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ASCA Guest Observer Facility


Recipe for RDD Correction


The average dark current level for a given pixel is believed to be stable over timescale of months. Thus, by knowing this average (via Frame mode observations that are now taken regularly), the RDD effect can be largely removed on an event-by-event basis. This idea is implemented in the FTOOL correctrdd.

Correctrdd requires the 'RDD map' files, generated from a series of Frame mode calibration observations. In the early days of ASCA, Frame mode data were not taken regularly. The RDD map files exists for 4-CCD mode data only in 1993 Apr, 1994 July (except chip 0 on both SIS-0 and SIS-1), 1994 Nov, and in 1995 Mar. In 1995 Apr, Frame mode data were taken in 1, 2, and 4 CCD mode in all chips that are frequently used in the respective clocking modes. Starting in late 1995, we have started taking Frame mode data regularly.

The RDD map files are available at ftp://legacy.gsfc.nasa.gov/asca/data/trend/rev2/rddmap.

The file names have the form rdd9304_s0c0m4_t61_62.fits encoding the year and the month, instrument and chip numbers, the clocking mode, and the temperature range.

Running correctrdd is trivial enough, once the appropriate RDD maps have been obtained; the tricky issue is the interplay between RDD correction and DFE correction (the latter is the correction of time-dependent dark frame error due to, for example, the light leak). It appears that the order faintdfe - correctrdd - faint minimizes the errors due to this interplay. There is beta-test script, gordd, available for applying these tools in appropriate order to unscreened (.unf) files at htp://lheaftp.gsfc.nasa.gov/pub/mukai/gordd.

The result of gordd is a (set of) Bright2 mode data that can be reduced using, e.g., ascascreen.

A bug in correctrdd

It has been discovered that the FTOOLS v4.1 version of correctrdd writes out the RDDCOR_V keyword incorrectly. If the primary header of your corrected event file has:

RDDCOR_V=                    T / Version of RDDcorrect algorithm used
then you may need to modify it, using fv or fmodhead to:
RDDCOR_V=                    1 / Version of RDDcorrect algorithm used

This keyword is read by sisrmg. On some systems, T is interpreted as 0, which corresponds to non-RDD-corrected data; on others, T is interpreted as 1, which corresponds to RDD-corrected data, as intended. SIS response matrices for RDD-corrected data should have:

RQRDCV  =                    1 / Version of RDD correction s/w applied to data
If this is the case, no further action is necessary. If the value is 0, you need to modify the RDD-corrected event file(s), as indicated above, and re-run sisrmg.
This file was last modified on Monday, 13-Sep-1999 15:37:07 EDT
Curator: Michael Arida (SP Sys); arida@milkyway.gsfc.nasa.gov
HEASARC Guest Observer Facility
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This file was last modified on Monday, 13-Sep-1999 15:37:07 EDT

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