Check for aspect offset and fix

The following short thread will determine if your observation has an aspect offset which should be corrected. If this is the case, a set of CIAO commands will be presented which will modify the event file header keywords and thereby remove the offset. This thread can be run on any observation taken with any detector.

NOTE: This tool is not needed for observation data which was processed or reprocessed after 2003-Dec-31. In this case the current aspect alignment calibration has already been applied. Check the DATE FITS header keyword in the event file to determine the processing date.

Caveat: Chandra observations taken after about 2006-Dec-01 and processed on or before 2007-May-14 are subject to a systematic aspect offset of approximately 0.4 arcsec. New calibration files to correct this offset were installed effective 2007-May-14 and affected observations were reprocessed to remove the offset. Observers analyzing these observations should ensure that the processing date is after 2007-May-14.

Testing of this thread is documented here. Please report any errors or suspicious results.

A short Perl script fix_batch is now available which allows users to run this thread from the Unix command line. A description and instructions are given within the file.

Thread

  1. Copy your event file (evt_file) to what will be the new aspect-corrected version
    • If the file is unzipped already
      cp evt_file aspcorr_evt2.fits
    • If the file is zipped
      gunzip --stdout evt_file > aspcorr_evt2.fits
  2. In the text box below, cut and paste below the results of the following command. Remember you need to be in the CIAO environoment. 
    dmlist aspcorr_evt2.fits header,raw,clean | egrep 'TCTYP|TCRVL|ASCDSVER|DATE|OBS_ID'

  3. Submit request --- processing may take a minute or two


COMMENTS, QUESTIONS, ERRORS: Please send mail to Aspect Help