[daip] Fwd: Try to understand the initial amplitude calibration?

[Arnaud Collioud]

Hi Amy, hi Eric,

First of all, I wish you a Happy New Year, full of successes and joys!

I sent you about a month ago a message (see below) with tons of questions about amplitude calibration in AIPS. I pretty sure you were extremely busy with the change in AIPS version and many other things. So I do no know if you had time to look at it.

If you do not have time to answer, could you please just tell where I could find relevant info about my interrogations? 
Of course, any help would be greatly appreciated!

Cheers,
Arnaud  


Début du message réexpédié :

> De: Arnaud Collioud <Arnaud.Collioud at obs.u-bordeaux1.fr>
> Objet: Try to understand the initial amplitude calibration?
> Date: 9 décembre 2014 15:53:45 UTC+1
> À: Eric Greisen <egreisen at nrao.edu>, Amy Mioduszewski <amiodusz at nrao.edu>
> 
> Hi Amy and Eric,
> 
> I use AIPS to calibrate VLBI data (i.e. RDV experiments), and I am currently trying to understand how tsys/gc info become calibration info.
> 
> #1/ ANTAB
> 
> TY1/GC1 are already filled with VLBA info, but I have to load tsys/gc for the other antennas using ANTAB. Until recently, I directly used tsys resulting from the EVN antabfs.pl script, which extracts tsys info from logs.
> But sometimes there are no tsys values for some sources (which implies no solution).
> 
> - Should I provide (a least) one tsys value per scan?
> What I am doing for the moment is to keep the same value for the tsys until it changes, then I keep the new value, and so on (kind of "step function"). I set the "interpolation" interval to 20 seconds, to be sure to have a tsys value in each scan (which lasts from 30 to 300 seconds).
> 
> - The adverb OFFSET may provide a way to avoid this lack of tsys value, but I am not sure about how it practically works. 
> Does it take the tsys value within the [scan_start-OFFSET ; scan_end+OFFSET]? 
> What if there are several tsys values within this range?
> What is "the best" value that I should take? I suppose that if the value is large, you will look at tsys values from different sources?
> 
> - How ANTAB knows which tsys values correspond to which source? Does it look into the NX table?
> 
> #2/ APCAL
> (used without opacity correction)
> 
> As far as I understand, as soon as there is a tsys/gc value, APCAL produces an entry in the SN table. 
> So if I can secure the presence of a tsys value, there always will be a generated solution (regardless there is really data or not in the uvfits file).
> Is that correct?
> 
> #3/ CLCAL
> 
> I use CLCAL with these parameters:
> sources ''
> opcode 'CALP'
> interpol 'SELN'
> cutoff 0
> samptype ''
> gainver 1
> gainuse 2
> snver 1 (generated from ANTAB)
> 
> With these, solutions in the SN table are written to the output CL table with no smoothing, but with an interpolation by source.
> So what happens if, for a given source:
> - there are entries in CL1 but not in SN1?
> - there are entries only in SN1?
> 
> And finally, just to be sure:
> Do DOBLANK and DOBTWEEN always apply? Or just when the smoothing is enabled (SAMPTYPE)?
> 
> 
> Sorry to bother you with this long list of questions!
> I just want to be sure to get the best of my data, and avoid to loose some due to a misunderstanding in the AIPS process.
> Thank you!
> 
> Cheers,
> Arnaud
> 
> 

______________________________________________________________
COLLIOUD Arnaud
Laboratoire d'Astrophysique de Bordeaux
2 rue de l'Observatoire, BP 89, 33271 Floirac Cedex (FRANCE)
Tel: +33 5 57 77 61 19     Fax: +33 5 57 77 61 00

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