[daip] [Fwd: Re: VLA-AB465-data]

[Eric Greisen]

Data Analysts wrote:
> 
> 
> ------------------------------------------------------------------------
> 
> Subject:
> Re: VLA-AB465-data
> From:
> trevino <trevino at iram.es>
> Date:
> Tue, 24 Jan 2012 17:55:32 +0100
> To:
> Data Analysts <analysts at aoc.nrao.edu>
> 
> To:
> Data Analysts <analysts at aoc.nrao.edu>
> 
> 
> 
> Thanks for your rapid answer!
> 
> Is there any way to calculate this dates?
> 
> REgarding the strange velocity (340 km/s) in the LISTR, I would like to 
> know if there is a way to confirm if this was the velocity really used? 
> I mean, the line appears well detected, and it should be around 10 km/s. 
> I would like to know if there is a way to check the real observed 
> frequency, and confirm which is the correct velocity scale.
> 
> Furthermore, in the header of the image, the channel of reference for 
> frequency is number 1 (out of 31). Typically the reference channel 
> should be number 16, isn't it? For the velocity axis, the 340km/s is 
> located in the reference channel 128! All this make me think that there 
> could be a problem with the labelling of the hird axis.

I have talked to some old hands in VLA line work and they say that in 
the old days it was possible to observe with e.g. 255 channels but to 
select only some smaller number to be written out.  This is probably 
what has happened with this data beginning with 1988 jan 23.

There seems to be no information in the data format to tell FILLM about
this selection!  I suspect that the reference channel should be 16 and 
have no idea what the source velocities should be.  I notice that the
velocities in the earlier, wider-band data vary between 0 for 
calibrators and some chosen (near 0) values for sources.  You can fix 
the reference pixel with PUTHEAD KEYWORD = 'CRPIX3' but what the source 
velocities were I don't know.

PRTAB will show you in the CL table a column called DOPOFF which will 
tell you the Doppler offset in Hz for the various sources.  It shows 
some as tracked and others as not tracked.

Eric Greisen