[daip] Fwd: Re: Getting Velocity Information in AIPS

[Eric Greisen]

-------- Forwarded Message --------
Subject: Re: [daip] Getting Velocity Information in AIPS
Date: Fri, 17 Aug 2018 13:42:23 -0600
From: Eric Greisen <egreisen at nrao.edu>
Organization: National Radio Astronomy Observatory
To: ingyin.zaw at nyu.edu

On 08/17/2018 10:01 AM, Ingyin Zaw wrote:
> Hello,
> 
> I am doing spectral line work and would like to know the velocity of 
> slices in the data cube. What AIPS task can I use to tell me that. The 
> header and CNTR, etc. just give the frequency of the slices.
> 

The place to have done this is in SETJY with adverb value VCAL.  Then 
the velocity is carried along in the processing as an additional part of 
the header.  Verb ALTSW then switches between frequency and velocity in 
the spectral axis in the header.

It is easier to compute such things in the UV space - since one needs to 
have an antenna (or earth center) for reference and a single time (first 
observation).  Note that if Doppler tracking is not turned on, at the 
telescope as in the old VLA, or after the fact with CVEL, then the 
velocity varies during the observation.  Furthermore, even with velocity 
tracking, the velocity of the image is only correct at the center pixel 
and then varies (usually only slightly) away from that pixel.

Eric Greisen