Page History

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• In gtselect, the application of the data selections are made using the extended filename format of cfitsio.
  • For Pass 6 data, event class cuts were specified in via a filter string like this:

    Code Block
    "EVENT_CLASS >= 2"

    This was possible because the event classes were nested and the desired events could be selected using this filter.
  • For Pass7 data, the event classes are not nested at the less restrictive event selections, so it was decided that the EVENT_CLASS field would instead be interpreted as a bit field, with each bit indicating membership of an event to the corresponding event class. To select a given event class, a bit mask with the corresponding bit set is applied to the EVENT_CLASS variable:

    Code Block
    "((EVENT_CLASS/4)%2 == 1)"

    cfitsio does have the ability to apply a bit mask using "EVENT_CLASS & 4", but that requires the column in the FT1 file to be defined as a bit field. In the current definition, it is an integer.

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• Acceptance cone cut centered at RA, Dec = 166.1138, 38.2088 J2000 degrees with a 15 deg radius:

  Code Block
  DSTYP1 = 'POS(RA,DEC)' DSUNI1 = 'deg ' DSVAL1 = 'CIRCLE(166.1138,38.2088,15)'

• Time range cuts. These are encoded in the GTI extension.

  Code Block
  DSTYP2 = 'TIME ' DSUNI2 = 's ' DSVAL2 = 'TABLE ' DSREF2 = ':GTI '

• Event class selection. The bit-mask selection is defined using this "BIT_MASK" function that has three arguments:
  • FT1 column name
  • Bit to set in the mask
  • Pass version as given in the PASS_VER keyword in the FT1 EVENTS extension. This needs to be encoded in the BIT_MASK function designation so that it is propagated to downstream files (such as exposure maps) so that the IRF handling code can determine the IRFs to use based on the event selection and the
    information in CALDB.

    Code Block
    DSTYP3 = 'BIT_MASK(EVENT_CLASS,2,P7V6)' DSUNI3 = 'DIMENSIONLESS' DSVAL3 = '1:1 '

• Energy range selection.

  Code Block
  DSTYP4 = 'ENERGY ' DSUNI4 = 'MeV ' DSVAL4 = '100:300000'

• Zenith angle cut. This is needed by gtmktime to determine the time intervals in the FT2 file for which the the acceptance cone is completely contained within the maximum zenith angle. The accepted intervals are then expressed as GTIs. This cut is not strictly required by the likelihood tools (aside from gtmktime), but it is generally needed to remove Earth limb contamination.

  Code Block
  DSTYP5 = 'ZENITH_ANGLE'

  
  DSUNI5 = 'deg '
  DSVAL5 = '0:105 ' code

  DSUNI5 = 'deg ' DSVAL5 = '0:105 '