AIPS HELP file for DIGICOR in 31DEC22
As of Sun Aug 14 13:32:39 2022
Type: Adverb (Logical, > 0 => true)
Use: Standard data selection adverb. True (1) means apply
the digital corrections to VLBA data in FITLD.
False (-1,0) means do not apply the corrections.
Null value: false (-1)
FITLD....Reads FITS files to load images or UV (IDI or UVFITS) data
to disk. DIGICOR use for IDI only: Data processed in
digital correlators such as the VLBA correlator and DiFX
are affected by a variety of phenomena that we
collectively refer to as digital effects. FITLD is able to
apply the corrections needed to scale the data to the
values that would be returned by a purely analogue
correlator. DIGICOR = -1 will turn off the application of
these correction, DIGICOR = 0 or 1 will apply them to both
cross and total power, DIGICOR = 2 will apply the
corrections to cross-power and only to total power if
zero-padding was used in the correlator. The latter case
is designed to deal with a situation in which the
total-power spectrum is dominated by multiple, strong
spectral lines. In that case, if zero-padding was not
used, the reconstructed total-power spectrum, after an
FFT, is inaccurate at some level. In all continuum cases
DIGICOR = 1 should be the default. In nearly all line
cases DIGICOR = 1 is fine. only if the Tant in the line is
about equal to the Tsys should DIGICOR = 2 be used. One
word of caution, for data correlated prior to Aug 1 1994
the correlator was incorrectly scaling the data by a
factor of 1.57 (i.e. PI/2) . So for that data, DIGICOR can
be used but a BFACTOR of 0.6369 (i.e. 1/1.57) should be
applied in APCAL or to the final images. Unfortunately
FITLD had no knowledge of the date of correlation so
cannot perform this correction. The code detects data
from the DiFX correlator and applies a different (1.0)
saturation correction in that case.
**** FITLD no longer alters DIGICOR for non-VLBA arrays,
**** so set it carefully. Before 5 Feb 2013, it changed
**** DIGICICOR to -1 if the array name was not VLBA.
**** Note that DiFX correlators may write VLBA as the
**** array even when no VLBA antennas are involved.