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

[Amy Mioduszewski]

So sorry, business and holidays intervened.  See answers below.

On 01/09/2015 07:57 AM, Arnaud Collioud wrote:
> 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).
>>
This depends on whether each Tsys corresponds to a specific source, or if you 
don't care what source the Tsys is assigned to.  If you have sparse coverage 
then you probably just want to interpolate.  After you run ANTAB and APCAL you 
have an SN table and which you apply to a CL table with CLCAL, CLCAL can and 
will interpolate a SN table.  If you do not use INTERPOL "SELF" or "SELN" they 
it will happily interpolate between sources and you can even give it an 
interpolation limit with CUTOFF.  Now this is probably not advisable for 
antennas like the VLBA which will have good Tsys for each source, so I would run 
CLCAL twice, one with the antennas with good Tsys info and "SELF" then once
for the ones with sparser coverage with a different INTERPOL.

>> - 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]?
For each time in the Tsys input file ANTAB looks for in the NX table and search 
for which scan the Tsys value falls in from (start time of scan - OFFSET) to 
(end time of scan + OFFSET).  So I don't think OFFSET will help.  ANTAB just 
picks a scan for each Tsys value.

>> What if there are several tsys values within this range?
What worries me is what if there is multiple scans withing the range.  I think 
ANTAB would just pick the first one.  I think ANTAB is not where you should be 
looking for a solution for this.

>> 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?
Well the best value would be a combination of the same source and similar 
elevation.  If you are wandering all over the sky then it is probably not a good 
idea to just interpolate the Tsys.  Although at some level you would just take 
that as an approximation and fix it up the amplitude self-cal.

>>
>> - How ANTAB knows which tsys values correspond to which source? Does it look into the NX table
See the answer for OFFSET.

>>
>> #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?
Yes, bar weird things like the wrong subarray or different freqid.

>>
>> #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?
I answered some of this in my first comment.  CLCAL interpolates to all the 
entries in CL1, for the same source if the interpol is SELN.  So if you are 
using SELF or SELN if it comes across a CL entry that has no corresponding SN 
entry for the same source then that CL entry will be flagged.  This is only if 
there are no SN entries for that source.

>> - there are entries only in SN1?
It depends, if there are entries in SN1 with no corresponding source/freqid etc 
in the CL1 then nothing happens they are ignored.  If there is a corresponding 
source and you are using SELF or other INTERPOL other than SELN then those 
entries will be part of the interpolation that CLCAL does.


>>
>> And finally, just to be sure:
>> Do DOBLANK and DOBTWEEN always apply? Or just when the smoothing is enabled (SAMPTYPE)?
These only apply when smoothing.

Hope that helps!

Amy

>>
>>
>> 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
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