroscopy, 1-D spatially resolved spectra, and spectra of multiple point sources within its  ∼ 4 arcmin field of best focus .
Since the ultimate spectral resolution can only be achieved for point sources, the prime candidates for study in our Galaxy mainly comprise stellar coronae, white dwarf atmospheres, X-ray binaries, and cataclysmic variables. Extragalactic sources include relatively bright active galactic nuclei (AGN) and cooling flows in clusters of galaxies.

9.1.2  Heritage

Flat transmission gratings were flown aboard Einstein and EXOSAT. The LETG grating elements are produced using a technique similar to that used for production of the EXOSAT gratings. However, the LETG shares only basic operating principles with earlier instruments. Advanced grating technology has enabled the achievement of greater efficiency and increased dispersion. The Rowland geometry (see Figure 8.4) of the grating plate and spectroscopic arrays reduces dispersed image aberrations and hence contributes to improved spectral resolution.

9.1.3  Operating principles

When inserted behind the HRMA, the LETG diffracts X-rays into a dispersed spectrum according to the grating diffraction relation, mλ = psinθ, where m is the integer order number, λ the photon wavelength, p the spatial period of the grating lines, and θ the dispersion angle. Parameters are summarized in Table 9.1. The grating facets are mounted on an aluminum support plate which has been machined so that the centers of individual grating facets lie on a Rowland torus. The grating facets are aligned to produce a single dispersed image. Spectral resolution is determined, among other factors, by grating line density, line density variations, HRMA point-spread function, pointing stability, alignment accuracy, pixel size of the readout detector, and detector geometry.

9.1.4  Physical configuration

When the LETG is used, the Grating Element Support Structure (GESS ), an aluminum frame approximately 110 cm in diameter and 6 cm thick, is inserted  ∼ 300 mm behind the exit aperture of the HRMA and 1.4 m behind the HRMA mid-plane. The GESS holds approximately 180 trapezoidal grating modules, which measure about 13 × 50 mm. A design drawing of the full GESS is shown in Figure 9.1; a closer view, showing some mounted modules, is seen in Figure 9.2. Figure 9.3 shows empty grating modules mounted on the GESS. Each grating module holds three circular grating facets, each of which comprises approximately 80 of the triangular grating elements seen in Figure 9.4.
images/gess-1.png
Figure 9.1: LETG Grating Element Support Structure, a machined aluminum plate approximately 110 cm in diameter that holds grating modules on a Rowland torus behind the Chandra mirrors.
Link to pdf for Figure 9.1
images/gess-2.png
Figure 9.2: Detail of the LETG Grating Element Support Structure showing grating modules mounted on the inner annulus.
Link to pdf for Figure 9.2
images/gess-modules.png
Figure 9.3: A closeup view of the LETG GESS showing nearly three complete grating modules. Each module holds three circular grating facets, and each facet contains approximately 80 triangular grating elements.
Link to pdf for Figure 9.3
In contrast to the HETG gratings, which have a thin polyimide substrate, the LETG gratings are free-standing wires held by a support mesh. Within each grating facet the grating bars are supported by perpendicular "fine support" bars and triangular "coarse support" bars. The parameters of these structures are given in Table 9.1. A schematic of the grating structure is shown in Figure 9.4. Both the fine and coarse grating supports act as long-period transmission gratings themselves. The fine support produces a dispersion pattern perpendicular to the grating dispersion direction and the coarse support produces a six-pointed star pattern. These are discussed in more detail in Section 9.4 (see Figures 9.22 and 9.26, respectively).
images/letg-facet.png
Figure 9.4: LETG  facet structure schematic showing the basic shape of the individual grating elements and the relative sizes of the support structures. The upper view shows the complete grating element, which comprises the triangular coarse support, the vertical fine supporting bars, and the (horizontal) grating bars. The grating bars themselves are not shown to scale. In the upper view every 50th grating bar is drawn, in the lower view every 10th bar.
Link to pdf for Figure 9.4
Since the gratings are produced from a single master mask, there is negligible variation in the period between facets. The thickness of the gold of the grating bars on top of the support mesh determines the "phasing," or efficiency of redistribution of photons into each spectral order in wavelengths where the gold is partially transparent. The thickness is designed to optimize the 1st order response at energies of interest.
To reduce aberrations, the GESS is shaped to follow the Rowland torus. The basics of the Rowland geometry are shown in Figure 8.4. The primary readout detector (HRC-S ) is made of three tilted array segments which also follow the Rowland circle in the image plane (see Figures 7.2 and 9.5). Because the detector array elements are flat, the distance from the Rowland circle changes with position, and so the spectral resolution changes very slightly with wavelength. The secondary readout detector, ACIS-S, has 6 CCDs, each of which is only one-quarter as long as an HRC-S segment, so the ACIS-S array follows the Rowland circle even more closely.
images/hrcs_dev_rowland.png
Figure 9.5: The front surfaces of the HRC-S detector segments and their relationship to the Rowland circle are shown schematically. The scalloped line beneath them is the difference between the detector surface and the Rowland circle.
Link to pdf for Figure 9.5

9.2  Calibration

9.2.1  Pre-launch Calibration

Prior to assembly, individual grating elements were tested using a visual light spectrograph at the MPE. Laboratory calibration of grating period and resolution was performed for individual grating elements at optical wavelengths, and extrapolated to the X-ray range. Grating efficiencies at X-ray wavelengths were modeled using near-infrared spectrophotometry, and verified by X-ray measurements of a sample of facets. Grating facet and module alignment was also tested. LETGS efficiency, resolution, and line response function were tested at the X-ray Calibration Facility at MSFC in Huntsville, AL for both ACIS-S and HRC-S configurations. Absolute energy scale and off-axis response were also measured. Efficiency and the Line Spread Function (LSF) of the LETG and HRMA/LETG subsystem were characterized using a detector system designed for HRMA calibration, the HRMA X-ray Detection System (HXDS). Details may be found in the XRCF Final Report at http://cxc.harvard.edu/cal/Hrma/XRCF_Report/

9.2.2  In Flight Calibration

In-flight calibration of the LETGS, along with its primary detector, the HRC-S, is planned and executed by the CXC LETG team. LETG first-light and focus observations were of the active late-type binary Capella whose coronal spectrum is rich in narrow spectral lines (see Figure 9.25). Spectra of Capella and the late-type star Procyon (F5 IV) are used in calibration of the LETG dispersion relation, resolving power, line response function, and grating-detector alignment.
Calibration of the LETGS effective area (EA) and HRC-S relative quantum efficiency (QE) at energies above the C-K edge (0.28 keV, 44 Å) relies primarily on observations of the quasar continuum sources PKS 2155-304 and 3C 273, as well as Capella. PKS 2155-304 is also regularly observed with LETG/ACIS-S to monitor contamination buildup on ACIS (see Section 6.5.1) and for cross calibration with other X-ray missions. For calibration of the LETG/HRC-S EA at longer wavelengths, observations of the hot DA white dwarfs HZ 43 and Sirius B are used.
In addition to these objects, the supernova remnant G21.5-0.9 is used for HRC-S broadband effective area verification studies. The active late type eclipsing binary AR Lac is observed for calibration of the HRC-S spatial characteristics, including resolution (point spread function) and gain variation. Finally, the bright A0 V star Vega is observed with the HRC-S to monitor detector optical and UV susceptibility (see Section 7.8); until recently, Betelgeuse was also observed to monitor the HRC-S's optical response. Early in the mission, most of these objects were observed at least twice per year to monitor LETGS operation, but calibration observations are now less frequent. Targets and monitoring frequencies are listed in Table 9.2.
Table 9.2: Planned LETGS Calibration Monitoring Observations
Target Freq. Purpose
(yr−1)
Capella 1 LETG/HRC-S (ACIS-S for AO9) LRF, dispersion relation, QE, EA
PKS 2155-304 2 LETG/ACIS-S QE, ACIS-S contam, cross calib with XMM, Suzaku
HZ 43 1 LETG/HRC-S EA
AR Lac 1 HRC-S spatial resolution, gain
G21.5-09 1 HRC-S gain, broadband QE, EA
Vega 4 HRC-S UV susceptibility

9.3  LETGS Performance

9.3.1  Usage

Overview

The primary use of the LETG is for on-axis observations of point sources, which produce a zero-order image and a dispersed spectrum. Typical LETGS observations range from a few tens to several hundred ksec; substantially longer observations may require a Large or Very Large proposal. No real-time observations are allowed, and to reduce the (small) risk that the grating mechanism might fail, its frequency of use is minimized by grouping grating observations into consecutive time blocks whenever possible.
Since use of the LETG attenuates the undispersed flux by roughly an order of magnitude, excessive counting rates should not generally be a concern with respect to exceeding detector count rate limits or telemetry saturation. However, some bright sources (e.g. Sco X-1) if observed for long exposure times could cause significant charge depletion in the HRC MCPs-see Chapter 7, especially Section 7.13. Some observers may find it useful to insert the LETG for imaging observations simply to reduce the detected photon counting rate. Even moderate rates may cause pileup problems when using ACIS-S (see Sections 6.15 and 9.4.1/ACIS-S).

Detectors

In standard operation, the LETGS uses the HRC-S as its detector, covering a wavelength range of approximately -165 to +175 Å in 1st order for on-axis sources. This wavelength range can be shifted somewhat by offset pointing, but image quality degrades substantially beyond about 4′. The HRC-S does not have sufficient energy resolution to allow sorting of overlapping spectral orders.
In rare cases it might be useful to use the HRC-S Low Energy Suppression Filter (LESF), as discussed in Section 9.4, in order to obtain a predominantly higher-order (m > 1) spectrum. The LESF is a region on the HRC-S UV/Ion Shield (UVIS) where the aluminum coating is relatively thick, and corresponds to the upper part of the "T" in Figure 7.1. Note that the Al coating on the LESF is thicker on the outer plates than on the central plate. See Figures 9.17 and 9.19 for the effect of the LESF on 1st and higher order effective areas.
The ACIS-S detector can be used instead of the HRC-S, but effective wavelength coverage is reduced because of its smaller size in the dispersion direction (ACIS-S is only half as long as the HRC-S) and the fact that the two outermost chips (S0 and S5) have essentially zero QE for detecting 1st order LETG photons (see Figure 9.6). Another consideration is that ACIS has lower temporal resolution than HRC, which may be important when observing periodic or rapidly varying sources. In some cases, however, those disadvantages may be outweighed by the lower effective background rate and intrinsic energy resolution of ACIS, which can be used to separate diffraction orders. Note that the CTI-degraded energy resolution of the ACIS FI CCDs (Section 6.7) does not pose a problem for LETG point-source observations, since the source position can be placed close to the ACIS readout, where the energy resolution is the best.
images/letg_perf_acissQE.png
Figure 9.6: Comparison of QEs of ACIS FI and BI chips (and the HRC-S) when used with the LETG. QEs shown above are for Y-offset = +1.5′ (0 for HRC-S). Y-offsets may be chosen to tailor the coverage of BI chips (S1 and S3; top/blue QE curve), which have higher QE at low energies than the FI chips (S0,S2,S4,S5; bottom/red curve). Dashed (green) line is for HRC-S, thick solid curves are for ACIS. Vertical lines indicate ACIS chip gaps (with dither, 1.5 Å). For clarity, -3 to +3 Å is not shown for ACIS. See Section 9.4.2/Offset Pointing for more information regarding the choice of Y-offset.
Link to pdf for Figure 9.6
In some specialized cases, the HRC-I may also be used with the LETG. A detailed discussion of the various merits of LETGS detector choices from a point of view of proposal planning is given in Section 9.4.

Off-Axis and Multiple Sources

Because the LETGS is essentially an objective-grating system, it is possible to do multi-object spectroscopy, although as noted above, the point-spread function degrades rapidly off axis. To include or reject secondary sources, or to avoid overlapping diffraction from multiple sources, observers may specify the orientation (roll angle) of the grating dispersion direction on the sky (see Chapter 3). Observations of extended sources are also possible, but at the expense of resolving power and with the loss of the simple relation between position and energy. In angular extent, the standard HRC-S spectroscopy readout region is 3.37′ x 101′, and the full ACIS-S array covers 8.3′×50.6′. In special cases, a different HRC-S detector "window" (up to twice as wide in cross-dispersion) may be selected, as described in Section 9.4.

9.3.2  Wavelength Coverage and Dispersion Relation

The active extent of the HRC-S in the dispersion direction is 296 mm, almost exactly twice that for the ACIS-S. The nominal 0th-order aimpoints for each detector are slightly offset from the detector center so that gaps between the three HRC-S segments (six ACIS-S segments) will occur at different wavelengths in negative and positive orders. A Y-axis (dispersion axis) offset of +1.5' is usually used with LETG/ACIS-S observations to shift coverage of longer wavelengths onto the backside-illuminated S1 and S3 chips, which have higher QE at low energies than the other ACIS-S chips.
With a dispersion of 1.148 Å/mm for the LETG, the standard wavelength range of the LETGS with HRC-S is −164 Å to +176 Å. Physical coverage with ACIS-S extends from −87 to +84 Å when using Yoffset=+1.5', but the poor low-E response of the outlying front-illuminated chips limits the effective wavelength range to −60 Å for negative 1st order and less than about +40 Å for the positive order (see Figure 9.6). Outlying chips may be useful, however, for collecting higher-order spectra.

Off-Axis Pointing and Detector Gaps

Wavelength coverage can be adjusted (increasing the wavelength range on one side and decreasing on the other) by changing the central offset (the observatory `y' coordinate - see the discussion in Chapter 3), although spectral resolution degrades rather quickly beyond about 4′. From the information in Table 9.1, one can derive the relationship between angular offset (in the dispersion direction) and wavelength as 3.36 Å per arcminute, so an offset of 10′ would stretch the positive order HRC-S coverage to approximately 210 Å (60 eV). While the vast majority of LETG observations have been made with offsets of less than 2′, flight LETG calibration data have been collected at 5′ off-axis (for resolution testing) and 10′ off-axis (for effective area calibration).
As noted in Section 9.3.2, there are gaps between detector segments which create corresponding gaps in the wavelength coverage of each order. The gaps in + and − orders do not overlap so that the combined wavelength coverage is continuous. The location of the gaps (neglecting the effects of dither) are listed in Table 9.3, which also lists the location of the HRC-S UV/Ion Shield inner "T" filter edge. Dithering the spacecraft will partially smooth these gaps, but observers may wish to adjust the source pointing if a favorite line falls in a gap, or to tune the wavelength coverage of the higher-QE back-illuminated (S1 and S3) ACIS-S chips. Standard HRC dither amplitude (full width, in both directions) is 40′′ (1.95 mm), which covers 2.3 Å, and standard ACIS dither is 16′′ (0.78 mm), or 0.9 Å.
Please see the "Checking your LETG/ACIS Obscat Setup" web page on the LETGS Observer Information Page (http://cxc.harvard.edu/cal/Letg/). An interactive tool available for visualizing spectral feature placement on the ACIS array as a function of Yoffset and source redshift is available there.
Detector Section Energy Wavelength
(eV) (Å)
HRC-S  UVIS Inner T (thick Al) (690) - 690 (18) - 18 
HRC-S  seg-1 (neg. mλ) (75) - (220) (164) - (56)
HRC-S  seg0 (240) - 200 (51) - 62 
HRC-S  seg+1 (pos. mλ) 185 - 70 67 - 176
ACIS-S  S0 (neg. mλ) (142) - (210) (87.1) - (58.9)
ACIS-S  S1 (Back-illuminated) (212) - (410) (58.5) - (30.2)
ACIS-S  S2 (417) - (7700) (29.7) - (1.6) 
ACIS-S  S3 (Back-illuminated) (11000) - 459 (1.1) - 27.0
ACIS-S  S4 451 - 223 27.5 - 55.7
ACIS-S  S5 (pos. mλ) 221 - 147 56.2 - 84.3
Table 9.3: LETG Position-Dependent Spectral Coverage (1st Order). Energies and wavelengths for the negative order are given in parentheses. Listed values are for the most commonly used pointing (on-axis for HRC-S and Y-offset = +1.5' for ACIS-S) without dither. Standard dithering affects 1.1 Å on the edge of each HRC-S segment and 0.45 Å on the edge of each ACIS chip. Typical uncertainties are of order 0.3 Å and arise from aimpoint drifts and target acquisition errors. Energies and wavelengths for the negative order are listed in parentheses. Note that the QE for FI ACIS chips is much lower than for BI chips at low energies, rendering S0, S4, and S5 of limited use (see Figure 9.6). Please see http://cxc.harvard.edu/cal/Letg/ACIS_params for up-to-date and detailed information regarding offset pointing and wavelength ranges.

Dispersion Calibration

As of this revision, overall wavelength calibration is accurate to a level of a few parts in 10000 across the full wavelength range of the instrument. Based on ongoing analyses of in-flight calibration observations of narrow line sources, the Rowland diameter has remained stable since launch.
The RMS deviation between observed and predicted wavelengths for a set of relatively unblended lines observed in Capella spectra amounts to 0.013 Å, after correction for the relative spacecraft and Capella radial velocity differences. Through analysis of accumulated calibration and GO observations, some remaining differences between predicted and observed line wavelengths have been found to be caused by small event position errors at some locations on the HRC-S detector.
Position errors occur in both dispersion and cross-dispersion axes, though for spectroscopy the latter are usually not important because data are generally summed in the cross-dispersion direction. The magnitude of position errors in the dispersion axis for the central HRC-S plate range from 0 to  ∼ 0.05 Å, with typical errors of  ∼ 0.01-0.02 Å . The outer plates ( > 60 Å ) tend to exhibit larger errors of typically  ∼ 0.02-0.08 Å. The size of the position errors changes over spatial scales of the HRC readout taps, which are 1.646 mm apart (see Chapter 7 for details of the HRC). Spacecraft dither moves dispersed monochromatic photons of a given order over a region of the detector that is roughly 2mm square. Within any given dither region, then, monochromatic light will fall on detector regions that have different position determination errors. As a result, a narrow spectral line could suffer some distortion of its line profile and/or a small shift of its apparent wavelength. Such distortions or shifts could occur at the spacecraft dither frequency (see Chapter 5); observers should therefore exercise caution in interpretation of such periodic effects. Care should also be taken when interpreting the results of combined spectra from + and - orders, since these effects are not symmetrical about 0th order.
The position errors appear to be stable in detector coordinates and to repeat in different observations for which the aim points are very similar. An empirical correction for the effect based on accumulated calibration and GO observations has been derived and is implemented in CALDB 3.2.0 as part of the HRC-S degap map. Details on the derivation of the wavelength corrections can be found under "Corrections to the Dispersion Relation" on the LETG Observer Information web page. After correction, the RMS deviation between observed and predicted wavelengths lines observed in Capella are reduced from 0.013 to 0.010 Å across the entire wavelength range of the instrument. Corrections for the outer plates ( > 60 Å) are much less effective owing to a lack of reference lines with adequate signal-to-noise ratio. For the central plate alone, the RMS deviation amounts to 0.006 Å .
For the purposes of observation planning, it should be assumed that individual observed line wavelengths could be in error by up to about 0.02 Å for λ < 60 Å, and 0.05 Å for λ > 60 Å. Despite the observed repetition in pattern from observation to observation, observers are reminded that the exact wavelength error for any given line depends on the exact position of the target on the detector. Small differences in actual aim point that occur naturally between observations as a result of uncertainties in aspect and target acquisition (see Chapter 5) mean that wavelength shifts for a specific line are not generally repeatable from one observation to another and might also be subject to small secular trends.

9.3.3  Resolving Power

The dominant contribution to the LETGS line response function (LRF) and instrument resolving power is the HRMA point-spread function (PSF), which is  ∼ 25 μm FWHM, depending on energy. The next most important factor is the detector PSF, which is  ∼ 20 μm FWHM for the HRC-S, with 6.43-μm-wide pixels; ACIS pixels are 24 μm wide. Uncertainties in correcting photon event positions for the observatory aspect, which occurs during ground data processing, adds a small contribution of order a few μm. Finally, the small errors in event position determination resulting from HRC-S imaging non-linearities described in Section 9.3.2 can lead to some distortion of spectral line profiles. The size of these effects has not been fully quantified as yet, although they generally affect the line FWHM by less than 25%.
When all these effects are combined, the LETGS line response function is generally  ∼ 40 μm FWHM. With a conversion of 1.148 Å/mm for the LETG, a good figure of merit for LETGS resolution is therefore 0.05 Å. Because the three segments of the HRC-S can not perfectly follow the Rowland circle (see Figure 9.5), however, resolution varies slightly along the detector, and is lowest near the ends of each detector segment. Resolution degradation is almost negligible when using the ACIS-S, since its six segments more closely follow the Rowland circle, although the coarser ACIS pixel size (24 μm vs.  ∼ 40-μm-FWHM LRF) means that line profiles are barely adequately sampled. A plot of LETGS resolving power for an on-axis point source, based on results from an observation of Capella, is shown in Figure 9.7.
images/letg_res_nre.png
Figure 9.7: LETG spectral resolving power, as derived from observations of Capella (Obs ID's 1248, 1009, 58) and Procyon (Obs ID's 63, 1461) with the HRC-S. The analysis is based only on spectral lines thought not to be affected significantly by blending at the LETGS resolution. Measured line widths were corrected for source orbital, rotational, and thermal motions. The dashed line is an optimistic error budget prediction calculated from pre-flight models and instrument parameters. The conservative solid curve is based on in-flight values of aspect, focus, and grating period uniformity. The deviations from approximate linearity near ±60 Å and at the longest wavelengths arise from deviations of the HRC surface from the Rowland circle (see Figure 9.5). Deviations in the experimental data from a smooth curve are likely caused by hidden blends not predicted by the radiative loss model and by detector imaging nonlinearities discussed in Section 9.3.2.
Link to pdf for Figure 9.7
Plots of fits to the LETG+HRC-S LRF at zeroth order and of Fe XVII and XVIII lines at  ∼ 17 and 94 Å are given in Figures 9.8 and 9.9. The fitted form is a Moffat function:
I(λ) = [ 1 +
λ
λc

 2

 
 ]−β
(9.1)
which is a fairly good approximation of the LETG/HRC-S LRF. Figure 9.10 illustrates the χ2 of fits to the zeroth order profile vs. β, and shows a best-fit profile with an index of  ∼ 2.5. (Note that β = 1.0 yields a Lorentzian profile.) The best fit to the very high signal-to-noise-ratio 0th order profile is far from being statistically satisfactory. However, spectral lines seen in first order generally contain orders of magnitude fewer counts than in that calibration spectrum's 0th order, and the Moffat function nearly always provides a good match. Line response functions can also be generated within CIAO in the RMF FITS format. These are based on ray-trace simulations using the MARX program and generally match observed line profiles to a level of 10% or better once intrinsic source broadening terms have been taken into account. Observers wishing to use line profile shapes as a diagnostic tool should keep in mind that, in the case of LETG+HRC-S observations, non-linearities in the HRC-S imaging can lead to significant distortions of observed line profiles (see Section 9.3.2).
images/letg_zero_PSF.png
Figure 9.8: Observed LETG zeroth-order LRF from in-flight calibration observations of the active late-type binary Capella. The model profile, the continuous curve, is a Moffat function (see equation 9.1) corresponding to the best-fit value of β = 2.5. The reduced χ2 of 11.8 certainly indicates a statistically poor fit to these very high S/N data, but the beta model is a good approximation to the LRF for weaker dispersed lines.
Link to pdf for Figure 9.8
images/letg_2lines_fits.png
Figure 9.9: LETGS line response function as illustrated by two bright Fe lines (Fe XVII at  ∼  17 Å and Fe XVIII at  ∼  94 Å ) using in-flight calibration observations of Capella. The solid curves are best-fit Moffat functions with β = 2.5.
Link to pdf for Figure 9.9
images/letg_beta_chi.png
Figure 9.10: LETGS zeroth order profile goodness of fit vs. β, showing a best-fit profile with an index of  ∼  2.5 (see equation 9.1).
Link to pdf for Figure 9.10

Extended Sources

If a source is extended, there is no longer a unique mapping between the position of an event in the focal plane and wavelength, and this results in the apparent degradation of spectral resolving power. For very large sources, the grating resolution may be no better than the intrinsic ACIS energy resolution.
The effect of increased source size on the apparent LETG spectral resolving power has been simulated using the MARX program, and results are shown in Figure 9.11. Another illustration of the effect of source extent may be seen in Figure 9.30 (Section 9.4), which shows model spectra over a small wavelength range.
images/fig_ext_leg.png
Figure 9.11: LETG spectral resolving power for extended sources. The predicted LETG resolving power (E/∆E) is shown versus wavelength for several source sizes. The MARX simulator has been used, and the source is represented by a β model (Equation 9.2). For comparison, the spectral resolution for ACIS front illuminated CCD chips is shown (thick solid line). Note that the ACIS/FI curve does not include the effects of CTI, which progressively degrades resolution away from the readout edge (most of this degradation can be compensated for in data analysis).
Link to pdf for Figure 9.11
In each case, extended sources were modeled using a Beta model for the surface brightness profile. Beta models are often used to describe the distribution of emission in galaxies and clusters of galaxies, and have an identical form to the Moffat function used to describe the line profile above, except that the intensity dependence is radial:

I(r) = [ 1 +
r
rc

 2

 
 ]−β
(9.2)
where I(r) is the surface brightness, r is the radius, and rc characterizes the source extent. β was set to a typical value of 0.75 for the simulations. rc is very close to the radius of a uniform disk fit to a source and was varied in the simulation.

Off-Axis Sources

Similarly, for sources off axis, the increased point-spread function decreases the spectral resolving power. The effect on off-axis sources has been simulated with MARX and is shown in Figure 9.12.
Figure 9.12: LETG spectral resolving power for off-axis sources. The predicted LETG resolving power (E/∆E) is shown versus wavelength for various off-axis distances. For comparison, the spectral resolution for ACIS  front-illuminated CCD chips is shown. Note that the ACIS/FI curve does not include the effects of CTI, which progressively degrades resolution away from the readout edge.
Link to pdf for Figure 9.12
As with extended sources, an ACIS pulse-height spectrum may, in extreme cases, provide energy resolution comparable to or better than the LETG for a source far off axis.

9.3.4  Grating Efficiency

Fine and Coarse Support Structure Diffraction

As explained in Section 9.1, the LETG has fine and coarse support structures which are periodic in nature, and have their own diffraction characteristics. The fine support structure disperses photons perpendicularly to the main spectrum, with about 1/26 the dispersion of the main grating. The coarse support is a triangular grid, and creates a very small hexagonal diffraction pattern which is generally only discernible in 0th order or for very bright lines. Examples of this secondary diffraction are visible in Figures 9.22, 9.23, and 9.26, all in Section 9.3.7.
The two support structures each diffract roughly 10% of the X-ray power, but the coarse-support diffraction pattern is so small that essentially all its photons are collected along with the primary spectrum during spectral-region extraction in data analysis. A significant fraction of the fine-support diffraction pattern, however, may lie outside the spectral extraction region, resulting in a loss of several percent of the total X-ray intensity (see, e.g., Figure 9.22). The fractional retention of X-ray power in the source extraction region is referred to as the spectral extraction efficiency and is discussed in Section 9.3.5.

Total Efficiencies

The 0th, 1st, and selected higher-order grating efficiencies , based on a rhomboidal grating bar analytical model and verified by ground calibration, are shown in Figure 9.13. The efficiency for each order is defined as the diffracted flux with the grating assembly in place divided by the flux if the grating assembly were not in place. Plotted values are for the total diffraction efficiency (including photons diffracted by the coarse and fine support structures), with negative and positive orders summed. Even orders are generally weaker than odd orders up through roughly 6th order.
images/letg_effic_new.png
Figure 9.13: LETG grating efficiency. Summed positive and negative order efficiency is plotted versus wavelength. The top panel shows the zeroth and odd orders; bottom panel shows 1st and even orders. The summed efficiency of orders 6 through 25 is also included in both panels. Plotted values include all support structure diffraction. Due to this inclusion, these curves overestimate the efficiency within the extraction window by a few percent. Features near 6 and 80 Å are due to the partial transparency of the gold grating material at these wavelengths.
Link to pdf for Figure 9.13
The wiggles near 80 Å, and the stronger features near 6 Å, arise from partial transparency of the gold grating material to X-ray photons. Note that there are no absorption-edge features from C, N, or O in the LETG efficiency as there are in the HETG, because the LETG does not use a polyimide support film.

9.3.5  Effective Area

The LETGS effective area for any diffraction order is equal to the product of the HRMA effective area, the net LETG efficiency for that order, and the overall detector efficiency (which varies slightly depending on exactly where the diffracted spectrum falls on the detector). All these quantities vary with wavelength.
Of these three contributors, the HRMA (see Chapter 4) is the best calibrated within the LETGS energy band. Next best is the LETG efficiency, based on both calibration data and theory. The major contributor to the effective area uncertainty lies in the efficiency of the HRC-S, especially at longer wavelengths ( > 44 Å ; < 0.28 keV). The HRMA+HRC-S effective area at those wavelengths could not be adequately calibrated on the ground. In-flight calibration (see Section 9.2.2), particularly of the net 1st order effective area, has provided the best and most extensive data, and the effective area is now believed to be accurate to a level of approximately 15% or better across the most of the bandpass. The effective area in the wavelength range 44-70 Å (0.28-0.17 keV) remains the least well-known, with uncertainties thought to be 20% or less.
Effective areas for orders 2-7 have been calibrated relative to 1st order to an accuracy of  ∼ 10% (5% for 3rd order). Uncertainties for orders beyond 7th may be larger but are generally unimportant.

Instrument Spectral Features

In addition to fixed-position detector features (primarily detector segment gaps-see Section 9.3.2) there are instrumental spectral features which occur at fixed energies because of absorption edges in the materials comprising the HRMA, LETG, and HRC-S or ACIS-S. The edges are tabulated in Table 9.4 and can be seen in the effective area curves (such as Figure 9.17) as decreases or increases in effective area depending on whether the material is part of the mirror, the filter, or the detector. Every effort has been made to adequately calibrate Chandra over its entire energy range, but it should be understood that effective areas near absorption edges are extremely difficult to quantify with complete accuracy and uncertainties in these regions are inevitably higher.
Table 9.4: Instrumental Absorption Edges
Instrument Element Edge Energy Wavelength
(keV) (Å)
HRC Cs L 5.714 2.170
HRC  Cs L 5.359 2.313
HRC  I L 5.188 2.390
HRC  Cs L 5.012 2.474
HRC  I L 4.852 2.555
HRC  I L 4.557 2.721
LETG Au M 3.425 3.620
LETG Au M 3.148 3.938
HRMA Ir M 2.909 4.262
LETG Au M 2.743 4.520
HRMA Ir M 2.550 4.862
LETG Au M 2.247 5.518
LETG Au M 2.230 5.560
HRMA Ir M 2.156 5.750
HRMA Ir M 2.089 5.935
ACIS Si K 1.839 6.742
HRC, ACIS  Al K 1.559 7.953
HRC  Cs M 1.211 10.24
HRC  I M 1.072 11.56
HRC  Cs M 1.071 11.58
HRC  Cs M 1.003 12.36
HRC  I M 0.931 13.32
HRC  I M 0.875 14.17
HRC  Cs M 0.7405 16.74
HRC  Cs M 0.7266 17.06
HRC  I M 0.6308 19.65
HRC  I M 0.6193 20.02
HRC,ACIS  O K 0.532 23.30
HRMA Ir N 0.496 25.0
HRC  N K 0.407 30.5
HRC,ACIS  C K 0.284 43.6
HRC  Al L 0.073 170

Spectral Extraction Efficiency

In practice, it is impossible to "put back" photons which undergo secondary diffraction (from the coarse and fine support structures) in a real observation. Instead, one defines an extraction region for the observed spectrum and adjusts the derived spectral intensities to account for the fraction of total events that are contained within the extraction region.
images/letg_wads.png
Figure 9.14: A MARX simulation of a flat spectrum illustrating the broadening of the LETG+HRC-S  profile in the cross-dispersion direction, and showing the "bow-tie" spectral extraction window. The cross-dispersion profile of an LETG+ACIS-S spectrum is approximately constant across its smaller wavelength range and the default extraction window is rectangular.
Link to pdf for Figure 9.14
images/letg_extract_ao5.png
Figure 9.15: The spectral extraction efficiency for 1st order, representing the fraction of the power diffracted into the LETG 1st order that is enclosed within the default CIAO "bow-tie" spectral extraction window as a function of 1st order wavelength. The "missing" power at long wavelengths is primarily due to diffraction of photons outside of the extraction region by the fine support structure, while scattering losses dominate at shorter wavelengths. These extraction efficiencies were determined using MARX raytrace simulations. In-flight data, though noisier, have been found to agree with these efficiencies to a level of about 5% or better. The uncertainty in the effective area calibration resulting from the extraction efficiency alone is therefore less than 1%.
Link to pdf for Figure 9.14
The default extraction region for the LETG+ACIS-S configuration is a rectangle; that for the LETG+HRC-S configuration is "bow-tie" shaped, comprising a central rectangle abutted to outer regions whose widths flare linearly with increasing dispersion distance (see Figure 9.14). The shape of the bow-tie has been optimized to match the astigmatic cross-dispersion that is a feature of Rowland-circle geometry, with the goal of including as much of the diffracted spectrum as possible while minimizing the included background. Extraction efficiency varies from  ∼ 90% near zero order to more than 95% at the longest wavelengths and is illustrated in Figure 9.15. Effective areas provided in this chapter and on the LETG Observer Information web page are based on this extraction region. CIAO permits adjustment of the bow-tie parameters by the user, but the user should be aware that such changes could change the effective extraction efficiency. The extraction efficiency for default parameters is included in CALDB versions 2.12 and higher.

Zeroth and First-Order Effective Areas

Although the HRC-S is the default detector for the LETG, other detector configurations are possible. Figures 9.16 and 9.17 show effective areas for the LETG 0th and 1st orders, respectively, when using the HRC-S, HRC-S with LESF, or ACIS-S as the readout detector. Based upon these and other plots, the various tradeoffs as to the use of each detector are thoroughly discussed in Section 9.4.
images/letg_0thO_v0110.png
Figure 9.16: LETGS 0th-order effective area for an on-axis point source for the LETG with HRC-S and ACIS-S detectors. The 0th-order effective area for the HRC-S/LESF combination is the same as for the HRC-S. LETG+ACIS-S areas were computed using an effective area model that included the effects of contamination build-up extrapolated to the level expected in mid-May 2009.
Link to pdf for Figure 9.16
Figure 9.17: LETGS 1st-order effective area for an on-axis point source, with HRC-S, HRC-S LESF and ACIS-S  detector configurations with log and linear scaling. Positive and negative orders are summed. LETG+ACIS-S areas were computed using an effective area model that included the effects of contamination build-up extrapolated to the level expected in mid-May 2009. Note that the vertical scale of the linear plot has been truncated.
Link to pdf for Figure 9.17(top) Link to pdf for Figure 9.17(bottom)

Off-Axis and Extended Sources

Differences in the LETGS effective area for off-axis and significantly extended sources compared to the on-axis point source case are primarily determined by the HRMA vignetting function (see Chapter 4).

High-Order Diffraction Effective Areas

Although the LETG (and HETG) have been designed to reduce complications from higher-order diffraction by suppressing even orders, many grating spectra will have overlapping diffraction orders. When ACIS-S is used as the detector, its intrinsic energy resolution can be used to separate orders. The situation is more complicated, however, with HRC-S, which has very little energy resolution. Detector options and various data analysis techniques are described in Section 9.4.
The relative contribution of higher-order photons with different detector configurations can be estimated by inspection of Figures 9.18, 9.19, and 9.20. As an example, say an observer plans to use the LETG/HRC-S configuration and wants to determine the intensity of a line at 45 Å, but knows that line may be blended with the 3rd order of a 15 Å line which has 10 times the emitted intensity of the 45 Å line. Looking at Figure 9.18, we read the 1st- and 3rd-order curves at mλ = 45 Å and see that the 3rd-order value is about one-tenth the 1st-order value. Multiplying by 10 (the ratio of the emitted intensities of the 15 and 45 Å lines), we compute that  ∼ 50% of the feature at mλ = 45 Å will come from the 15 Å line. A fuller explanation, with color figures and more examples for the LETG/HRC-S with line and continuum sources, can be found at http://cxc.harvard.edu/ciao/threads/hrcsletg_orders/.
images/letg_hrcs_ea_orders_mlam.png images/letg_hrcs_ea_orders_lam.png
Figure 9.18: The combined HRMA/LETG/HRC-S effective area, illustrating the relative strengths of 1st and higher orders. Positive and negative orders are summed. In the label for the bottom axis of the top panel, m is the order number. See the text for an example of how to determine the relative strength of overlapping lines from different orders, and http://cxc.harvard.edu/ciao/threads/hrcsletg_orders/ for color figures and further information.
Link to pdf for Figure 9.18(top)
Link to pdf for Figure 9.18(bottom)
images/letg_lesf_ea_orders_v0110.png
Figure 9.19: The combined HRMA/LETG/HRC-S/LESF effective areas for 1st and higher orders. Positive and negative orders are summed.
Link to pdf for Figure 9.19
images/letg_acis_ea_orders_v0110.png
Figure 9.20: The combined HRMA/LETG/ACIS-S effective areas for 1st and higher orders. LETG+ACIS-S areas were computed using an effective area model that included the effects of contamination build-up extrapolated to the level expected in mid-May 2009. Positive and negative orders are summed.
Link to pdf for Figure 9.20

9.3.6  Background

The LETG is always used in conjunction with a focal-plane detector, so the LETG spectra will exhibit that detector's intrinsic, environmental, and cosmic background. The components of the background of the HRC are discussed in Section 7.10. The quiescent background rate over the full detector is close to the 184 cts s−1 telemetry limit. Imposition of the HRC-S spectroscopy window reduces the rate to about 120 cts s−1, as discussed below.

HRC-S Exposure Windows, Deadtime, and Timing Resolution

To avoid constant telemetry saturation, the HRC-S is operated in a default, windowed down "edge-blanking" configuration, in which data from only 6 of the 12 coarse taps in the center of the detector in the cross-dispersion direction are telemetered (see Section 7.10.2). The edge-blanking creates an active detector area slightly less than 10 mm, or 3.4 arcmin, in the cross-dispersion direction. This window easily accommodates the (dithered) dispersed spectra of point sources; other windows may be specified for extended sources or other special cases. In the default configuration, the total quiescent background rate is currently near its maximum of 120 cts s−1. This rate will slowly decrease toward  ∼ 60 cts s−1 as we approach Solar Maximum around 2012.
As long as the total counting rate is below the 184 cts s−1 telemetry limit, detector deadtime is negligible (and recorded as a function of time in the secondary science .dtf files-net exposure time is recorded in the image FITS file header). During background "flares" arising from an increased flux of solar wind particles, however, the background rate may rise above the 184 cts s−1 telemetry limit. During these times detector deadtime may become significant. Current data processing algorithms correct for this deadtime with a typical accuracy of  ∼ 10% or better.
Time resolution approaching 16 μsec can be achieved with the HRC if the data rate is below telemetry saturation. To leave ample margin for telemetry in case of background flares, a special Imaging Mode (see Section 7.11) is used for high-resolution timing observations. This mode utilizes only the central region of the HRC-S detector and provides a field of view of approximately 7′ × 30′. However, if telemetry saturation does take place then the time resolution with HRC-S is approximately the average time between events.

HRC-S Background Reduction via On-Ground Data Filtering

The quiescent BG rate in HRC-S Level 1 data is currently 8.8 ×10−5 cts s−1 arcsec−2, or 0.15 counts per pixel in 100 ks. The extent of a dispersed line in the LETGS spectrum is roughly 0.07 Å (9.5 pixels) wide and the spectral extraction region is 29 to 87 pixels wide in the cross-dispersion direction, yielding roughly 45 to 125 background counts beneath the line in a 100 ks exposure. However, the HRC-S pulse height distribution is sufficiently narrow that a large fraction of pulse-height space can be excluded from the data to further reduce the background, which has a relatively broad pulse-height distribution.
Since gain varies by more than a factor of two across the HRC-S, a gain map has been created which allows the calculation of position-independent pulse height (PI) values from PHA values as part of standard data processing. The relationship between photon wavelength and mean PI value has been determined from laboratory and flight data so that PI/wavelength filtering can be applied to LETG spectra to substantially reduce the background level using standard CIAO tools (see "Background Reduction and Position-Dependent Gain" on the LETG Observer Information web page). Roughly 60% of the background is removed in this way with a < 1% loss of (1st order) X rays, using a "light" PI filter. Because of a gradual decrease in the HRC-S gain (see the LETG web page) and localized pulse-height anomalies, observers should exercise extreme caution in using the "medium" or "heavy" filters. An improved gain map and pulse-height filter based on scaled SUMAMPS values (which vary more smoothly than PHA) that uses a finer spatial grid and accounts for temporal gain changes will be available in 2008.
images/bg_effect_2006.png
Figure 9.21: The LETG+HRC-S background from an observation of Capella. Spectra were derived from standard `bowtie' background extraction region (see Figure 9.14). The curves from the top down correspond to: background spectrum with no PI filtering; minimal filtering (PHA=255 events removed) with 0% X-ray event loss; light filtering with 0.5% X-ray loss; 2% X-ray loss; and 5-15% X-ray loss. X-ray losses are for 1st order; losses will be slightly higher for higher orders, especially with heavier filtering. PI filtering can not applied at wavelengths shorter than 6 Å. The units of the y axis are total events/dispersion-pixel (summed in the cross dispersion direction), where one HRC pixel is 6.43 μm wide (0.0074 Å). The exposure time was 85 ks.
images/bg_effect_2006.pdf Figure 9.21
Relevance for Higher Orders  
The mean of the pulse-height distribution increases weakly with photon energy, such that a factor of two difference in energy corresponds to a shift in the mean of  ∼ 12%, which is about one-half sigma. Therefore, the mean pulse height of 4th order will lie about one sigma away from the 1st order mean. The PI filters are a few to several sigma wide, however, so that only about 5% of 8th order is removed by the light filter. Order separation, even using the heavy filter, is therefore not of any practical use.
Relevance for Observation Planning  
There are two backgrounds relevant for the LETG/HRC-S: the first is the unfiltered rate, which is roughly 120 cts s−1 during quiescence, but can rise during background `flares' to cause telemetry saturation when the total (background plus sources) rate reaches 184 cts s−1; the other is the filtered background rate, leading to  ∼ 21-38 counts in a 0.07-Å spectral bin per 100,000 s integration during quiescence, which may be used for estimating signal-to-noise (see also the discussion in Section 9.5.1 and Figure 9.28 for the spectral dependence).
There is a third background rate which will be of interest when high time resolution (sub-msec) is required, which is the counting rate before any on-board screening is applied. See Sections 7.11 and 7.14.1 for more information on the HRC-S Timing Mode.

ACIS-S Background

As with the HRC-S detector, background rates in ACIS are somewhat higher than expected, but lower than in the HRC. Pulse-height filtering applied during order separation further reduces the effective ACIS-S background to extremely low levels when used with gratings. The reader is directed to Chapter 6 for further discussion.

9.3.7  Sample Data

Figure 9.22 is a detector image of an 85 ksec LETGS observation of Capella (ObsID 1248). The central 30 mm of the dispersion axis and the full extent of the telemetered cross-dispersion window (9.9 mm) is shown. The image is in angular grating coordinates (TG_D, TG_R), which have been converted to Å. The lines radiating from zeroth order above and below the primary dispersion axis are due to fine-support structure diffraction. Star-shaped coarse support structure diffraction is seen around zeroth order. Figure 9.23 is a close-up of the bright Fe XVII, Fe XVIII, and O VIII lines between  ∼ 15-17 Å, in which many orders of fine support diffracted flux can be seen.
images/letg_cap_crossdisp.png
Figure 9.22: HRC-S detector image of LETGS observation of Capella. In order to illustrate the stretching of the cross-dispersion axis, both axes are in Å with 1.148 Å/mm; only the central 30 mm of the central plate is shown. The full extent of the telemetered six-tap cross-dispersion window is shown and measures 9.9 mm. The areas of reduced background at top and bottom are due to dither effects. Star-shaped coarse support structure diffraction is seen around zeroth order, and "cat's whiskers" fine support structure diffraction is seen above and below the primary dispersion axis, as well as in the vertical line through 0th order.
Link to pdf for Figure 9.22
images/letg_cap_fexvii.png
Figure 9.23: Detail of Figure 9.22, showing the LETG/HRC-S image of bright lines in Capella. Both axes are in Å with 1.148 Å/mm. The Fe XVII lines at  ∼  15 and 17 Å are the brightest in the LETG Capella spectrum. Faint features above and below the primary spectrum are due to fine support structure diffraction.
Link to pdf for Figure 9.23
Figure 9.24 is an HRC-S image of a second Capella observation (ObsID 1420, 30 ksec), showing positive order dispersion. The increasing cross-dispersion extent of lines at longer wavelengths is due to astigmatism in the HRMA/LETG system (see also Figure 9.14). The positive order HRC-S  plate gap is seen at  ∼  63 Å. An extracted Capella spectrum (ObsID 62435, 32 ksec), is shown in Figure 9.25. Positive and negative order flux has been summed.
images/letg_cap_image.png
Figure 9.24: HRC-S detector image of a Capella observation, showing positive order dispersion. Both axes are in Å with 1.148 Å/mm. The increasing cross-dispersion extent of lines at longer wavelengths is due to astigmatism in the HRMA/LETG system. The positive-order HRC-S plate gap is at  ∼  63 Å.
Link to pdf for Figure 9.24
images/letg_cap_spec.png
Figure 9.25: Extracted LETGS spectrum of Capella with some line identifications (from Brinkman et al. 2000, ApJ, 530, L111). Many of the lines visible between 40 and 60 Å are 3rd order dispersion of the strong features seen in 1st order in uppermost panel.
Link to pdf for Figure 9.25
Figure 9.26 is a zeroth-order image of summed Sirius AB observations (ObsIDs 1421, 1452, 1459) with a total exposure time of 23 ksec. The star-shaped pattern is due to coarse support structure diffraction. Sirius A and B are separated by  ∼ 4′′. The flux from Sirius A is due to the small, but non-zero, UV response of the detector.
images/letg_sirius.png
Figure 9.26: LETG/HRC-S zeroth order image of Sirius A and B. The two stars are separated by  ∼ 4′′. Flux from Sirius A is due to the small but finite UV response of the detector. The star-shaped structure is due to coarse support diffraction.
Link to pdf for Figure 9.26

9.4  Observation Planning

The purpose of this Section is to provide further information directly related to planning LETGS observations that is not explicitly presented in Sections 9.1 and 9.3, and to reiterate the most relevant issues of instrument performance that should be considered when preparing an observing proposal.

9.4.1  Detector Choices

The best choice of detector will depend on the exact application; some considerations are listed below. For further details concerning the HRC and ACIS detectors, refer to Chapters 7 and 6, respectively. We remind readers that contamination build-up on the ACIS detector has significantly reduced the effective area of the LETG+ACIS-S combination for wavelengths > 20 Å  compared to that at launch.
HRC-S  

   Summary   HRC-S is probably the best detector choice for spectroscopic observations in which one or more of the following observational goals apply:
-signal longward of 25 Å is of significant interest (see Figure 9.17);
-the highest time resolution is required.
HRC-I  

   Summary   HRC-I is possibly the best detector choice for sources in which signal longward of 73 Å (0.17 keV) is not of primary interest and accurate effective area knowledge for > 44 Å ( < 0.28 keV) is not a strong concern, and in addition one or more of the following observational goals apply:
-the source is very weak with interesting spectral features at wavelengths beyond the limit of the LETG/ACIS-S coverage;
-high resolution timing is not required;
-a larger detector area in the cross-dispersion direction than is provided by the HRC-S is required.
ACIS-S  

   Summary   ACIS-S is possibly the best detector choice for sources for which signal longward of 25 Å (0.5 keV) is of little interest and one or more of the following observational goals apply:
-Particular spectral features of interest occur where the LETG+ACIS-S effective area is higher than that of LETG+HRC-S
-High time resolution beyond the 3.2 s exposure of TE mode (less if a subarray is used), or the 2.85 ms of CC mode (if applicable), is not important
-A low resolution 0th order spectrum from the S3 BI chip is of high scientific value, in addition to the dispersed LETG  spectrum
-Order separation is important
-Pileup can either be avoided or mitigated or is not likely to be a problem.

9.4.2  Other Focal Plane Detector Considerations

Instrument Features and Gaps

Attention should be paid to the locations of instrument edge features and detector gaps to make sure that spectral features required to achieve science goals are not compromised by these. Calibration in the vicinities of these features is generally much more uncertain. These features and gaps are listed for both HRMA+LETG+HRC-S and HRMA+LETG+ACIS-S  combinations in Tables 9.3 and 9.4. Note that intrinsic instrumental features, such as edges, are not affected by dithering and offset pointing (see below), but chip gaps in ACIS-S and HRC-S plate gaps, as well as the boundaries between "thick" and "thin" regions of Al that make the "T" shape of the HRC-S UVIS, are.

Dither

The standard LETG+HRC-S dither amplitude is 20 arcsec (40 arcsec peak-to-peak; 2 mm in the focal plane or 2.3 Å ) and that of LETG+ACIS-S is 8 arcsec (16 arcsec peak-to-peak; 0.8 mm or 0.9 Å ), in both axes. Spectral features in dispersed LETG spectra will experience the same dither pattern, and allowance for the size of the dither must be made when considering if spectral features of interest will encounter detector gaps.
In special cases, different dither amplitudes can be specified by the observer, though it must be kept in mind that detector safety constraints, such as accumulated dose/pore in the HRC (see Section 7.13), must not be violated.

SIM-Z Offsets

The SIM permits movement of the focal plane detectors in the spacecraft z direction (perpendicular to the LETG dispersion axis). This can be used to better position a source on the ACIS-S or HRC detectors, for example to accommodate multiple sources, or to place a source over the HRC-S LESF filter region. The nominal aim point for the LESF requires a SIM-Z offset of +7 mm.
In the case of LETG+ACIS-S, a standard SIM-Z offset of -8 mm is applied to point source observations, unless otherwise requested by the observer, in order to place the source closer to the ACIS readout. In the case of extended sources, this offset might not be desirable as it could place part of the source off the detector. The effects of spacecraft dither should always be considered when choosing a SIM-Z offset.

Offset Pointing

As of Feb 2007, the default Z-offset when using ACIS-S (with or without a grating) has been changed from 0 to -0.25 arcmin (-15 arcsec) in order to put the aimpoint closer to the best focus (highest resolution) position. This Z-offset value will be used unless otherwise requested by the observer.
Pointing off-axis in the observatory y axis can be used to change the wavelengths at which detector gaps occur, or to change the wavelength corresponding to the ends of the detectors. Examples of offset pointings are shown in Chapter 3. When choosing offsets, an increase of +1 arcmin in Yoffset corresponds to a shift of +3.36 Å in wavelength. As an example, by invoking a +2 arcmin offset pointing (see Chapter 3 for the convention), the long wavelength cut-off of the HRC-S can be extended in the + order from approximately 176 Å for on-axis pointing to 183 Å. This, of course, is obtained at the expense of a commensurate shortening of coverage in the − order.
Offset pointing leads to degradation of the PSF, and consequently the spectral resolution-see Figure 9.12. For offsets of 2 arcmin or less this degradation is minimal. For offsets of > 4 arcmin, spatial and spectral resolution will be considerably degraded.
In the case of the LETG+ACIS-S configuration, certain offsets might be useful, e.g., in order to place features of interest on (or off) backside illuminated chips for better low energy quantum efficiency. Table 9.3 and Figure 9.6 can be used to determine what offsets are required. There is also an extremely useful visualization tool on the `Checking Your LETG/ACIS-S Obscat Setup' page (http://cxc.harvard.edu/cal/Letg/ACIS_params), which incorporates the latest detector aimpoint calibration (see Section 6.10). Four particularly important Y-offsets are the following:

ACIS-S Modes

When using the LETG+ACIS-S configuration, a mode must be selected for the ACIS detector. The ACIS detector is very flexible but deciding the best set-up can be complicated. Prospective observers considering using ACIS-S for the focal plane detector are urged to read Chapter 6 carefully. The most common modes used for LETG+ACIS-S observations are those using sub-arrays. The shorter frame times of subarrays can be a good way to both increase the time resolution and decrease pileup. Care must be taken when defining subarrays to make sure that the choice of SIM-Z plus any offset pointing in the z direction places the source comfortably inside the subarray. Modes with 256 rows ([1/4] subarray) or fewer are recommended for observations of point (or small) sources, provided that the source position is known to within a few arcseconds. Subarrays permit the use of shorter frametimes, and thus less pileup. As an example, a [1/8] subarray (using 6 chips and with the standard SIM-Z offset of -8 mm) requires a frametime of 0.7 s (vs 3.2 s for the full array). See Section 6.12.1 for further information on frametimes.
When using subarrays, observers will also need to select a Start Row so the spectrum will be properly centered. The telescope aimpoint drifts slowly over time (see Section 6.10), but currently the optimal Start Row value for a [1/8] subarray (using 6 chips with the standard SIM-Z value of -8 mm) is row 123; for a [1/4] subarray, use row 59. Other subarray configurations may be chosen to maximize the number of rows for a given frametime, e.g., 138 rows on 3 chips with a frametime of 0.5 s. Please see "Checking your LETG/ACIS-S Obscat Setup" (http://cxc.harvard.edu/cal/Letg/ACIS_params) for details and the most current information.

Optional ACIS-S Chips

As described in more detail in Section 6.20.1, the continuing gradual degradation of the Chandra thermal environment means that a steadily increasing fraction of observations are experiencing scheduling constraints with regard to the allowable exposure time per orbit. One way to increase the scheduling flexibility of LETG+ACIS-S observations is to use fewer than the full array of 6 CCDs, which reduces the thermal loading of the ACIS power controller.
Beginning in Cycle 8, users may denote some ACIS chips as "Optional." In cases where thermal limits might otherwise be breached, Optional chips will be turned off for an observation; this allows Mission Planning more flexibility when setting up observing schedules and makes the process more efficient. Problematic observations with Optional chips are thus less likely to be split into multiple pieces than they would be if all the chips are required. It is expected that only a handful of all Chandra observations during Cycle 9 will have Optional chips turned off, but this fraction will steadily rise over time.
In deciding which if any ACIS-S chips to mark as Optional in LETG observations, note that chips S0 and S5 have negligible QE for LETG 1st order dispersed photons; those outermost chips are only useful for collecting higher-order spectra, or perhaps in other very special circumstances. The web page "Checking your LETG/ACIS-S Obscat Setup" (http://cxc.harvard.edu/cal/Letg/ACIS_params) and the Spectrum Visualizer tool linked there are helpful in determining which CCDs may be unnecessary.

HRC-S Windowing

As described in Section 9.3.6, the HRC-S has a default spectroscopic "window" defined that limits the detector area from which events are telemetered to the ground. The window is a rectangle based on coarse position tap boundaries; the default rectangle comprises the central 6 taps in the cross-dispersion direction (corresponding to  ∼ 9.9 mm) and the whole detector length in the dispersion direction.
However, this window can be defined to suit special observational goals, such as if the source is extended and the width of the readout region must be increased (with increased risk of telemetry saturation during background flares).
Likewise, a smaller detector region can be used if, for example, the source is very bright and telemetry saturation is a concern. In such a case, the number of cross-dispersion taps could be reduced, since the source only dithers across 2 or 3 taps. There would be less area for the background extraction region, but with a bright source this would not be a problem.
One can also specify that a subset of taps along the dispersion direction be used, particularly if the source spectrum is expected to be cut off at long wavelengths and higher-order spectra are not needed. The most commonly used (though still rare) non-default configuration is simply to use only the central plate with the standard 6 cross-dispersion taps, usually referred to as the Imaging Mode.
When considering defining a special HRC-S window, it is reasonable to assume that the detector background is spatially uniform for the purposes of computing the total source plus background count rate. The telemetry capacity of 184 cts s−1 should be kept in mind to avoid telemetry saturation by using a window that is too big.

9.4.3  General Considerations

Complications from Other Sources

Field sources coincident with the target source dispersed spectrum should be avoided. While some flexibility in roll angle of order several arcminutes can often be accomodated in observation execution, this avoidance is most rigorously accomplished by imposing a roll angle constraint (see below). Note that it is also desirable to retain a pristine region either side of the dispersed spectrum to enable an accurate estimation of the background within the spectral extraction window.
In some circumstances, photons from bright sources outside of the direct field of view of the HRC or ACIS might be dispersed by the LETG onto the detector.
Particular attention should be paid to optically-bright and UV-bright sources, even if these are some distance off-axis. The ACIS-S and HRC-S filters are much more transparent to optical and UV light than are those of HRC-I and ACIS-I (the HRC-S central "T" segment is closer in performance to that of HRC-I, but has completely different thicknesses of polyimide and Al layers). As an example, an observation of the bright A0 V star Vega (V=0.03) in one of the outer HRC-S UVIS segments gave a count rate of about 475 count s−1.
The energy resolution of the ACIS-S detector enables removal by filtering of all photons except those in a fairly narrow wavelength or energy range corresponding to the wavelength or energy of photons in a spectrum dispersed by the LETG. This means that contamination of the dispersed spectrum by, for example, the 0th order or dispersed spectrum of other sources might not be a significant problem.
However, a much better solution to problems of source contamination is, if it is possible within other observation constraints, to choose a roll angle (Chapter 3) that avoids the source contamination issue.

Roll Angle Considerations

Roll angle constraints can be specified to avoid contamination by off-axis sources, as described above, or to help separate the dispersed spectra of multiple sources in the cross-dispersion direction. The maximum separation between dispersed LETG spectra of two sources is obviously one that places the sources in a line perpendicular to the dispersion axis.
Owing to spacecraft thermal degradation and increasing difficulty in scheduling constrained observations, proposers are reminded that only a very limited number of constrained observations can be accomodated. Technical justification for requested constraints must be provided. It is also important to remember that roll angle constraints will also impose restrictions on the dates of target availability as discussed in Chapter 3. Exact restrictions depend on celestial position. Their impact can be examined using the observation visualizer tool, downloadable from the CIAO home page at http://cxc.harvard.edu/ciao/.

High Order Throughput

It is expected that the majority of observations with the LETG will make use of the HRC-S as the readout detector because of its wavelength coverage and high quantum efficiency at long wavelengths. Since the HRC-S has very little energy resolution, the overlapping of spectral orders could be a significant issue and prospective observers should assess the degree to which their observation might be affected. The following list summarizes some considerations:

   Scientific Utility:   Higher spectral orders provide higher spectral resolving power than the 1st order spectrum by the approximate factor of the order number m. For observations in which features are expected to be seen in higher orders, this capability could be scientifically useful.

   LESF:   At the time of writing, the LESF (the region of thicker Al coating on the HRC-S UVIS) remains untested in flight, but could be useful for obtaining a spectrum containing mostly higher order flux.

   Source Spectrum:   For some sources higher orders will contain very little flux and will not be an issue. Typical examples are hot white dwarfs or relatively cool stellar coronae with T ∼ 106 K. Sources whose spectra are fairly weak in the region where the effective area of the LETG+HRC-S is highest ( ∼ 8-20 Å ; 1.5-0.6 keV) but gain in strength toward longer wavelengths will also be less affected by higher order throughput. Typical examples are blackbody-type spectra with temperatures T ∼ 106 K or less, such as might characterise novae or isolated neutron stars.

   Estimates:   Figure 9.18 can be used to estimate high order contamination. PIMMS can be used for gross estimates of higher order count rates; the PIMMS higher order calculation uses an effective area curve for orders m > 1 combined.

   Instrumental Capabilities:   Order separation is straightforward with ACIS-S. With HRC-S orders cannot be separated.

   Deconvolution and Forward Modeling:   By folding a spectral model through an LETG -HRC-S instrument response that includes all significant higher orders (generally ≤ 10), the whole spectrum can be modeled at once. The capability to simultaneously specify the response matrices and effective areas (and extraction efficiencies) for multiple orders is available within the current version of Sherpa/CIAO. Combined response matrices including orders 1 through 6 are presently available under the LETG+HRC-S Effective Area page and may be used with current versions of CIAO and other spectral analysis software such as XSPEC and SPEX. While these latter response matrices are adequate for the analysis of continuum sources, note that they do not include the small-scale wavelength distortions discussed in Section 9.3.2, and hence care must be taken in analyzing line dominated spectra.
An alternative approach is "bootstrap" modeling, where the analysis begins by only considering the shortest wavelengths. The approach assumes that contamination from the higher orders is minimal at the highest energies. The high energy data are used to deduce the higher order contribution at somewhat lower energies which is subtracted, and the process is repeated, working down in energy. The bootstrap approach is independent of an assumed source-model, but it may suffer from the amplification of noise in the case of weak sources and has mathematical problems in distributing photons into successively higher resolution orders. The latter concern renders the bootstrap method better for continuum sources than for those dominated by spectral lines.

ISM Absorption

The long wavelength cutoff of the LETGS in tandem with the HRC-S  detector of  ∼ 175 Å (which can be extended with offset pointing as described in Section 9.4.2), reaches well into the extreme ultraviolet (EUV). In this wavelength regime, the spectra of even very nearby sources with relatively low ISM absorbing columns can be appreciably attenuated by H and He bound-free photoionizing transitions. Therefore observers should be aware that the effective long wavelength cutoff for anything but the nearest sources ( ∼ 100 parsec or less) will probably be determined by ISM absorption. It is also important to remember that neutral and once-ionized He can dominate the ISM absorption cross-section in the 44-200 Å (0.28-0.062 keV) range, and consideration of the neutral H absorption alone is generally not sufficient. Shortward of the C edge near 44 Å (0.28 keV), metals become the dominant absorbers. For illustration, the ISM transmittance for a "typical" mixture of neutral and ionized H and He with H:He:He+ ratios of 1 : 0.1 : 0.01 is shown in Figure 9.27 for the 5-200 Å range with different values of the neutral hydrogen column.