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Chapter 7
HRC: High Resolution Camera

7.1  Introduction and Instrument Layout

A schematic of the HRC layout is shown in Figure 7.1, and a summary of the characteristics is given in Table 7.1. A cross-section of the HRC-S layout and the relationships to the optical axis and the LETG Rowland circle are shown in Figure 7.2.

Figure 7.1: A schematic of the HRC focal plane geometry as viewed along the optical axis from the telescope towards the focal plane.

Figure 7.2: A schematic cross-section of the HRC-S MCP (not to scale). The HRC-S is shifted 0.1 mm forward of the tangent plane, so the Rowland circle intersects each segment at two points.

The Instrument Principal Investigator is Dr. Stephen S. Murray of the Smithsonian Astrophysical Observatory.

7.2  Basic Principles

Figure 7.3: A schematic of the HRC Microchannel-Plate detector.

The electron cloud - typically about 2×10⁷ electrons per photon - that emerges from the output MCP is accelerated towards a position-sensitive charge detector. The HRC employs two types of charge detectors: the HRC-I uses a crossed grid charge detector , while the HRC-S uses a hybrid where one axis is comprised of wires and the other has gold lines deposited on a ceramic substrate. Adjacent wires (or lines) are resistively connected and every eighth wire is attached to a charge-sensitive amplifier, referred to as a "tap", as illustrated in Figure 7.4.

The X-ray position is determined by calculating the centroid of the charge cloud exiting the rear MCP via the "three tap algorithm". In short, the three tap algorithm determines the charge cloud centroid using a combination of digital and analog electronics and off-line processing. Fast discriminators and logic circuits first determine a "coarse" position, which is based on the amplifier with maximum detected charge. Analog switches then select the three amplifiers centered on that coarse position and steer them to analog-to-digital converters. The coarse position and three digitized values are then telemetered to the ground and used off-line to calculate the event position. This process is performed for each axis. The reconstructed X-ray position can then be written as the sum of a coarse position and a charge centroid term centered on the coarse position:

where cp is the coarse position, Q_cpi+1 is the charge measured on the cp_i+1 tap, and ∆ is the distance between taps. Since the charge cloud extends beyond the two outer taps, each of the outer amplifiers underestimates the amount of charge needed to calculate the true centroid. For an event perfectly centered on the middle tap, the amount of charge missed by the two outer taps cancel in the equation. If however, the event position is not over the center of a tap, the fractional amount of missing charge is different and produces a small systematic error in the reconstructed position. The small systematic positional error combined with the coarse position logic produce "gaps" in the HRC images. These gaps are perfectly aligned with the detector axes and correspond to positions exactly half-way between amplifier taps. The gaps are systematic and are removed in data processing.

Figure 7.4: Schematic representation of event position determination for one axis of the crossed grid charge detector (CGCD). The electron cloud is divided between several amplifiers. The position of the event relative to the central coarse position is calculated from the difference between the signals on either side of the coarse position divided by the sum of the three signals.

For more details concerning the HRC see Murray & Chappell (1989) and Zombeck et al. (1995).

7.2.1  Aimpoints

7.3  Shutters

7.4  Dither

7.5  Spatial Resolution & Encircled Energy

Approximately 90% of the encircled energy lies within a 14 pixel diameter region (1.8 arcsec) from the center pixel for the observation of AR Lac shown in Figure 7.6. The measured PSF is as good or better than the simulations because a very conservative pre-flight estimate of the aspect solution was used in the simulations. Analysis of AR Lac and Capella observations carried out at different parts of the detector show that a hook-like feature has developed since 2003 (see Figure 7.5; Juda & Karovska 2010). This feature contains ≈ 5% of the flux, and is located ≈ 0.8" from the centroid, towards the origin corner of the detector coordinate system, i.e., in the direction towards the ACIS detectors. This does not appear to be a HRC-specific feature (Kashyap 2010; see also the CIAO caveats page on the PSF artifact: http://cxc.harvard.edu/ciao/caveats/psf_artifact.html).

Figure 7.5: Deconvolved images of the calibration point source AR Lac, observed on-axis once each year from 1999 till 2010 (see Juda & Karovska 2010). The images are displayed congruent with the detector (U,V) coordinate system. The vertical white bar is 1 arcsec in length. A hook-like feature is present in all data acquired since 2002; manifestations of such features may also be present in earlier data (Karovska 2011).

The imaging resolution of the HRC-I degrades off-axis for two reasons: the HRMA PSF increases in size with increasing off-axis angle, and the deviation increases between the flat HRC-I detection surface and the curved HRMA focal surface. The off-axis imaging behavior of the HRC-I is shown in Figure 7.7. The nominal best-focus of the HRC-I is chosen to provide the best image quality in the center of the field-of-view.

Figure 7.6: The predicted and observed fractional encircled energy as a function of radius for an on-axis point source observed with the HRMA/HRC-I. The calculations (at two energies, 0.277 keV and 6.40 keV) include a very conservative estimate of the aspect solution (FWHM = 20 μm (0.41")). Flight data from an observation of AR Lac are also shown.

Figure 7.7: Encircled energy as a function of source off-axis angle for 50% and 90% encircled energy for 1.49 and 6.40 keV for the combined HRMA/HRC-I. A conservative contribution from the aspect solution is included (FWHM = 20 μm (0.41")). A plot for the HRC-S would be almost identical since the PSFs of the two instruments are virtually identical.

7.6  Non-Dispersive Energy Resolution

Figure 7.8: Pulse height versus energy for the HRC-I detector (top) and the centroid of the pulse height distribution versus energy (bottom). These data were obtained at SAO during flat field, normal-incidence-illumination tests. The voltage settings have been changed in-flight and thus the applicability of these data is questionable. They are presented here only for illustrative purposes.

Figure 7.9: The color-color grid for a power-law spectral model, as calculated for the HRC-I  (left) and the HRC-S (right). The PI channels are grouped into three bands, S=50:130, M=131:200, and H=201:280 for the HRC-I and S=30:100, M=101:150, and H=151:300 for the HRC-S, and their logarithmic ratios are plotted along the two axes for specific values of the model parameters α (the index of the power-law function) and N_H (the absorbing column density in units of 10²² cm⁻²). Lines of constant α are depicted in red shades, and lines of constant N_H in blue shades.

Figure 7.10: As in Figure 7.9, for an APED thermal model at relatively low temperatures. The loci of constant plasma temperature (kT) are in shades of red and are labeled by their value in keV.

Figure 7.11: As in Figure 7.10, for a set of higher plasma temperatures.

Figure 7.12: As in Figure 7.9, for a blackbody model. The loci of constant temperature (kT) are in shades of red and labeled by their value in keV.

7.7  Gain Variations

Figure 7.13: Monitoring the gain and gain correction across the HRC-I detector. The mean scaled sum of the amplifier signals (SUMAMPS) for AR Lac observations, carried out at various times and numerous locations across the HRC-I detector, are shown. The scaled SUMAMPS replace PHA. There also exist intrinsic variations in the spectrum which have not been accounted for in this figure. The values from data obtained at different years are shown with different symbols. Note the steady decline in the mean scaled SUMAMPS with time for all pointings; the gain maps in the CALDB  can be used to renormalize them such that they are equivalent to an on-axis observation made in October 1999. The horizontal dotted line represents the gain corrected Pulse Invariant (PI) values and the shaded band represents the 1σ scatter on them.

Figure 7.14: Monitoring the gain and gain correction across the HRC-S  detector. The mean scaled SUMAMPS at each epoch (solid points) declined with time at all pointings from the beginning of the mission (square symbols) till before the voltage change (circle symbols). After the voltage was increased in March 2012, the mean SUMAMPS incraeased (diamond symbols) to nearly the same level as at the beginning of the mission.

7.8  UV/Ion Shields

The shields suppress out-of-band (outside the X-ray band) radiation from the ultraviolet through the visible. The detector response to out-of-band light for an object in its field-of-view is a possible source of unwanted signal. Suppressing out-of-band radiation is particularly important for observing sources which have bright XUV and UV fluxes. The HRC has strongly reduced sensitivity in this spectral region, as shown in Figure 7.15. As part of the in-flight calibration program the bright A star Vega (A0V, U=0.02, B=0.03, V=0.03) was observed with both the HRC-I and HRC-S. The predicted count rate for HRC-I was 7×10⁻⁴ cts s⁻¹. From monitoring observations of Vega, an upper limit to the UV rate of 8 ×10⁻⁴ cts s⁻¹ is calculated (Pease et al. 2005). The image of Vega was also placed on three regions of the HRC-S  - the inner segment "T", the thin aluminum inner segment, and on one of the thin aluminum outer segments. The predicted count rates were 1, 400, and 2000 cts s⁻¹ respectively. The corresponding observed rates were 0.2, 240, and 475 cts s⁻¹. Sirius was observed with the HRC-S/LETGS in order to obtain a soft X-ray spectrum of Sirius B (white dwarf) and Sirius A (A1V, V=-1.46, B-V=0.01) was seen in zeroth order at about the expected count rate. Based on these sets of observations, the UV/Ion shields are performing as designed. Ongoing monitoring observations of Vega indicate no change in the UV response of HRC-I and HRC-S since launch. For a detailed discussion of the out-of-band response of the HRC to stars, see http://hea-www.harvard.edu/HRC/calib/palermopaper.ps , which allows one to determine the out-of-band count rate produced by a blackbody source with known T_eff, m_V, and N_H.

Scattered UV, FUV, and XUV light from the Sun or the bright Earth may cause a background dependence on viewing geometry. The spacecraft was designed to limit the contribution from stray scattered radiation to 0.01 cts cm⁻² s⁻¹ (2.4×10⁻⁷ cts arcsec⁻² s⁻¹) on the HRC. The imaged components of scattered radiation are dependent on the solar cycle, but are at most  ∼  0.01 cts cm⁻² s⁻¹ for most lines of sight.

Figure 7.15: The HRC-I (top) and the center section of the HRC-S (bottom) UV/Ion shield effective area as a function of wavelength.

7.9  Quantum Efficiency and Effective Area

The combined HRMA/HRC effective areas - the product of the HRMA effective area, the quantum efficiency of the HRC-I or the HRC-S and the transmission of the appropriate UV/Ion shield, integrated over the point spread function - are shown in Figure 7.16. Monitoring of the efficiency of both detectors is continuing. The charge extracted since launch has resulted in a small decrease in gain in both detectors, but this has had a negligible effect on the efficiency (See http://cxc.harvard.edu/cal/Hrc/ ). The HRC-S QE has been declining at a rate of <~1% per year. This decline is generally wavelength independent except for certain locations on the detector where the gain decline causes loss of photons below the lower level discriminator. The QE decline in the HRC-I is < 2% over the duration of the mission at long wavelengths.

Figure 7.16: The effective area of the HRMA/HRC-I (dashed line) and the central segment of the HRMA/HRC-S in imaging mode (solid line) integrated over the full PSF. Absorption edges are due to the iridium coating on the mirrors, the CsI MCP coating, and the polyimide/Al of the UVIS.

7.10  On-Orbit Background

7.10.1  HRC-I

Ground-based filtering further reduces the non-X-ray background in the HRC detectors (see Murray et al. 2000, Juda et al. 2000 and Wargelin et al. 2001; http://cxc.harvard.edu/cal/Letg/Hrc_bg/). After filtering the non-X-ray background for HRC-I data is reduced by  ∼ 40% while the corresponding reduction in X-ray events is less than a few percent. For the HRC-S, the non-X-ray background is decreased by  ∼ 50% and the X-ray loss is 1−2%. Furthermore, filtering makes the spatial distribution of the detector background flatter. Filtering also removes saturated events responsible for faint secondary "ghost" images (see Section 7.11).

7.10.2  HRC-S

The background can be further reduced in ground data processing by using pulse height filtering that preferentially selects X-rays over cosmic ray events. A reduction in background by a factor of about three is possible for dispersed spectra. Thus there are two relevant background rates for the HRC-S: a telemetry rate of 120 cts s⁻¹ and a post-processing rate for calculating signal to noise. The latter is discussed in detail in Section 9.3.6 (see especially Figure 9.22).

7.10.3  Temporally Variable Background

Figure 7.17: An example of the background variability during a  ∼ 30 ks HRC-I observation of the SNR G21.5-09 taken on 1999-10-25. The total event rate (middle) and valid event rate (bottom) show correlated bursts up to  ∼ 800 cts s⁻¹. The bursts are uniformly distributed over the detector. The anti-coincidence shield (top) exhibits no correlated enhancements. The total and valid rates differ by  ∼ 200 cts s⁻¹ due primarily to cosmic ray events that are vetoed and don't appear as valid events in the telemetry.

Figure 7.18: Average background spectra in the inner 10′ region of the detector, obtained from yearly observations towards AR Lac are shown. Times of high background flaring have been excluded. The magnitude and shape of the spectra vary considerably with time, but may be interpolated between epochs to estimate the background for any given observation.

Figure 7.19: The change in HRC-I stowed background rate with time. The total rate (black points and error bars, upper set) and the valid rate (red points and error bars, lower set) are shown. The HRC-S data are similar.

Figure 7.20: The range of PI that should be included to reduce the background by a given percentage is shown in the figure as a shaded band for an assumed absorbed power-law spectrum. All of the parameters included in the grid for Figure 7.9 are considered possible, and these PI ranges can be shrunk if more information is available for a particular source. The depth of the shading indicates how much of the source events are expected to be lost. The vertical dashed lines indicate the background reduction for source event losses of 1%, 5%, and 10%. As shown in Figure 7.18, the background varies with time. Here, for the sake of definiteness, we use the background from the year 2008.

Figure 7.21: As Figure 7.20, but for sources with thermal spectra, for parameter values depicted in Figures 7.10, 7.11.

Figure 7.22: As Figure 7.20, but for sources with blackbody spectra, for parameter values depicted in Figure 7.12.

7.11  Instrument Anomalies

The HRC has a hardware problem that corrupts the data from the position taps under a specific set of conditions: 1) the amplifier scale factor is switched to the least sensitive scale, 2) an even number of taps on the axis have signals that are above a set threshold, and 3) the event occurs on the negative side of the tap. When these conditions are met the tap signals are sampled while the amplifiers are still ringing after switching from the initial guess for the event coarse position to the correct one. The ringing results in offsets on the telemetered tap values from their true values, with the smallest signal of the triplet for an axis being most affected. When the event position is calculated from corrupted data, positions are incorrectly determined and can be off by a few pixels. This ringing is partially corrected for in ground processing (Juda et al. 2000). These corrections are implemented via the CIAO tool hrc_process_events. Observers, if they are concerned that the ringing may be producing artifacts, can apply additional filtering to remove events with AMP_SF=3.

A wiring error in the HRC causes the time of an event to be associated with the following event, which may or may not be telemetered. The result is an error in HRC event timing that degrades accuracy from about 16 microseconds to roughly the mean time between events. For example, if the trigger rate is 250 events/sec, then the average uncertainty in any time tag is less than 4 milliseconds.

The HRC team has developed a special operating mode that allows high precision timing to be achieved (see Section 7.14.1). This timing mode uses only the central segment of the HRC-S. Disabling the outer two segments lowers the total count rate by two-thirds, dropping it below the telemetry saturation limit for most sources. Thus, there is a high probability that all events will be telemetered. In this case, once the time tag of each event has been appropriately shifted in ground processing, the original timing accuracy (16 microseconds) can be recovered. When using this approach, it is prudent to be sure that the total count rate (source plus background) is somewhat below the telemetry saturation limit to avoid telemetry saturation due to statistical fluctuations in the count rate.

In addition to the primary science data for individual events, the rate of microchannel plate triggers (total rate) and triggers that pass on-board validity tests (valid rate) are telemetered to the ground. The valid rate is used to correct the primary rate for deadtime and telemetry saturation effects. As long as the primary rate is below saturation, the primary rate itself can be used to make the small ( < 1%) correction, since the event processing dead-time is known. However, when the event rate exceeds saturation, a fairly common occurrence because of background flaring from low energy protons, the valid rate is necessary to correct the event rate. Unfortunately, the total and valid event rates are overestimated by about 15% for normal operation of the HRC-S . This problem is caused by an overshoot in occasional large trigger pulses, resulting in double counting in the total and valid event on-board scalers. The primary science event is not affected, since once event processing starts with the initial trigger pulse, a gate rejects further pulses until processing is complete. The HRC-I does not have this overshoot problem. The HRC-S valid event rate is corrected in standard processing, using the fraction of event pulse amplitudes that are above a given (segment dependent) threshold.

7.12  Calibration

7.13  Operational considerations and constraints

In addition to the general Chandra observatory level constraints (Chapter 3), there are a few HRC-specific considerations and constraints that must be taken into account when planning an observation.

7.13.1  Total Count limits

7.13.2  Count rate limits

Telemetry Limit

1. Bright target:
   • Insert either the LETG or HETG and analyze the zeroth-order image. This solution may be so dramatic as to substantially increase the required observing time.
   • Offset aimpoint. To be effective, this solution may result in substantially reduced spatial resolution.
2. Bright nearby source
   • Depending on the proximity, an appropriate choice of roll angle and/or offset can position the offending source(s) off the detector. Flux from bright sources could be blocked with the HRC shutters, but note that only one blade is functional and this option is unavailable in the standard setup.
   • Request a rectangular window for on-board data so that events produced by the nearby bright source(s) do not contribute to the telemetry limit.

Linearity limit

It is important to be aware that avoiding telemetry saturation does not guarantee that linearity limits are not exceeded. There are only three approaches to assure that the linearity limit is not exceeded:

• Offset aimpoint to smear the image out.
• Insert a transmission grating to reduce the flux and offset aimpoint (if also necessary).
• Defocus - this option is not recommended and is only mentioned for completeness.

7.14  Observing with HRC - Operating Modes

7.14.1  Timing Mode

The HRC-S, when used in this mode, provides about a 6 x 30 arcmin field of view.

7.14.2  Edge and Center Blanking

7.15  References

General

David, L.P., Harnden, F.R. Jr., Kearns, K.E, and Zombeck, M.V., The ROSAT High Resolution Imager (HRI) Calibration Report, revised (1999).
http://hea-www.harvard.edu/rosat/hricalrep.html

Fraser, G., "X-ray Detectors in Astronomy", 1989, Cambridge University Press.

Giacconi, R., et al., 1979, Ap. J., 230, 540.

Murray, S.S., Chappell, J.H., Elvis, M.S., Forman, W.R., Grindlay, J.E., Harnden, F.R., Jones, C.F., Maccacaro, T., Tananbaum, H.D., Vaiana, G.S., Pounds, K.A., Fraser, G.W., and Henry, J.P., "The AXAF High Resolution Camera (HRC) and its use for observations of Distant Clusters of galaxies" Astro. Lett. Comm., 26, 113-125, 1987.

Murray, S.S., et al., "In-flight Performance of the Chandra High Resolution Camera", SPIE, 4012, 2000. http://hea-www.harvard.edu/HRC/calib/ssmspie2000.ps

Zombeck, M.V., Chappell, J. H , Kenter, A, Moore, R., W., Murray, S. S., Fraser, G.W., Serio, S.,"The High Resolution Camera (HRC) on the Advanced X-ray Astrophysics Facility (AXAF)", Proc. SPIE, 2518, 96, 1995.

Position modeling, de-gap corrections, and event screening

Juda, M., & Karovska, M., "Chandra's Ultimate Angular Resolution: Studies of the HRC-I Point Spread Function", AAS/HEAD 2010. http://hea-www.harvard.edu/ juda/memos/HEAD2010/HEAD2010_poster.html

Karovska, M., 2011, "Followup Study of the PSF Asymmetry", CXC Memo, Jun 2011.

Kashyap, V., et al., 2005, "HRC-S Degap Corrections".
http://cxc.harvard.edu/cal/Letg/Hrc_disp/degap.html

Kashyap, V., "Analysis of Chandra PSF feature using ACIS data", CXC Memo, Oct 2010. http://cxc.harvard.edu/cal/Hrc/PSF/acis_psf_2010oct.html

Kenter, A., "Degap as a Transformation of Probability Distribution Problem", 3/1/99. http://hea-www.harvard.edu/HRC/calib/degap.ps

Murray, S.S., Chappell, J.H., 1989, SPIE 1159, 460-475. "Position Modeling for the AXAF High resolution Camera (HRC)"

Murray, S.S., et al., "Event Screening for the Chandra X-ray Observatory High Resolution Camera (HRC)", SPIE Proceedings, 4140, 2000.
http://hea-www.harvard.edu/HRC/calib/event_screening.ps

http://cxc.harvard.edu/ciao/caveats/psf_artifact.html (CIAO Caveats page on the PSF Artifact)

Count rate limitations and linearity

Juda, M and Dobrzycki, A, "HRC Deadtime and Telemetry Saturation", 6/18/99.
http://cxc.harvard.edu/contrib/juda/memos/tlm_sat.html

Juda, M., "Telemetered vs. Processed Events", memo, 12/7/01.
http://cxc.harvard.edu/contrib/juda/memos/proc2valid/index.html

Juda, M., "HRC-S Double Pulse Fraction", memo, 6/27/02.
http://cxc.harvard.edu/contrib/juda/memos/proc2valid/pha_fraction.html

Kenter, A.T., Chappell, J.H. Kobayashi,K.,Kraft,R.P., Meehan, G.R., Murray, S.S., Zombeck, M.V., Fraser, G.W., Pearson, J.F., Lees, J.E., Brunton, A.N. and Pearce, S.E. Barbera, M., Collura, A., Serio, S., "Performance and Calibration of the AXAF High Resolution Camera I " SPIE 3114, 1997.

Pease, D.P., & Donnelly, H., memo, 5/1998.
http://cxc.harvard.edu/cal/Hrc/detailed_info.html#ctrt_lin

Zombeck, M. V., "Secondary Science Rate Double Counting", memo, 2/12/02.
http://hea-www.harvard.edu/HRC/calib/doublecount.html

Calibration

http://cxc.harvard.edu/cal (CXC calibration site)

http://hea-www.harvard.edu/HRC/calib/calib.html (HRC IPI Team calibration site)

http://cxc.harvard.edu/cal/Hrc/(HRC CXC Cal team site)

Juda, M., 2012, CXC Memo, "Pile-up" Effect on the HRC PSF
http://cxc.cfa.harvard.edu/contrib/juda/memos/hrc_pileup/index.html

Kenter, A.T., Chappell, J., Kobayashi, K., Kraft, R.P., Meehan, G.R., Murray, S.S., Zombeck, M.V., "Performance and Calibration of the AXAF High Resolution Camera: I. Imaging Readout", SPIE, 3114, 26, 1997.
http://hea-www.harvard.edu/HRC/calib/spie97_kenter.ps

Kenter, A., et al., "In-flight Performance and Calibration of the Chandra High Resolution Camera Spectroscopic Readout (HRC-I)" SPIE, 4012, 2000.
http://hea-www.harvard.edu/HRC/calib/hrci.spie2000.ps

Kraft, R.P., Chappell, J., Kenter, A.T., Kobayashi, K., Meehan, G.R., Murray, S.S., Zombeck, M.V., "Performance and Calibration of the AXAF High Resolution Camera: II. the Spectroscopic Detector", SPIE, 3114, 53, 1997.
http://hea-www.harvard.edu/HRC/calib/spie97_kraft.ps

Kraft, R., et al., "In-flight Performance and Calibration of the Chandra High Resolution Camera Spectroscopic Readout (HRC-S)" SPIE, 4012, 2000.
http://hea-www.harvard.edu/HRC/calib/hrcs.spie2000.ps

Meehan, G.R., Murray, S.S. , Zombeck, M.V., Kraft, R.P., Kobayashi, K., Chappell, J.H., and. Kenter, A.T., "Calibration of the UV/Ion Shields for the AXAF High Resolution Camera", SPIE, 3114, 74, 1997.
http://hea-www.harvard.edu/HRC/calib/spie97_meehan.ps

Meehan, G, "Calibration of the HRC-I UV/Ion Shield", 10/13/99.
http://hea-www.harvard.edu/HRC/calib/hrci_cal_report.ps

Meehan, G.,"Calibration of the HRC-S UV/Ion Shields", 10/13/99.
http://hea-www.harvard.edu/HRC/calib/hrcs_cal_report.ps

Murray, S. S.; Chappell, J.H.; Kenter, A. T.; Kobayashi, K.; Kraft, R. P.; Meehan, G. R.; Zombeck, M. V.; Fraser, G. W.; Pearson, J. F.; Lees, J. E.; Brunton, A. N.; Pearce, S, E.; Barbera, M.; Collura, A.; Serio, S., "AXAF High-Resolution Camera (HRC): calibration and recalibration at XRCF and beyond", SPIE, 3114, 11, 1997.

Background

Isobe, T., and Juda, M., memo, 9/11/2007.
http://cxc.harvard.edu/contrib/cxchrc/Stowed_study/hrc_stowed_position_study.html

Isobe, T., and Juda, M., 2007, "High Resolution Camera Stowed Background Study", Proc. of Chandra Calibration Workshop, October 2007, Huntsville, AL.
http://cxc.harvard.edu/ccw/proceedings/07_proc/presentations/isobe/

Isobe, T., and Juda, M., 2009, "How to Create a Background Map for an Observation", memo, 1/27/2009.
http://cxc.harvard.edu/contrib/cxchrc/Stowed_study/hrci_image_correction.html

Isobe, T., and Juda, M., 2009, "How to Create a Background Map for an Observation", Proc. of Chandra Calibration Review, September 2009, Boston, MA.
http://cxc.harvard.edu/ccr/proceedings/09_proc/presentations/isobe/

Juda, M., "Time History of the HRC Background", memo, 5/22/01.
http://cxc.harvard.edu/contrib/juda/memos/hrc_bkg/time_history.html

Juda, M., "HRC Rates and High Solar Activity", memo, 5/21/01.
http://cxc.harvard.edu/contrib/juda/memos/hrc_bkg/high_solar.html

Juda, M., et al., "Characteristics of the On-Orbit Background of the Chandra X-ray Observatory High Resolution Camera", Proc. SPIE 4851, August 2002
http://cxc.harvard.edu/contrib/juda/memos/spie2002/spie2002.html, http://cxc.harvard.edu/contrib/juda/memos/spie2002/spie2002.ps

Detector coordinate systems

McDowell, J., "Coordinate Systems for Analysis of On-orbit Chandra Data, Paper I: Imaging", http://cxc.harvard.edu/contrib/jcm/ncoords.ps

Counts lifetime

Kenter, A.T., K.A. Flanagan, G. Meehan, S.S. Murray, M.V. Zombeck, G.W. Fraser, J.F. Pearson, J.E. Lees, A.N. Brunton, and S.E. Pearce, "Microchannel plate testing and evaluation for the AXAF high resolution camera (HRC)", Proc. SPIE, 2518, 356, 1995.

Gain, spectral response, out-of-band response

Kashyap, V., Posson-Brown, J., 2005, "Spectral Response of the HRC-I", Chandra Calibration Workshop, Oct 31-Nov 1 2005,
http://cxc.harvard.edu/ccw/proceedings/05_proc/presentations/kashyap2/

Kashyap, V. Posson-Brown, J., 2009, "The Imaging and Spectral Performance of the HRC", Chandra Calibration Review, 2009.14,
http://cxc.harvard.edu/ccr/proceedings/09_proc/presentations/kashyap/

Pease, D.O., Drake, J.J., & Kashyap, V.L., 2005, "The Darkest Bright Star: Chandra X-Ray Observations of Vega", ApJ, 636, 426

Pease, D., Kashyap, V., Drake, J., Juda, M., 2005, "Monitoring the HRC-S UV Rate: Observations of Vega", CXC Memo, May 2005,
http://cxc.harvard.edu/cal/Hrc/Documents/hrcs_vega_05.ps

Posson-Brown, J., Kashyap, V., 2005, "Monitoring the Optical/UV Transmission of the HRC with Betelgeuse", CXC Memo, June 2005,
http://cxc.harvard.edu/cal/Hrc/Documents/betelgeuse.ps

Posson-Brown, J., Kashyap, V., 2005, "Monitoring the Optical/UV Transmission of the HRC with Betelgeuse", Chandra Calibration Workshop, Oct 31-Nov 1 2005,
http://cxc.harvard.edu/ccw/proceedings/05_proc/presentations/possonbrown/

Posson-Brown, J., Kashyap, V., 2009, "SUMAMPS-based gain maps and RMF for the HRC-I", Chandra Calibration Review, 2009.16,
http://cxc.harvard.edu/ccr/proceedings/09_proc/presentations/possonbrown2/

Wargelin, B., 2012, CXC Memo, HRC-S Voltage Change,
http://cxc.cfa.harvard.edu/cal/Letg/newHRCShv/

Wilton, C., Posson-Brown, J., Juda, M., Kashyap, V., 2005, "The HRC-I Gain Map", Chandra Calibration Workshop, Oct 31-Nov 1 2005 ,
http://cxc.harvard.edu/ccw/proceedings/05_proc/presentations/wilton/

Zombeck, M.V., "HRC-I out of band response."
http://hea-www.harvard.edu/HRC/calib/hrci_cal.html

Zombeck, M.V., "HRC-S out of band response."
http://hea-www.harvard.edu/HRC/calib/hrcs_cal.html

Zombeck, M.V., et al., "Vega calibration observations."
http://hea-www.harvard.edu/HRC/calib/vega/vega.html

Zombeck, M.V., et al., "The Out-of-band Responses of the HRC on Chandra", X-ray 2000 Proceedings, Palermo, 2000.
http://hea-www.harvard.edu/HRC/calib/palermopaper.ps

Zombeck, M. V., "Response of the HRC to Vega", memo, 10/28/02.
http://hea-www.harvard.edu/HRC/calib/vega/vega_trend.html

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