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Spectral Line Processing and FITS files

When fits reads a visibility FITS file, it can use one of two schemes for computing the observatory's radial velocity. Possible schemes are:

1. fits assumes that a velocity of a given channel remains constant. This means that one of the following three conditions is satisfied:
   • Correcting shifts have been performed on the spectra by offline software to account for the Earth's motion (e.g. by AIPS task CVEL or various tasks in Miriad).
   • The velocity resolution is sufficiently coarse that Doppler tracking is not required.
   • Doppler tracking has been performed during the observation (not ATCA data).
   If any one of these is satisfied, then given the velocity of a particular channel, fits can determine the corresponding observatory radial velocity that this would imply.

   You can either allow fits to retrieve the needed velocity information from the FITS file, or you can give this information explicitly using the velocity keyword. The default behaviour (if you do not set the velocity keyword at all) is to use the information in the FITS file. You will need to explicitly enter the velocity information to fits if the information in the FITS file is incorrect.

   The velocity keyword gives three values: the velocity definition and rest frame, the velocity of a particular channel, and the channel velocity in km/s. Possible values for the velocity definition and rest frame are lsr (LSR velocity, radio definition), bary (barycentric velocity, radio definition), optlsr (LSR velocity, optical definition), optbary (barycentric velocity, optical definition) and obs (velocity relative to the observatory - not of much use). For example, to indicate that channel 257 has an LSR velocity (radio definition) of 4310 km/s, use

       velocity=lsr,4310,257
   

   Specifying the velocity of a channel for a multi-source FITS file is not a particular meaningful thing to do (unless you are interested in only one source).

2. fits assumes that the velocity of a particular channel varies as a result of the Earth's motion. This will be the case for ATCA observations and so should generally be used (unless CVEL has been used in AIPS, or similar corrections performed in Miriad). In this case, fits will compute the Earth's radial velocity as a function of time. The model of the Earth's motion used by fits is accurate to better than 0.005 km/s- which is adequate for most radio-astronomical applications. fits will compute the radial velocity from its Earth model if a velocity frame and definition is given in the velocity keyword (either lsr, bary, optlsr, optbary or obs, as before), but the velocity and reference channel are omitted. For example, fits will use its Earth model to determine the observatory's radial velocity with respect to the LSR if given:

       velocity=lsr
   

It is recommended that the second approach is used for ATCA data that has not already been corrected for Earth motion. This is especially so if the velocity information in the FITS file is incorrect.