[daip] Reference Frequencies

[Eric Greisen]

Laura Chomiuk writes:
 > 
 > Hello,
 >    I am trying to reduce some archival data from 1986, and I am trying to 
 > understand if the frequencies given in the image headers and in LISTR 
 > 'scans' output are the central frequencies or the begining of my bandpass. 
 > This data is 20 cm, 50 MHz bandwidth, single channel continuum data. 
 > 
 > Frequency Table summary
 > FQID IF#      Freq(GHz)      BW(kHz)   Ch.Sep(kHz)  Sideband
 >    1   1       1.46490000   50000.0039  50000.0039      1
 >        2       1.41490000   50000.0039  50000.0039      1
 > 
 > Is freq above the central frequency for each channel?

In FITS coordinates, the stated coordinate is at the center of each
voxel.  It may be moved by having a non-integer reference pixel.  In
normal VLA data we do not do that, so these frequencies are nominally
at the center of your band.  I say nominally, because if the bandshape
is not symmetric about the center, then the effective frequency will
diverge from the one stated.  True astrometry should never be done
with wide, asymmetric bandshapes (and the VLA at 50 MHz is
asymmetric).

Eric Greisen