[daip] POSSM velocity calculation

[Eric Greisen]

Mayumi Sato wrote:
> Dear Eric,
> 
> Thank you for your reply.
> We did not do ALTDEF for either of multi-source and SPLIT files.
> Clearly the IMHEAD I sent you show that ALTSW fails for the
> multi-source file saying "USE ALTDEF" but ALTSW went through for SPLIT
> file (however, which got a wrong velocity).
> 
> To check this, I re-ran SPLIT for the multi-source file and ran POSSM
> on both files without doing anything with ALTDEF or ALTSW, but there
> still remains the same velocity difference.
> I suspect the 64 channel shift / ALTDEF problem occurs either in
> SPLITting the files (without ALTDEF on multi-source file) or in POSSM.
> 
> When we imaged from the SPLIT files, the ISPEC spectra (with ALTSW but
> without ALTDEF) showed the right velocity as in multi-source POSSM -
> only SPLIT+POSSM got the wrong velocity.


I have discovered that SPLIT and SPLAT make the assumption that the 
alternate reference pixel in the header of the multi-source file is 
correct.  They do not make that assumption about rest frequency, LSR 
velocity, and will in future not make that assumption about the 
reference pixel.  A puthead with keyword-'altrpix'; and keyv=64 will fix 
the single source file.

Eric Greisen