[daip] VCAL in SETJY

[Andy Biggs]

Hi.

I have VLBI observations of an HI absorber that was initially discovered 
with the GBT. If I compare the two spectra, they look very similar, but 
they appear to be offset in velocity by ~300m/s. The attachment shows 
the spectra as well as their difference.

Now, the vertical scaling is arbitrary as there's a bright source of 
unknown flux density in the GBT beam that's diluting the apparent 
optical depth. The way I've scaled the spectra is as if there's an 
additional component missed by the VLBI array (certainly possible) but a 
velocity offset remains a possibility. I therefore want to be sure that 
the velocity scales are correct. I'm talking to GBT people, but I'd also 
like to check that what I did in AIPS with the VLBI data is correct, 
especially as I'm not an experienced spectral-line VLBI observer.

Basically, I used SETJY with its VCAL option to calculate the velocity 
at the reference pixel and then ran CVEL. That's it really - seems very 
straightforward. The reference pixel is also close to the line so there 
should be no issues with the velocity increment being a function of 
frequency in the optical definition. This all sounds fine?

What I've also done is to try and calculate the geocentric to 
barycentric velocity conversion using some IDL code from the astrolib 
library and compare this to the AIPS value. The latter isn't reported, 
but I can calculate it as I know what the velocity at the reference 
pixel is in the geocentric frame.

In the IDL code, there's an input that I don't entirely understand 
("input epoch of mean equinox of dvelh and dvelb") where dvel are the 
heliocentric and barycentric velocities. I assume though that it should 
be set to the date of the observation and doing that causes the AIPS and 
IDL values to differ by only 5m/s which is of course excellent.

So, AIPS seems to be doing the right thing, but it would be great if you 
could reassure me that what I'm doing sounds okay.

Thanks,

Andy

-- 

Andy Biggs
ARC Astronomer, ALMA Regional Centre
European Southern Observatory
Karl-Schwarzschild-Straße 2
D-85748 Garching
Germany

tel. +49 89 3200 6471
fax. +49 89 3200 6898
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