This page is intended to aid proposers and observers as they select instrumental parameters for LETG/ACIS-S observations, specifically with regard to pointing offsets and subarray configurations. Most of the information presented here is also in the Proposer's Observatory Guide (see especially Table 9.3 and section 9.4.2/Offset Pointing). The additional contributions of this page are specific subarray configurations and links to the Spectrum Visualization Tool.

Contents

• Recommended Z-SIM
• Default Z-Offset
• Choosing Y-Offset
  • Spectrum Visualization Tool
• ACIS Subarrays and Optional Chips

Recommended Z-SIM

The default of Z-SIM = -8 mm is strongly recommended. This puts the spectrum close to the ACIS readout, which reduces the effects of CTI-induced energy resolution degradation. With the current (late 2012) ACIS-S aimpoint (Yoffset=0, Zoffset=0) of (chipx,chipy)=(224,490) and a pixel size of 23.987 microns, an 8 mm shift moves the dispersed spectrum down to row 490-(8000/23.987) = 156. The standard default Z-Offset of -15" moves the spectrum up by 31 rows to row 187, and the subarray configurations listed below center the subarray on that row.

Originally, -8 mm was the largest Z-SIM value allowed with ACIS-S (because of fiducial light selection procedures), but that restriction was lifted in late 2005. In principle, a larger Z-SIM value can be used to put the spectrum as close to the readout as desired, but this yields only a very slight improvement in CCD energy resolution and, more importantly, the dispersed spectrum will fall closer to the edge of the ACIS optical blocking filter. Contamination increases rapidly near the filter edges and is less well calibrated than at the default location. Furthermore, dithering samples varying thicknesses of contaminant and therefore leads to periodicity in the source lightcurve; this effect becomes larger toward the filter edges. A few calibration measurements, usually to study contamination absorption, have been conducted at non-default Z-SIM values, including -11.5 mm.

Default Z-Offset

Choosing Y-Offset

Because of aimpoint drift, particularly following a safe-mode in the summer of 2011, Y-offset=0 now puts 0th order near the boundary between nodes 0 and 1 on the S3 chip, so that it dithers across the boundary and slightly degrades the quality of the 0th-order spectrum. To avoid this, one should use Y-offset=+9" (+0.15'), which is the default offset for ACIS-S observations without a grating. Y-offset values may also be chosen to tailor the coverage of the Back-Illuminated (BI) chips, S1 and S3. As seen in the figure below, the BI chips have much higher QE at low energies than the Front-Illuminated (FI) chips.

The Spectrum Visualization Tool displays where spectral features fall on the ACIS-S detector as a function of Y-offset and source redshift. One arcminute of Y-offset corresponds to a shift of 3.36 Å. Keep in mind that the PSF degrades significantly for offsets of more than a few arcmin. The following offsets are of particular interest:

• 0'   Use this offset to obtain the highest possible spectral resolution and if dithering 0th order across node boundaries is not a concern.
• +0.15'   Use this offset to obtain high spectral resolution and avoid dithering 0th order across node boundaries.
• +1.50'     This offset puts the major oxygen emission lines and absorption features (without redshift) safely on the S3 chip instead of on the lower-QE S4 chip or in the chip gap. Spectral resolution is degraded by 20%. Most LETG/ACIS-S Calibration and GO observations use this offset.
• +1.55'     This is the largest offset that can be used and still have 0th order ~completely on S3 (using the standard 16" dither), while allowing an adequate margin (5") for errors in target placement and using a 5-pixel (2.5") radius circle around 0th order. (Note that aimpoint scatter, which is closely related to fiducial light scatter, increased significantly after the 2011 safe mode. If obtaining "all" of 0th order is essential, observers may wish to increase the 2.5" extraction circle radius and/or the 5" aimpoint error margin assumed here.)
• +1.91'     This puts 0th order in the gap between S2 and S3; a small fraction of 0th order events will fall on each chip because of dither. There will be uninterrupted spectral coverage (apart from chip-edge dither effects around 28.5 Å) by the BI chips (S1 and S3) from 0 to 57 Å. Spectral resolution is degraded by ~25%.

Figure 1: LETGS 1st order effective area (EA) with ACIS-S and HRC-S; lower panel shows low-EA regions in more detail. The effects of dither and ACIS bad columns are explicitly included. Dotted lines mark ACIS chip boundaries, and HRC plate gaps appear near -53 and +65 Å. ACIS curve is for Y-offset=+1.5' (5.04 Å shift) and HRC curve is for Y-offset=0'. The ACIS EA is taken from mid-2012. Note the abrupt fall in LETG+ACIS EA beyond ~28 Å as longer wavelengths fall on the Front-Illuminated S4 chip, whereas the Back-Illuminated S1 chip provides useful EA well beyond 50 Å. Y-offsets may be chosen to tailor the coverage of BI chips (S1 and S3), which have significantly higher QE at low energies than the FI chips.

ACIS Subarrays and Optional Chips

A 1/8 subarray will work well in most cases, and is the best calibrated LETG/ACIS configuration. If the source position is not well known or the source is extended, a larger subarray may be needed. The advantages of using a smaller subarray are a faster readout (shorter frametime) and thus less pileup in 0th order and the dispersed spectrum.

Also note from the figure above that S0 and S5 are useless for detecting 1st-order photons, although they may be useful for collecting higher order spectra in some cases. Because of increasingly stringent thermal requirements, the S0 chip will be turned off unless there are compelling reasons for its use. It is further recommended that the S5 chip also be turned off or marked as Optional. Optional Chips can be turned off at the discretion of Mission Planning schedulers if thermal constraints would otherwise be violated. Using fewer chips also allows a slightly shorter frametime (and/or more rows) to be used.

The tables below list subarray parameters for the traditional 1/2, 1/4, and 1/8 subarrays, and for optimized subarrays that maximize the number of rows for a given frametime (sometimes with a 1-row margin), which may be helpful when pileup in 0th order is a concern. The listed Start row values will center the image in the cross-dispersion direction as closely as possible, and are based on the latest calibration of aimpoint drift.

The default Z-SIM value of -8 mm, which puts the spectrum close to the ACIS readout and minimizes the effects of CTI-induced energy resolution degradation, is assumed, as is the default Z-offset value of -0.25'.   Subarray Type = "Custom" in all cases.

Frametime for m active CCDs, using n rows starting with row q, is given by the equation

T(msec) = 41.12*m + 2.85*n + 0.040*m*q - 32.99

and rounding up to the nearest 0.1 sec.

Last modified: 10/24/12

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