[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 find your statement about imaging and atlsw rather hard to believe in 
that the header information of the split file is very explicit and shows 
that 10 km/s occurs and channel 0.

I will look into this further - in particular, perhaps SPLIT is dropping 
the reference channel.  POSSM is very hard to figure out and I do not 
know what options you specified to it.  The combination of whether you 
did altsw before running possm and the value of aparm(7) is relevant. 
But a simple reading of the headers made the possm plot you sent for the 
split file match the parameters that are in its header.  Those 
parameters are used by IMAGR and all downstream tasks unless you change 
them.  Note that aips does not compute the Earth's rotation velocity not 
the volecity about the Sun.  We depend on the on-line systems or the 
user to provide the connection between frequency and velocity in the 
desired frame of reference.

Eric Greisen