7. High Resolution Camera (HRC)

7.1 Introduction and Instrument Layout

The High Resolution Camera (HRC ) is a microchannel plate (MCP ) instrument comprised of two detectors, one optimized for imaging (HRC-I ), and one (HRC-S ) serves as a readout for the Low Energy Transmission Grating (LETG ) discussed in Chapter 9. The HRC-I provides the largest field-of-view ( $\sim 30' \times 30'$ ) of any detector aboard Chandra , and its response extends to energies below the sensitivity of the ACIS (Chapter 6), albeit without comparable spectral resolution. The time resolution of the HRC detectors (16 $\mu$ sec) is the best on the observatory, but can only be exploited under certain conditions as discussed in Section 7.10.

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, the relationship to the optical axis and to the LETG Rowland circle is 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.
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**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.
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The HRC is a direct descendant of the Einstein (Giacconi et al. 1979) and ROSAT High Resolution Imagers (HRIs) (David et al. 1996). The ROSAT HRI had the same coating (CsI) as the HRC .

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

**Table 7.1:** HRC Parameters
Focal Plane Arrays
HRC-I :	CsI-coated MCP pair	$90\times90$ mm coated
		( $93\times93$ mm open)
HRC-S :	CsI-coated MCP pairs	3- $100\times20$ mm
Field of view	HRC-I :	$\sim 30\times30$ arcmin
	HRC-S :	$6\times99$ arcmin
MCP Bias angle:		$6^\circ$
UV/Ion Shields:
	HRC-I :	5520 Å Polyimide, 763 Å Al
	HRC-S :
	Inner segment	2750 Å Polyimide, 307 Å Al
	Inner segment ``T''	2750 Å Polyimide, 793 Å Al
	Outer segment	2090 Å Polyimide, 304 Å Al
	Outer segment (LESF)	2125 Å Polyimide, 1966 Å Al
Spatial resolution	FWHM	$\sim 20\mu$ m, $\sim 0.4$ arcsec

	HRC-I : pore size	10 $\mu$ m
	HRC-S : pore size	12.5 $\mu$ m
	HRC-I : pore spacing	12.5 $\mu$ m
	HRC-S : pore spacing	15 $\mu$ m
	pixel size (electronic readout)	$6.42938\mu\rm m$
		[0.13175 arcsec pixel $^{-1}$ ]
Energy range:		keV
Spectral resolution	$\Delta E/E$	$\sim 1$ @1keV
MCP Quantum efficiency		30% @ 1.0 keV
		10% @ 8.0 keV
On-Axis Effective Area:	HRC-I , @ .277 keV	$133\,\rm cm^2$
	HRC-I , @ 1 keV	$227\,\rm cm^2$
Time resolution		16 $\mu$ sec (see Section 7.10)
Limiting Sensitivity	point source, 3 $\sigma$ detection in $3\times10^5$ s	$9 \times 10^{-16}\rm erg\,cm^{-2}\,s^{-1}$
	(power law spectrum: $\alpha$ = 1.4,
	$\rm N_H = 3 \times 10^{20}$ cm $^{-2}$ )
On-orbit	HRC-I	9 $\times 10^{-6}$ cts s^-1 arcsec $^{-2}$
quiescent background	HRC-S	1.8 $\times 10^{-4}$ cts s^-1 ${\rm (res.\ elm.)^{-1}}$
(prior to ground processing)		(0.07Å $\times$ 0.1 mm)
Intrinsic dead time		50 $\mu$ s
Constraints:	telemetry limit	184cts s^-1
	maximum counts/observation/aimpoint	450000 cts
	linearity limit (on-axis point source)
	HRC-I	$\sim$ 5cts s^-1 (2 cts s^-1 $\rm pore^{-1}$ )
	HRC-S	$\sim$ 25cts s^-1 (10 cts s^-1 $\rm pore^{-1}$ )

7.2 Basic Principles

Figure 7.3 illustrates the features of the HRC MCP s. X-rays enter through an UV/Ion shield, necessary in order to reduce/avoid signals from UV light, ions, and low energy electrons. Most of these X-rays are then absorbed in the CsI-coated walls of the first (input) of two consecutive MCP s. The axes of the millions of tubes that comprise the input and output MCPs are not parallel to the optical axis but are canted (``biased'') at an angle of $6^\circ$ , but in opposite directions as shown. This bias is introduced to improve the probability of an interaction. The CsI coating serves to enhance the photoemission over that from a bare MCP . The resulting photoelectrons are then accelerated by an applied electric field. The next interaction with the walls releases several secondary electrons and so on until a cascade of electrons is produced.

**Figure 7.3:** A schematic of the HRC Micro-channel-Plate detector.
$\begin{figure}\centering \scalebox{0.5}{\includegraphics{../../ogplots/snap.ps}}\end{figure}$

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One purpose of the second (output) MCP is to provide additional gain. In addition, reversing the direction of the second MCP s bias angle with respect to the first removes a clear path for positive ions, and hence reduces the possibility of (positive) ion feedback - wherein an accelerated ion moving in the opposite direction as that of the electrons ends up causing the release of electrons and starts the process all over again.

The electron cloud -- typically about $2\times10^7$ 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: HRC-I uses a crossed grid charge detector; the HRC-S uses a hybrid where one axis is comprised of wires, while 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:

$\begin{displaymath} pos = cp_{i} + (\frac{Q_{cp_{i+1}} - Q_{cp_{i-1}}}{Q_{cp_{i-1}} + Q_{cp_ {i}} + Q_{cp_{i+1}}})\times \Delta \end{displaymath}$

(7.1)

where

is the coarse position, $Q_{cp_{i+1}}$ is the charge measured on the $cp_{i+1}$ tap, and $\Delta$ 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.
$\begin{figure}\centering \scalebox{0.5}{\includegraphics{../../ogplots/cgrid1.eps}} \end{figure}$

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The three-tap position algorithm described above can be improved upon by making use of the predictability of the shape of the charge cloud exiting the rear MCP. The spatial distribution of the charge cloud leaving the rear of the 2nd MCP has a very specific shape for X-ray induced events. This shape has often been modeled as the combination of a Gaussian and a Lorentzian distribution. Due to this specific shape, it has been observed and simulated via Monte Carlo techniques that the fine position term:

$\begin{displaymath} (\frac{Q_{cp_{i+1}} - Q_{cp_{i-1}}}{Q_{cp_{i-1}} + Q_{cp_{i}} + Q_{cp_{i+1}}}) \end{displaymath}$

(7.2)

and the complementary term:

$\begin{displaymath} (\frac{Q_{cp_{i}}}{Q_{cp_{i-1}} + Q_{cp_{i}} + Q_{cp_{i+1}}}) \end{displaymath}$

(7.3)

are highly correlated. In fact, the shape of a scatter plot of the two quantities for X-ray induced events closely describes a hyperbola. Non X-ray events, primarily those due to the passage of charged particles, produce charge distributions that are often larger and are spatially extended and complex. As such, it is possible to remove many non-X-ray background events by screening those events that do not fit the hyperbola. Furthermore, since the charge distribution is centrally peaked, the complement $Q_{cp_{i}}$ term is larger and less susceptible to noise induced errors than the $Q_{cp_{i+1}} - Q_{cp_{i-1}}$ difference term. It is therefore possible to use the complement term, and the best fit hyperbolic locus to correct those events where instrumental noise has compromised the three-tap fine position. A much more detailed explanation of this technique is presented in Murray, et al. (2000).

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

7.2.1 Aimpoints

The aimpoints are the positions on the instrument where the flux from a point source with no commanded offsets is placed. Note that the on-axis position is offset approximately 20'' from the aimpoint. There are two nominal aimpoints as indicated in Figure 7.1 - one at the approximate center of the HRC-I , and the other slightly off-center on HRC-S . The HRC-S aimpoint Z-offset places the LETG -dispersed image along the centerline of the two white rectangles in the diagram. The HRC-S aimpoint Y-offset is slightly off-center, so that the boundaries between the three HRC-S segments correspond to different wavelengths of the grating-dispersed spectrum (See Chapter 9 for details).

7.3 Dither

The spacecraft is dithered during all observations in a Lissajous figure. For observations with the HRC , the dither amplitude is 40 arcsec peak-to-peak, with nominal periods of 1087 (in Y) and 768 (in Z) seconds. Dither serves the purpose of smoothing pixel-to-pixel variations in the response. The dither also eliminates gaps in coverage caused by the HRC-S inter-segment spaces near -50 A and +60 A (see Figure 7.3). The effects of dither are removed during high-level ground processing of the data.

7.4 Spatial Resolution & Encircled Energy

Imaging with the HRC is best performed with the HRC-I because of the much lower background (Section 7.8) and larger field of view. The intrinsic PSF of the HRC is well modeled by a gaussian with a FWHM of $\sim 20\mu$ m ( $\sim$ 0.4 arcsec). The HRC pixels, determined by the electronic readout and not the pore size, are $6.42938 \mu$ m (0.13175 arcsec). The HRC response is thus well matched to the intrinsic HRMA resolution (Chapter 4).

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 7.5. 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.

The imaging resolution of the HRC-I /HRMA combination degrades off-axis for two reasons: the HRMA PSF increases 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 /HRMA is shown in Figure 7.6. 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:** 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 $\mu$ m (0.41)). Flight data from an observation of AR Lac are also shown.
$\begin{figure}\centering \scalebox{0.6}{\includegraphics{../../ogplots/hrma_ee_aspect_hrc_i_point_obs_guide.ps}} \end{figure}$

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**Figure:** 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 $\mu$ m (0.41)). A plot for the combined **HRMA** /HRC-S would be almost identical since the PSFs of the two instruments are virtually identical and independent of off-axis angle.
$\scalebox{0.6}{\rotatebox{0}{\includegraphics{../../ogplots/hrma_ee_offaxis_hrci.ps}}}$

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7.5 Energy Resolution

The pulse-height amplitude of each event is telemetered. However, the energy resolution is poor. Figure 7.7 shows the HRC-I pulse height distributions for six energies obtained during sub-assembly calibration. The pulse-height distributions for the HRC-S detector are somewhat narrower. There is significant spatial variation in the gain across both instruments.

**Figure 7.7:** The pulse height versus energy for the HRC-I detector and the centroid of the pulse height distribution versus energy. This data set was 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 and they are presented here for purposes of illustration only.
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7.6 UV/Ion Shields

The placement, composition, and thickness of the various UV/ion shields (filters) are shown in Figure 7.1. Tables and plots of the UVIS transmission as a function of energy can be found at http://hea-www.harvard.edu/HRC/calib/uvismodel.html.

The shields are useful in suppressing 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 7.8. 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 $7\times 10^{-4}$ cts s^-1 . The observed upper limit is $1\times 10^{-3}$ cts s^-1 . 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 upon these sets of observations, the UV/Ion shields are performing as designed. Other stars will be observed as part of future calibration activities. For a detailed discussion of the out-of-band response of the HRC to stars, the reader is referred to http://hea-www.harvard.edu/HRC/calib/palermopaper.ps. Plots of the out-of-band count rate for any star for which T $_{eff}$ , V, and N are known, are given.

Scattered UV, far-UV (FUV), and extreme-UV (XUV) light from the Sun or bright Earth may cause a background strongly dependent on viewing geometry. The spacecraft was designed to limit the contribution from stray scattered radiation to 0.01 cts cm $^{-2}$ s $^{-1}$ ( $2.4\times10^{-7}$ cts arcsec $^{-2}$ s $^{-1}$ ) on the HRC . The imaged components of scattered radiation are dependent on the solar cycle, but are at most $\sim$ 0.01 ${\rm cts~cm^{-2}~s^{-1}}$ for most lines of sight.

**Figure 7.8:** The HRC-I (top) and the center section of the HRC-S (bottom) UV/Ion shield effective area as a function of wavelength.
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7.7 Quantum Efficiency and Effective Area

The quantum efficiency of the HRC detector is the product of the appropriate UV/Ion shield transmission and the quantum efficiency of the CsI coated MCP . Pre-flight flat field measurements show a 10% variation in the QE across the HRC-I . The HRC-S also exhibits QE variations of the same magnitude, where the complex structure of the HRC-S UVIS contributes to the spatial variations.

The combined HRMA /HRC-I and -S effective areas - the product of the HRMA effective area, the quantum efficiency of the HRC and the transmission of the appropriate UV/Ion shield - are shown, integrated over the point spread function, in Figure 7.9. See also the discussion in the HRMA chapter (Section 4.2.2, and Figures 4.2 and 4.3.) Analysis of data from in-flight calibrations is ongoing (See http://cxc.harvard.edu/cal/Hrc/).

**Figure:** 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 of the telescope, the CsI MCP coating, and the polyimide/Al of the UVIS. The results of the low energy in-flight QE calibration have been incorporated into these curves.
$\begin{figure}\centering {\scalebox{0.65}{\includegraphics{../../ogplots/hrc_onaxis_ea_pog.ps}}} \end{figure}$

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7.8 On-Orbit Background

7.8.1 HRC-I

The HRC-I total counting rate on orbit is about 250 cts s^-1 due mostly to cosmic ray events which are detected in the anti-coincidence shield (antico) and flagged. The observed cosmic ray event rate is about twice what was assumed pre-launch and corresponds to $\sim$ 2 ${\rm cts~cm^{-2}~s^{-1}}$ . Because a rate of 250 cts s^-1 saturates the telemetry limit of 184 events sec ${\rm ^{-1}}$ , the on-board veto function has been activated. This reduces the valid event rate to about 50 c/s over the field yielding a background telemetered rate of 10 ${\rm ^{-5}}$ ${\rm cts~s^{-1}~arcsec^{-2}}$ . The background is generally flat, or at least smoothly varying over the field with no more than a 20% difference between the center (higher) and edges (lower) of the detector. Note, the total event rate remains unchanged, but detector events in coincidence with antico events no longer enter the telemetry data stream. Before launch the expected rate, after vetoing the effects of cosmic rays, was 10-20 cts s^-1 composed of mainly the internal rate of the MCP s (10-15 cts s^-1 ), and a small contribution from cosmic rays due to antico inefficiency. There is additional background in the HRC-I that is not well understood. However, for point source detection and exposure times of 100 ks or less the background is virtually negligible. For studies of extended objects of low surface brightness, however, even this relatively low rate can become significant depending on the specific details of the feature.

Ground based data analysis screening techniques have been developed to further reduce the non-X-ray background in the HRC detectors. These techniques are described in detail in Murray et al. (2000), Juda et al. (2000) and Wargelin et al. (http://cxc.harvard.edu/cal/Letg/Hrc_bg/). The net result is that for the HRC-I, non-X-ray background is reduced by $\sim$ 40 $\%$ compared to non-screened data while the corresponding reduction in X-ray events is reduced by less than a few percent. For the HRC-S, non-X-ray background is decreased by $\sim$ 50 $\%$ and the X-ray loss is 1 to 2 $\%$ . Furthermore, the screening process makes the spatial distribution of the detector background more uniform. The screening algorithms have the added benefit of removing saturated events which were responsible for a faint secondary ``ghost'' images (see Section 7.10).

The HRC IPI team continues to work on the screening/processing techniques and to develop new ones. These event screening procedures have been incorporated by the Data Systems Group into the standard CXO/HRC data pipeline and are transparent to the users. Proposers interested in the details of the screening procedures are encouraged to read the aforementioned papers and references contained therein.

7.8.2 HRC-S

The anti-coincidence shield of the HRC-S is not working because of a timing error in the electronics. The error is not correctable. As a result the event rate is very high and exceeds the telemetry rate limit. To cope with this problem the HRC Team has defined a ``spectroscopy region'' which is about 1/2 of the full width and extends along the full length of the HRC-S detector. The spectroscopy region is $\sim 10$ mm x 300 mm in size. The region is imposed by using the edge blanking feature of the electronics. With this change, the telemetered quiescent background rate is about 85 cts s^-1 .

As with the HRC-I , this background can be further reduced in ground data processing by using pulse height filtering that preferentially selects X-rays over the cosmic ray events. A reduction in background of a factor of about three for dispersed spectra is possible. Thus there are two relevant background rates for the HRC-S: a telemetry rate of 85 cts s $^{-1}$ and a post-processing rate for calculating signal to noise. The latter is discussed in detail in Chapter 9 (see especially Figure 9.28).

7.8.3 Temporally Variable Background

Both the HRC-I and HRC-S experience occasional fluctuations in the background due to charged particles. These times of enhanced background are typically short (a few minutes to a few tens of minutes) and are anywhere from a factor a 2 to a factor of 10 over the quiescent rates. The increased background appears to be uniformly distributed over the detector and introduces no apparent image artifacts. On the average it seems that no more than about 20% of the observing time is affected by these events, and they are easily recognized in the secondary science rate data and so can be filtered out if desired. An example of this behavior is shown in Figure 7.10. The reader is referred to Juda et al. (2002) for more information on the HRC background.

**Figure 7.10:** An example of the background variability during a $\sim$ 30 ks HRC-I observation of the SNR G21.5-09 taken on 1999-10-25. The total event rate and the valid event rate show correlated bursts up to $\sim$ 800 cts s^-1 . The data in the bursts are uniformly distributed over the detector. The anti-coincidence shield exhibits no correlated enhancements. The total rate and the valid rate differ by $\sim$ 200 cts s^-1 due primarily to cosmic ray events that are vetoed and don't appear as valid events in the telemetry.
$\begin{figure}\centering \scalebox{0.7}{\includegraphics{../../ogplots/bkg_rate_variability_run.2.ps}} \end{figure}$

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7.9 Source Sensitivity

Examples of the minimum flux detectable by the HRC-I in a 1 arcsec radius circle on-axis and for a 20 $\arcsec$ diameter extended source are shown as a function of observing time and spectral form in Figure 7.11.

**Figure 7.11:** Predicted minimum detectable flux as a function of exposure time using HRC-I at best focus for two sample point sources of differing spectra and one sample extended source in the 0.08 to 10 keV bandpass. We require a minimum of 5 source counts of a 3 sigma measurement above background for detection of a source. The spectral parameters are summarized in the figure. For the point sources, we have used a detect cell of 1 $\arcsec$ radius, which corresponds to 90 $\%$ of the encircled energy on-axis. The background is assumed to be $9 \times 10^{-6}$ cts arcsec $^{-2}$ s $^{-1}$ .
$\begin{figure}\centering \scalebox{0.5}{\includegraphics{../../ogplots/hrc_sens.ps}} \end{figure}$

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7.10 Instrument Anomalies

Initial observations with the HRC-I detector showed a faint secondary ``ghost'' image associated with the PSF. This ``ghost'' image was a displaced, weaker ( $\sim 3\%$ ) image $\sim$ on one side of every source in the HRC-I field of view. The cause of this imaging anomaly was determined to be saturation of fine position amplifiers. The previously mentioned event processing algorithms which are now part of the CXO/HRC data pipeline have reduced the relative intensity of the ghost image to $< 0.1 \%$ effectively eliminating this anomaly. A similar ``ghost'' image existed in the HRC-S but at a much reduced intensity.

A problem has been discovered in HRC event times. A wiring error in the detector causes the time of an event to be associated, not with that event, but with the following event, which may or may not be telemetered. The result is an error in HRC event timing such that the accuracy is degraded from about 16 microseconds to a time which is more or less the mean time between events. To date, the total HRC-I rate has been dominated by the background, and is about 250-300 events/sec. Thus, if the time tag of each event is shifted, it turns out (verified by laboratory tests and simulations) that the average uncertainty in the time of any event is just 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 worked out a special operating mode that allows high precision timing to be achieved (see Section 7.13.1). This new timing mode uses only the central segment of the HRC-S Disabling the outer two segments lowers the total counting 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. In using this approach, it is prudent to be sure that the total counting rate, source plus background, is somewhat below the telemetry saturation limit to avoid the effects of statistical fluctuations.

In addition to the primary science data for individual events, the rate of microchannel plate triggers (total rate) and triggers (valid rate) that pass on-board validity tests 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 not uncommon occurrence because of the flaring background 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 . The problem is caused by an overshoot in occasional large trigger pulses. This results 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 the overshoot problem. A scheme that corrects the HRC-S valid event rate based upon the fraction of event pulse amplitudes that are above a given threshold (segment dependent) has been developed but not yet implemented in the standard processing software.

7.11 Calibration

Calibration of the HRC included laboratory calibrations, a system-level ground calibration with the HRMA and HRC at the X-ray Calibration Facility (XRCF) at MSFC, and on-orbit calibration using celestial and on-board radioactive X-ray sources. The on-orbit calibration of the HRC is an on-going activity. All calibration data are, or will be, described in detail on our web site (http://cxc.harvard.edu/cal).

The user is urged to consult the WWW site and its pointers for the latest information.

7.12 Operational considerations and constraints

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

7.12.1 Total Count limits

Both the gain and the quantum efficiency are adversely affected by the total amount of charge extracted from the MCP at the point of extraction. To minimize such effects, the high voltage on the detector is lowered during passage through the radiation belts and at times of very high particle radiation. To limit the impact from X-ray sources themselves, a 450,000 count limit distributed over the dither pattern from an on-axis source at a given aimpoint has been imposed. Users anticipating to exceed this value should so note in the comments section of the RPS form when submitting their proposal. In this case, the CXC will establish new aimpoints as necessary. Offsets in the pointing may be imposed, if necessary, in order to limit the accumulated dose to a given region of the MCP .

7.12.2 Count rate limits

There are two counting rate limits:

7.12.2.1 Telemetry Limit

The maximum telemetered counting-rate is 184 cts s^-1 . This is a limitation on the total count rate received over the full field-of-view rather than for one individual source within the field. It is possible to exceed this limit and to subsequently correct the total count rate by using the secondary science rates, which keep track of the actual detected rate, to determine the deadtime correction (see Section 7.10). The resulting deadtime fraction increases rapidly with valid event rates above 184 cts s^-1 . For example, at 200 cts s^-1 the deadtime fraction is 8%, at 250 cts s^-1 26%, and at 300 cts s^-1 39%. In the following we list some methods of dealing with situations where the telemetry limit is exceeded.

Bright target:
- Insert either the LETG or the HETG as appropriate and perform the investigation viewing the zeroth-order image. This solution may be so dramatic as to substantially increase the required observing time.
- Offset point. To be effective, this solution may result in substantially reduced spatial resolution.
Bright nearby source
- Depending on the proximity, an appropriate choice of roll angle and/or aim point can position the problematical source (or sources) off the detector. At present, the HRC instrument is not allowing observation requests that require the use of the mechanical systems; the option of blocking bright sources with the HRC shutters is presently unavailable.
- Request a rectangular window for the on-board data so that events produced by the nearby bright source (or sources) do not contribute to the telemetry limit.

There are of course, other combinations and situations that can lead to telemetry saturation - numerous faint sources on the field, a too-bright extended source, etc.

7.12.2.2 Linearity limit

The HRC-I was found to be linear for incident photon rates up to $\sim 2$ cts s^-1 pore $^{-1}$ , which translates to $\sim 5$ cts s^-1 for an on-axis point source (see Kenter et al. 1997, Figure 7). The HRC-S was found to be linear for rates five times greater. (We note that in the case of the HRC-I there is no evidence that linearity does not extend beyond the stated limit, only that no data were taken for higher incident fluxes.)

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 oneself that the linearity limit is not exceeded:

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

7.13 Observing with HRC - Operating Modes

For many observations, it is necessary only to specify the instrument, the exposure time, and the target coordinates. No other parameters need to be provided. There are, however, a number of optional parameters that one might wish to consider invoking to optimize a particular observation. The observer is encouraged to make use of the various tools provided when planning an observation, and not to forget to include the background when estimating sensitivity.

7.13.1 Timing Mode

The HRC-S is normally operated in spectroscopy mode whereby signals from any of the three MCP segments can be recognized as triggers. An alternate mode of operation (timing) ties the signals from the outer segments to ground so that only signals from the center MCP generate triggers. A key distinction of this mode from using an edge-blanked region (described below) to select only the center MCP segment is that the timing mode selects events without using the on-board veto logic. This preferred method of doing high-precision timing observations reduces the active detector area thereby minimizing the total trigger rate. Provided that this rate is below telemetry saturation, all events will then be telemetered and the event time tags can be correctly assigned in ground processing (see Section 7.10).

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

7.13.2 Edge and Center Blanking

It is possible to define a rectangular region, other than the default region, on either HRC-I or HRC-S from which events either inside (edge-blanking) or outside (center-blanking) the region are selected for telemetering. This would be done, for example, to prevent events from a nearby bright source from contributing to telemetry (see section 7.12.2). If a proposer wishes to define such a rectangular region, she/he should state this request in the "Remarks" field of the RPS form in order to prompt discussions with a CXC Support Scientist.

7.13.3 Zero-order Blocking

The HRC has a pair of blades that were designed so that quadrant focusing and mirror-tilt measurements could be performed on-orbit. The blades can be positioned to block the zero-order image in a grating observation. The positions of the blades can also be adjusted to block other portions of the field.

NOTE: The operation of the shutters is presently unavailable as an observing option.

7.14 References

The following is a list of publications, documents, web pages, and memos containing information about the High Resolution Camera (HRC ).

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/rsdc_www/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. http://hea-www.harvard.edu/hrc_art/hrc_spie/hrcspie0.html

Position modeling, de-gap corrections, and event screening

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

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., 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

Juda, M., et al., ``Improving Chandra High Resolution Camera event positions via corrections to cross-grid charge detector signals'', SPIE Proceedings, 4140, 2000. http://hea-www.harvard.edu/HRC/calib/spie2000_tap_correction.ps

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

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.

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

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

9.2 Calibration

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

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

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

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

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.

8.2.3 Background

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.htm 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

Count life

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.

Out-of-band response

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

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

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

cxchelp@head-cfa.harvard.edu