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| Product | Status | Accuracy |
|---|---|---|
| LETG/HRC-S effective area | Updated 7/02 | 15% over whole range. Much improved edge structure. |
|
LETG efficiency (m=0,1,3) | Updated 3/00 | 10% -- 3rd/1st relative efficiency confirmed 8/04 |
|
LETG efficiency (m=2,4,5,6,7) |
Updated 8/04 in CALDB 2.28 | 10% relative to 1st order, 15% absolute. Incorporated new ACIS-S1 and S3 QEs. Minor changes for odd orders. 2nd order changes were the largest (-40% to +65%). |
| Bowtie spectral extraction region |
Completed 4/00, bug fixed 6/01 |
Spectral extraction efficiency accurate to 2% |
| HRC-S Degap Map | Updated 6/01 | Typically 1-2 pixels (0.007 Å), but with some deviations of 5-10 pixels. An update (ps) is under development. |
| Dispersion relation | Updated 7/00 | RMS 0.014 Å, but with some localized displacements of 5-10 pixels (0.037-0.074 Å). Efforts to improve this are ongoing. |
| Line Spread Function (RMFs) | Updated 5/01 | ACIS/LETG: OK for Sherpa, 1st order only HRC-S/LETG: Orders 1-6, OK for proposals, but not yet good enough for Sherpa |
| HRC-S gain map (PHA-->PI) |
Completed 4/00 | Relative accuracy of 5% on scales of 1/2-tap (0.95 Å without dither). Gain has declined slightly since launch but relative accuracy of gain map remains ~unchanged. Absolute gain changes do, however, have implications for pulse-height filtering (below). |
| HRC-S/LETG pulse-height (PI) filter |
Completed 2/01 | 50-75% background reduction using light filtering with no more than 1% X-ray event loss. More aggressive filtering is not recommended, particularly given temporal gain changes (above). An improved and time-dependent filter is anticipated in 2005. |
The LETG/HRC-S effective area (for both on-axis and off-axis sources) incorporates the LETG diffraction efficiency, HRC-S QE, and extraction region efficiency. This is what observers should use when analyzing spectra.
The gain of the HRC (both S and I) is very slowly decreasing. PI filtering algorithms, which remove 50-70% of the background in HRC-S/LETG spectra, will eventually need to account for the gain change, but so far, gain changes have had no significant effect on the effectiveness of the "Light" background filter. The next gain map will have a finer grid spacing (currently 1/2-tap) to enhance PI filtering effectiveness.
Now that data have been reprocessed using the new degap map, we are recalibrating the line spread function (LSF) and dispersion relation. With the new degap map, gaps between taps are typically one pixel or less, which should improve dispersion linearity. Even so, systematic wavelength residuals on the outer two HRC-S segments will probably remain, and so we will make ad hoc corrections for the next dispersion relation update, bringing the overall accuracy close to the 0.01% level.
An update to the degap map that accounts for much of the residual systematic deviations has been constructed, and is being tested. See Kashyap et al., 2004, SPIE 5488-62 (ps) for a preliminary report.
Further improvements in HRC-S performance (such as position accuracy
and background rejection) now hinge on finding ways
to correct or otherwise deal with
x-ray events that yield incorrect pulse-height signals.
This is only a very small fraction (couple percent) of events,
but they are not uniformly distributed over the detector, so
localized errors can be significant if we apply adjustments
or filters too aggressively.
Last modified: 10/15/04
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