[daip] Amplitude calibration of VLBI data

[Steven Tingay]

Hi Jim,

Thanks very much for your quick response, it is much appreciated.

However, after puzzling over the problem for the rest of the day 
yesterday, Shinji and I sorted it out and now understand what's going on.

The root cause was not a problem with AIPS or a problem with our FITS 
file.  The root cause was the different definitions of the (x,y,z) 
coordinate system that describes antenna positions used by the ATNF and 
NRAO.  The mismatch in the definition leads APCAL to think that the source 
is below the horizon for most of the observation, meaning that SN table 
solutions are not generated for these times.

Negating all the y coordinates in our antenna table, to become consistent 
with the definition used in AIPS, solved the problem immediately.

So, when generating our FITS file we will have to be mindful of the 
coordinate system we use for our antenna table, depending on if we want to 
reduce our data in MIRAIAD (ATNF definition) or AIPS (NRAO definition).

One comment though.  It would be nice, and not difficult to implement, if 
APCAL reported reasons why it does not generate SN table solutions during 
runtime.  For example, a message saying "No SN table entry generated: 
source below horizon" would have saved an enourmous amount of time in 
chasing down this problem.  Perhaps this could be added to APCAL?

Again, thanks for your email, but I'm glad we found the problem before it 
got added to your "things to do" list.  

Cheers, Steven   

##=====================================================##
Dr Steven Tingay		stingay at astro.swin.edu.au
Swinburne SKA Project Leader
Associate Professor

Centre for Astrophysics and Supercomputing
Swinburne University of Technology
Mail No H39
P.O. Box 218, Hawthorn, Vic. 3122, Australia

ph:  +61 3 9214 8758
fax: +61 3 9214 8797

http://astronomy.swin.edu.au/ska
##=====================================================##

On Thu, 8 Dec 2005, Jim Ulvestad wrote:

> Hi Steven,
> 
> Could you send us the following?
> 
> (1) FITS file of your complete set of tables, with TASAV, and an e-mail
> telling
> us which is which.
> (2) Your input file for ANTAB.
> (3) Your AIPS inputs for ANTAB and APCAL.
> (4) A snippet of data created with UVCOP, say 10 minutes or so from the
> start of the run.
> 
> Easiest thing is probably to put them on an ftp site or something.  We
> suspect any one of a number of problems, such as subtle errors in the
> input text file, problems with polarization labeling, or minor input errors.
> At least we can try to rule those out before worrying about the structure
> of the FITS file.
> 
> Amy Mioduszewski normally handles our VLBI problems, but she's on
> vacation.  Since I have another "job," you may need to be a little patient!
> 
> Please reply to daip, not just to me.
> 
> Cheers,
> 
> Jim U.
> 
> Steven Tingay wrote:
> 
> > Dear AIPS help,
> > 
> > I wonder if you might have some suggestions to help me with a problem I am
> > having producing amplitude calibration tables for a VLBI dataset.
> > 
> > The data are from three Australian antennas, correlated on a software
> > correlator that we have developed here at Swinburne.  The root cause of
> > our problems may well be the FITS files that we are producing.  However,
> > we have calibrated data from our correlator before and it worked fine.
> > And I can't spot anything obvious that is wrong with this particular
> > dataset.  It seems somthing more subtle is going on.
> > 
> > Here is a basic synopsis of the problem, the things I have tried, and a
> > few questions along the way:
> > 
> > 1. Without applying any amplitude calibration data, we can load and 
> > examine the data, fringe-fit it, export it, and image it.  Data are
> > present for all expected antennas, in the right time range, and with
> > the right polarisations/IFs (examining the data with PRTUV, LISTR,
> > UVPLT, POSSM etc)
> > 
> > 2. I can load the calibration file in the KEYIN format which contains the 
> > Tsys measurements and gains for each of the three antennas, using ANTAB
> > - it reads the three gain entries and all Tsys values.  I can
> > examine the Tsys data in the resultant TY table in SNPLT and it looks
> > fine, correct values in the correct time range to match the uv data.
> > As far as I can tell, using PRTAB, the resultant GC table also looks
> > fine.  Is there a better way to look at the contents of the GC table
> > than simply using PRTAB?
> > 
> > 3. I ran APCAL to produce an SN table from the TY and GC tables.  That is 
> > when things go wrong.  APCAL seems to run fine, but when I use SNPLT to
> > examine the amplitude corrections in the resultant SN table, only two
> > of the antennas have values in the SN table.  The data for the third
> > antenna is missing and seems to have not made it through APCAL.
> > 
> > 4. Previously we had a similar problem when the source coordinates 
> > recorded in the FITS file were wrong and APCAL did not write solutions
> > for a set of antennas because the source was deemed below the horizon.
> > However, we fixed that problem and all worked perfectly.  Is there any
> > other similar sort of issue that may be causing the problem I have just
> > described?  The source coordinates seem correct, the antenna positions
> > and mount types seem correct.  Is there something else perhaps that I
> > should be checking?
> > 
> > 5. I guess I've also seen this problem when an observation has sparse Tsys 
> > measurements and the measurements don't coincide with the times of
> > unflagged data.  This should not be the case here.  Tsys measurements
> > were made every few seconds and very little uv data scans are flagged.
> > There are both dense Tsys data and uv data.
> > 
> > 6. I briefly tried using ANCAL to read the calibration file and produce a 
> > CL table directly.  ANCAL complained about the format of one of the
> > Tsys entries but I could not see any problem that that entry in the
> > file or those around it.  So, I gave up on ANCAL.
> > 
> > Thanks for any advice you can give.
> > 
> > Cheers,
> > 
> > Steven
> > 
> > ##=====================================================## Dr Steven Tingay
> > stingay at astro.swin.edu.au Swinburne SKA Project Leader Associate Professor
> > 
> > Centre for Astrophysics and Supercomputing
> > Swinburne University of Technology
> > Mail No H39
> > P.O. Box 218, Hawthorn, Vic. 3122, Australia
> > 
> > ph:  +61 3 9214 8758
> > fax: +61 3 9214 8797
> > 
> > http://astronomy.swin.edu.au/ska
> > ##=====================================================##
> > 
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> > 
> 
>