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LETG and HRC-S data processing problems

LETG and HRC-S data processing problems

B. Wargelin, 2001 May 14


One major and two minor problems have recently been discovered in the Automated Processing (AP) of HRC-S and/or LETG data, and another minor problem in one of the optional offline pulse-height filters. Solutions to these problems have been or will soon be implemented, and data reprocessing has started. Observers are referred to the LETG/HRC-S CIAO analysis thread for instructions on how to reprocess data themselves.

Update: The AP problems were corrected in the DS6.1.0 software release, which has been used since 02 July 2001. Here's how to check the processing version and date of your data.

Summary of problems

1) The HRC "PHA ratio test", which is applied just before Level 2 in the AP pipeline, removes a large fraction of valid x-ray events in the HRC-S (up to 30%) on an 8-tap (13 mm, 15 Angstrom) period (see Filtering Errors). This test, which is part of the Status Bit filtering (bit 21 in the event file status field), compares the pha and sumamps values for each event. As described in a memo by M. Juda, the pha/sumamps ratio is not well-behaved in the HRC-S; the data processing problem arose when HRC-I parameters were used for the test.

Note that all HRC-S data are affected, not just grating spectra, since even in the "good" areas, roughly 5% of valid x-ray events are removed. The degree of loss may also vary across the dither pattern, particularly at some off-axis locations, introducing spurious periodicity into source lightcurves.

2) The background regions that go with the LETG "bowtie" spectral extraction are chopped off (in the cross-dispersion direction) beyond about 120 Angstroms, because of an incorrectly set parameter in tg_create_mask. This error occurs between level 1.0 and 1.5, and results in incomplete background subtraction at long wavelengths (see Background Region Error). LETG/ACIS-S data are not affected because those spectra extend to only about 60 Angstroms.

3) The "heavy" (nominal 5% x-ray event loss) HRC-S PI filter, which is NOT part of the AP pipeline and is applied between level 1.5 and level 2 at the discretion of the observer/analyst, removes more than the originally advertised 5% of valid x-ray events at long wavelengths, and should not be used to create publishable spectra (see Filtering Errors). Note that "light" and "medium" PI filtering are still valid.

4) The "degap map" for some areas of the HRC-S leads to spatial distortions in the degapped image. An illustration is shown from an off-axis calibration observation of Ar Lac. This problem does not affect data in the standard LETG spectroscopy region which uses the central 6 taps of the HRC-S, but may affect observations which use the full detector width, such as "timing mode" observations, if the source is extended or off-axis.


All problems (except #3, which does not arise in pipeline processing and for which warnings have been posted) have been corrected in the AP pipeline as of early May 2001. Reprocessing of all existing HRC-S data has begun, and is expected to be completed by July 2001. For observers who wish to reprocess data themselves, the corrections are as follows:

  1. HRC-S Status bit #21 -- Do not filter on this bit. See analysis thread for new status-bit filter command.
  2. LETG background region -- Change the value of width_factor_letg from 20 to 40 in tg_create_mask. See analysis thread for proper CIAO commands.
  3. Heavy PI filter -- Users are warned on the PI filter page and in the analysis thread that heavily filtered spectra will not be properly calibrated. In the longer term, a finer-scale gain map and improved filter will be developed.
  4. Degap map -- The new map is available on the HRC Degap Coefficients page and in CALDB v2.6 (June 7, 2001). Please see the CIAO 2.1 HRC-S Degap Correction thread for instructions on use.

Last modified: 10/01/02

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