[daip] Amplitude calibration of VLBI data

[Steven Tingay]

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
##=====================================================##