The content of the GO FITS file related to frequency information depends upon the value of OBSTYPE.
For OBSTYPE = ``LINE'', the velocity and rest-frequency related values specified by the observer are important. These keywords, shown with typical values, are as follows:
VELOCITY= 42500.0 / source velocity in m/s VELDEF = 'VRAD-LSR' / velocity definition and reference frame RESTFRQ = 9183000000.0 / line rest frequency in Hz RESTFRQ1= 9183000000.0 / rest frequency 1, always matches RESTFRQ RESTFRQ2= 9505000000.0 / rest frequency 2 RESTFRQ3= 9812000000.0 / rest frequency 3 RESTFRQ4= 8665300000.0 / rest frequency 4
The velocity definition and reference frame are encoded into the value of VELDEF according to the WCS paper III. See the discussion of the SOUVEL binary table in the GBT Tracking Local Oscillator FITS documentation at GBT/SPN/006 for more details on the possible values for the VELDEF keyword.
The RESTFRQn keywords were added at FITSVER 2.7. These are all of the rest frequency values that the user supplied in the order that they were supplied. The original RESTFRQ keyword is still being written and RESTFRQ1 should always match RESTFRQ. The n in RESTFRQn includes the integers 1 through 9 and the upper case letters A through Z as needed.
For OBSTYPE other than line (i.e. pulsar and continuum) the actual center frequency and bandwidth achieved at the backend should be obtained from the IF Manager FITS file. However, we provide a single keyword SKYFREQ which may be considered indicative of the planned center frequency:
SKYFREQ = 8929391685.963282 / intended sky center frequency in Hz