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 AIPS, 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:

  1. 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.)
  2. 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 AIPS (CLCAL and BPASS).