B.4 Multi-frequency observations
B.4.1 General frequency information
For any uv data file, the frequency listed in the header information is the sky frequency of the center of the band
(LINE or CH0) during the first scan of the observation. This is true regardless of whether you observed at a single
frequency or multiple frequencies. After you SPLIT a multi-source file into single-source files, the
frequency in the header refers to the sky frequency at the center of the band during the first scan on that
source.
Corrections for the Doppler shift due to the rotation of the Earth can be taken into account within 
, if the data
were observed at fixed frequency. Task CVEL may be used, but it requires that the spectra be well sampled in
frequency.
B.4.2 Multi-frequency uv files
A simple rule of thumb: If you want to calibrate sources together, load the data with the same value of FREQID in
FILLM. If you want to calibrate sources separately, give them different FREQIDs. For multi-frequency files, you
should be sure to assign a different qualifier (QUAL) to each observing frequency (or velocity) with the
OBSERVE program before taking the observations. There are essentially two types of multi-frequency
observations:
- Standard multi-frequency observations in which you want to do the entire calibration process
separately for each frequency. When reading in such data with FILLM, set CPARM(7) = 0. This sets a
different value of FREQID to data that differ by more than the maximum Doppler shift in a source in a
day. During calibration, you can control which data you process by choosing the appropriate values
for FREQID and QUAL. After calibration of each source/frequency, you may destroy the SN table to avoid
using it to calibrate sources with different FREQIDs. For each source/frequency, you should create a
new version of the calibration (CL) and bandpass (BP) tables (e.g., for the second source/frequency,
you will create version 3 of the CL table and version 2 of the BP table). (It principle, FREQIDs may
co-exist in single tables without interference, but if they are in carefully specified separate tables they
cannot interfere with each other.)
- Observations of, or affected by, Galactic emission or absorption, in which you want to combine data
at different frequencies to do the calibration. Normally, these are observations in which the calibrator
sources themselves are absorbed by Galactic HI around 0 velocity. It is extremely important that you
assign a different qualifier to each frequency with the OBSERVE program. Then load the data with
FILLM forcing a single value of FREQID by setting CPARM(7) = -1. In this case, the information that two
observations with the same FREQID have different frequencies will be contained only in the qualifiers.
Whatever data are loaded with the same value of FREQID will have the same reference frequency;
it should be possible to average over the observing frequencies using the appropriate programs in
(CLCAL and BPASS).