[daip] SPLAT, SPLIT reference frequency problem

[Michael Bietenholz]

I think both SPLAT and SPLIT (31DEC04) seem to get the frequency and
uvw scaling slightly wrong.  The effects are fairly small, but big
enough to worry about when you're doing precision astrometry.

1)  When averaging IF's together, the output reference frequency is the
    the center (average) freqency of IF1 in the original multi-file.
    It should be the center (average) frequency over all the IFs.
    This causes position shifts in any images made from such data,
    so is bad for anyone doing astrometry (even when bandwidth
    smearing is acceptably small).

2)  The values of u,v,w are in kilo-lambda at the reference frequency,
    thus it seems to me that the output values of u,v,w should scale
    exactly as the reference frequency.  This does not seem to be
    true if the input uv-file has a reference pixel along the FREQ
    axis not equal to 1.0.

    I think the output u,v,w values scale as the effective
    frequency of IF1,pixel=1.0 (FREQ axis).  SPLAT seems to put the
    output reference frequency at pixel=1.0 on the FREQ axis, and scales
    the frequency but not the values of u,v,w for any shift from the
    original reference pixel to 1.0.   If the input file has a reference
    frequency not at pixel=1 (which is often the case for VLBI data), the
    output values of u,v,w are wrong, again causing small shifts in position
    in any images etc. made from this data.


Notes:  by reference frequency I mean that in the uv-file header
as the "Coord value" of the FREQ axis.  I'm assuming that is the reference
frequency used for the calculation of u,v,w.

      michael bietenholz