Caveats Regarding CIAO and Archive Data

Aspect

Tom Aldcroft
10/12/01

Applicability

This document applies to observations processed with software version R4CU5UPD8.1 and later. This includes all reprocessed data. See below for information on extracting the processing version.

For data processed with prior versions, refer to the prior caveats page.

Caveats

There are two known aspect offsets which affect reprocessed data. In general, if you are concerned about absolute astrometry in your observation, it is recommended that you use the fix_offset thread. This thread is simple and will quickly determine if the observation is affected by an aspect offset. If so, the appropriate CIAO dmhedit commands will be supplied to modify the event header keywords and remove the offset.

This thread is now the preferred method for removing aspect offsets, and users no longer need be concerned with particular processing version numbers.

HRC-S offset

Some HRC-S observations have an aspect offset of 1.6 arcsec. This was discovered during analysis of the Chandra astrometric accuracy.

ACIS-I offset

Most reprocessed ACIS-I observations have an offset of up to 1.5 arcsec. The exact size is a function of when the observation was performed. This was discovered in a later, larger analysis of Chandra astrometric accuracy

Processing version

The version of processing software is listed in the header keyword ASCDSVER. This can be checked (for a file acisf00408N001_evt2.fits) with the command:

unix: dmlist acisf00408N001_evt2.fits head | grep ASCDSVER

which provides the output:

0007 ASCDSVER             R4CU5UPD8.1                    String       ASCDS version number

R4CU5UPD8.1 is the software version.

Last modified: 10/12/01

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