[daip] POSSM velocity calculation

[Eric Greisen]

Mayumi Sato wrote:
> Dear Eric,
> 
> Please find attached the IMHEAD of the two files (multi-source and
> SPLIT) called imh_multi.txt & imh_split.txt for the case of W49
> (043.17+0.01).
> I am also attaching POSSM spectra in ps files and the raw data in txt files.
> 
> Mayumi
> 
> 
>> Mark Reid wrote:
>>> I've encountered a problem in the velocity labeling when using POSSM.
>>>
>>> It works fine for a multi-source database.   But when I SPLIT the
>>> data, it seems to shift the velocity information by half the
>>> bandwidth.
>>>
>> Can you send me the headers of the two files (IMHEAD) - this seems unlikely.
>>  I used it on my paper last year and found I think a half channel error but
>> that was all.
>>
>> Eric
>>

Thank you for the attached information, I can now answer your query.

POSSM has done what you have told it to do.  In the multi-source file, 
you have not done an ATLDEF and so it is using the reference pixel 64 - 
presumably the source table contains the reference velocity (10 km/s) 
and rest frequency.  In the single source file, you have done an ALTDEF 
telling AIPS that the 10 km/s is at channel 0.  The print outs show 
exactly 64 channels different in the velocity due to the 64 channels 
difference in the velocity reference pixel you have provided with the 2 
files.

ERic Greisen