HRC Exposure Maps - 1999-2006

HRC-I and HRC-S Exposure Maps (1999-2006)

The entire y1999-y2005 (and the first 269 days of 2006) HRC data sets have been analyzed to determine individual year and cumulative exposure maps for the HRC-S and HRC-I detectors. In this context, ``exposure'' map is the total charge per unit area as a function of position extracted from the detectors. This quantity is important because the localized gain of an MCP detector decreases with the amount of charge extracted per unit area. This gain degradation is cumulative and irreversible. The only solution is to recover the gain by increasing the operating voltage, or to change the aim point. In either case the uniformity of response of the detector is affected.

These data only incorporate "valid events"; those that were not vetoed onboard. For the HRC-I, the anti-coincidence shield veto eliminates most of the cosmic ray induced events from the valid events. The HRC-S/LETG is normally operated with edge-blanking to lower the overall downlinked rate to the telemetry limit. Therefore, the HRC-I maps reflect primarily ``X-ray'' exposure and neglect the overall cosmic ray induced charge extraction which occurs uniformly over the detector's surface. We estimate (conservatively) that the latter exposure is < 5 x 10⁷ pC cm^-2. The HRC-S maps of the unblanked regions reflect charge extraction from both X-ray and cosmic ray events.

Histograms showing the number of pixels (2 x 2 and 0.5 x 0.5 arcsec²) with a given density of extracted charge (averaged over a pixel) and cumulative histograms showing the fraction of the detector exceeding a given density of extracted charge (averaged over a pixel) are also provided.

Laboratory measurements have shown that when approx. 3 x 10⁸ pC cm^-2 of charge is extracted, the modal gain will drop by approx. 10%. Given the small spatial extent of the HRMA PSF (approx. 50% of the imaged energy in an area of 2 x 10^-5 cm²) and the average charge per event of about 5 pC this threshold could be achieved after only 1000 events from a point source, if the observations were not dithered. The ``dither'' patterns are obvious in the resulting images. Note: a 10% gain depression will not result in a significant decrease in detection efficiency but represents the onset of irrevesible degradation.

The HRC electronics have been designed so that an event providing a charge of approx. 8 pC will saturate the 8 bit (0 to 255) PHA scale. Gain maps (average pha images, _gmi.fits) and image files (-img.fits) for the entire data set were created for the HRC detectors. Using a gain map, the charge extracted per 1.06 x 10^-4 cm^-2 region (2 arcsec x 2 arcsec) for the full field was calculated using the 8 pC per 255 PHA units scale factor and the total number of events detected in the region in order to derive the charge extracted per unit area (in pC cm^-2). A charge extraction map with a resolution of 0.5 arcsec x 0.5 arcsec was also made for an 8.5 arcmin x 8.5 arcmin field centered about the aim point. Maps of the full fields and the central regions of the detectors are provided below.

The cumulative images for both the HRC-S and HRC-I show some obvious features. Point sources are dithered and appear as Lissajous figures confined to 40 arcsec x 40 arcsec boxes. The large diffuse object is the SNR Cassiopeia-A. It is also apparent (see the fits files) that there was at least one observation in 1999 of a point source in non-dithered mode which resulted in a charge extraction of approx. 6 x 10⁹ pC cm^-2 for the HRC-I and approx. 2 x 10¹⁰ pC cm^-2 for the HRC-S. Evidence of a LETG observations with the HRC-I is also evident. Regular patterns of dithered point sources are evidence of the inflight calibration program.

HRC-I and HRC-S Exposure Maps (1999)

Description Map Surface Plot Distribution** Cum. Dist.**

HRC-I Charge Extraction fits ps ps ps

HRC-I Image fits

HRC-I Gain fits ps, gif

HRC-I Charge Extraction (central field) fits ps ps ps

HRC-I Image (central field) fits

HRC-I Gain (central field) fits ps, gif

HRC-S Charge Extraction fits ps ps ps

HRC-S Image fits

HRC-S Gain fits ps, gif

HRC-S Charge Extraction (central field) fits ps ps ps

HRC-S Image (central field) fits

HRC-S Gain (central field) fits ps, gif

** Units are 10⁷ pC cm^-2

HRC-I and HRC-S Exposure Maps (2000)

Description Map Surface Plot Distribution Cum. Dist.

HRC-I Charge Extraction fits ps ps ps

HRC-I Image fits

HRC-I Gain fits ps, gif

HRC-I Charge Extraction (central field) fits ps ps ps

HRC-I Image (central field) fits

HRC-I Gain (central field) fits ps, gif

HRC-S Charge Extraction fits ps ps ps

HRC-S Image fits

HRC-S Gain fits ps, gif

HRC-S Charge Extraction (central field) fits ps ps ps

HRC-S Image (central field) fits

HRC-S Gain (central field) fits ps, gif

HRC-I and HRC-S Exposure Maps (2001)

HRC-I and HRC-S Exposure Maps (2002)

HRC-I and HRC-S Exposure Maps (2003)

HRC-I and HRC-S Exposure Maps (2004)

HRC-I and HRC-S Exposure Maps (2005)

HRC-I and HRC-S Exposure Maps (2006)

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2000)

Description Map Surface Plot Distribution Cum. Dist.

HRC-I Charge Extraction fits ps ps ps

HRC-I Charge Extraction (central field) fits ps ps ps

HRC-S Charge Extraction fits ps ps ps

HRC-S Charge Extraction (central field) fits ps ps ps

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2001)

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2002)

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2003)

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2004)

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2005)

HRC-I and HRC-S Cumulative Charge Extraction Maps (1999 through 2006)

*To download a fits or ps file, click on the right mouse button and select "Save target as...".

Fits files can be displayed by using the program DS9. SAOImage DS9 is an astronomical imaging and data visualization application. DS9 supports FITS images and binary tables, multiple frame buffers, region manipulation, and many scale algorithms and colormaps. It provides for easy communication with external analysis tasks and is highly configurable and extensible. DS9 is a stand-alone application. It requires no installation or support files. Versions of DS9 currently exist for Sun Solaris, Linux, LinuxPPC, SGI, DEC Alpha, Windows 98/ME/NT/2000/XP, and MacOSX (darwin). All versions and platforms support a consistent set of GUI and functional capabilities.

The postscript (ps) files can be displayed with Ghostview. Ghostview can be downloaded from http://www.cs.wisc.edu/~ghost/gsview/get42.htm.

URL:http://hea-www.harvard.edu/HRC/trends/exposure_maps.html
Last update: 29 September 2006
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