SAS watchout

My observation has been made in EPIC multi-pointing mode, how can I process it correctly ?

In the standard processing of EPIC multi-pointing mode observations the actual position of the source, on the detector, in each pointing step is lost to the SAS. This can result in errors in the calculation of timing corrections and effective areas.

This will be solved in a future version of the SAS. In the meantime, the problem can be solved by taking the following manual steps. To complete step 2 you will need to use SAS version 15.0.1 or later.

  1. Calculate RA, DEC values for each pointing

    After the ODF has been ingested with odfingest and with the SAS_ODF variable still pointing to the directory containing the ODF, execute 
    
  > preqgti

    This will produce a FITS file called pointings.fit

    The POINTATT extension will contain the RA, DEC values that are needed for each pointing step.

  2. Run attcalc on the event file generated by each pointing setting the reference point to the values calculated in step 1

    e.g. 
      attcalc eventset=2165_0412601501_EPN_S601_ImagingEvts.ds
  atthkset=2165_0412601501_AttHk.ds refpointlabel=user nominalra="284.189875"
  nominaldec="-37.92775"

  3. Manually change the pointing position keywords in each event file to these values.

    This may be done with the 'fv' or 'fmodhead' tools from the ftools package

    e.g. for the example in step 2 
    
  > fmodhead "2165_0412601501_EPN_S601_ImagingEvts.ds[0]" h.dat
  > fmodhead "2165_0412601501_EPN_S601_ImagingEvts.ds[1]" h.dat

    where h.dat is a local text file that contains the two lines 
    
  > cat h.dat
  RA_PNT 284.189875
  DEC_PNT -37.92775

After these steps have been completed the event files may be used to produce time series, images and spectra in the normal way. The products generated from these files will then have the correct attitude and exposure keywords written in the header.