AIPS HELP file for BPASS5 in 31DEC25

As of Wed Mar 26 15:21:30 2025

HELP SECTION

BPASS5
Type: Adverb  (Scalar)
Use:  VLBARUN sends this value to BPASS as BPASSPRM(5) when not 0.
      Adverb is only known after RUN VLBAUTIL or RUN VLBARUN.

      BPASS5: Divide by channel 0 option. *** CHANGED MAY 2010 ***
          In general, use 0.1 when using multiple calibration sources.
          ICHANSEL default is inner 75 percent.

          If ~0 (-0.5 to 0.5,but not 0) the value of "channel
          0" is determined and applied (divided into the data)
          on a record-by-record basis, either from the average
          of channels specified by ICHANSEL or from an external
          file (if IN3NAME and IN3CLASS are specified).  If
          -1 (-1.5 to -0.5), the same is done but the
          amplitude of channel 0 is made 1.0 so a phase-only
          correction is made.  Note that model division is
          turned off if either of these options is selected.

          If -2 (< -1.5), divide by the average of channels
          specified by ICHANSEL (no external file) after the
          time averaging.  This option allows a normalized
          bandpass to be found correcting for any
          uncalibrated variations in the continuum.  If you
          have poor phase stability use 0 or -1, if you have
          decent phase stability use -2.

          If you have good phase stablility use +1.  Note
          that division by channel 0 adds biased noise
          because the channel-averaged amplitude is by its
          very nature biased;  the -2 option should be less
          biased since the data have been vector averaged in
          time first and so are less noisy. The -1 (phase
          only) division should avoid the amplitude bias but
          then the banddpasses are not normalized.  That can
          be corrected later with BPASSPRM(10) options.

          If > 0.5, do no normalization before determining the
          solutions.  This requires good stability during
          each integration period.  BPASSP(10)=3 still allows
          a normalized solution which may be even less biased
          since it is on an antenna rather than a baseline
          basis.

          Note also that a channel 0 may not be determined
          sensibly if the phase changes a lot across the pass
          band except by setting ICHANSEL to select a very few
          channels.

          ********  Note, if normalization is not done on a
          record-by-record basis, users must be very careful
          about averaging different calibration sources.  Any
          channel-dependent flagging will cause the different
          sources to be differently weighted in the partly
          flagged channels than in the completely unflagged
          ones.  The shape of the bandpass in this case will
          not be correct.  Therefore, if channel-dependent
          flagging is done and multiple calibrators averaged,
          the normalization must be done on a record basis.

Procedures:
   VLBABPSS....Computes spectral bandpass correction table.
   VLBARUN.....Applies amplitude and phase calibration procs to VLBA
               data.

EXPLAIN SECTION