[daip] Velocity - Frequency conversion in AIPS

[egreisen]

On 2021-02-26 12:44, Toby Brown via Daip wrote:
> Hi Eric,
> 
> I’m just checking back in on this again sorry. I’m trying to set
> the FITS standard key words CRVAL, CRPIX, CUNIT, and CDELT using the
> ALT* keywords in the headers.
> 
> However, which CRVAL and CRPIX correspond to ALTRVAL and ALTRPIX
> respectively, I can’t see where the units and channel width are
> defined in the alternate keywords. Are you able to point me in the
> right direction?
> 

The CDELT keyword needs to be computed from the frequency and frequency 
increment.  Velocities are in m/sec.  The "alternate" nomenclature is 
solely used for velocity and frequency so the increment and units are 
not recorded.  Actually AIPS does not record the units of anything - 
they are fixed by the axis type.  That saves us from furlongs per 
fortnight!

Eric