Sent to the VLBI email list on 15 August 2005


Dear PI,

You may or may not have noticed that the sensitivities for the phased VLA
in your recent experiment (see below) may be off from expected. Note that
single dish VLA observations should not be affected. This message is to
inform you how to recognize the possible phased VLA calibration problem and
how to solve for it. We sincerely apologize for the inconvenience.

Probably affected experiments, in alphabetical order:
bb190,  bb191,  bb191b, bb196,  bb204,  bb209a, bd103,  bf083,  bf084a, 
bf084b, bg141,  bg150,  bg155,  bh128,  bj054,  bk114b, bk114e, bl121, 
bm223a, bm223b, bn032,  br088c, br092a, br092b, br092c, br092d, br102a, 
br102b, br102c, bu030a, bu030b, bu030c, bw076,  bw080,  bw084,  gb049a, 
gb049b, gb049c, gb053,  gd017a, gd017b, gd018,  gg049a, gm048c, gm055a, 
gm059a, gt005,  ty001 

Probably unaffected:
bk110,  bk114a, bk114c, bk114d, bk119,  bn031,  bt075,  tf029a
(as they probably need to run ANTAB to get the VLA TY values included)

To recognize whether you are affected, look in your data's TY-table:

Figure out the antenna number for the VLA using PRTAN or TYPE ANTNUM('Y')
Then check if for antenna Y, the phased VLA and for the phased VLA only,
the 8th and 10th (TANT 1, TANT 2, assuming you have both polarizations)
columns are set to -1.0 (or any other negative value, which will be true
only if you run ANTAB). Note that if there are no TANT values in the TY 
table for the antenna number corresponding with the VLA, you need to run 
ANTAB anyway and you will not encounter this problem. Use for example

AIPS> default prtab; docrt 1; inext 'ty'; box 4 8 10
AIPS> indisk i; getname j; go prtab
      (and many returns until you get to the antenna number for the VLA)

If the columns are filled with a negative value, you should have nothing to
worry about. If they are labeled 'INDE' for the phased VLA, please read on.

--

Since late-2003 we have included the gain curve and Tsys values for the VLA
in most VLBA distribution tapes in order to make it more convenient for our
users; i.e. not to have to run ANTAB anymore and have the VLA behave just
as a VLBA antenna as much as possible. However, for the phased VLA, the
FITLD/VLBALOAD program did not recognize whether the VLA was used in single
dish or in phased array mode. This means that for the phased VLA, the
Tsys/Tant values were not properly converted into proper SN table gain
values in APCAL/VLBACALA, unless you used ANTAB to (re)load the VLA Tsys
values. To get a proper Tsys value, APCAL/VLBACALA uses the TANT columns of
the TY table to see if it is supposed to extract the source flux from the
SU table in order to convert the given Tsys/Tant in the TY table, and store
the appropriate gain solutions in the newly generated SN table.

To solve for the problem, unfortunately you probably have to recalibrate
your data from the start. If you haven't started loading the data from the
distribution tape(s) yet, you may want to fetch todays (15 Aug 2005) new
FITLD version using the midnight job (MNJ). If you got your data using the
archive ftp method, let me know and in a day or two there will be a new
file for you to download. Or you can still load the data into AIPS using
the old FITLD and/or archive file and follow the fix below.

If you have your data on disk inside AIPS, the easiest fix is as follows:
- delete (maybe after a TASAV) all SN and CL tables except for CL#1
- find the antenna number for Y (eg with task PRTAN)
- use TABED, with:
    default tabed; indisk i; getname j
    inext 'TY'; optype 'repl'; keyvalue=-1,0; outdisk=indisk
    aparm=8,0,0,2,4,[antenna number of phased VLA]; go tabed
  and once more for a second polarization with:
    aparm(1)=10; go tabed
- rerun VLBACALA or ACCOR/APCAL, etc

Again, we apologize for the extra work. 
Thanks to Josh Winn for pointing out this problem/feature/bug.
Please contact me if you have any further questions regarding this problem.

Note added after sending this message to the VLBI email list:

If you relied on self cal on either the target, or a nearby calibrator, 
and did not include the VLA in the list of antennas used for the flux 
scale, you should be ok without any recalibration.


Regards,

Loránt Sjouwerman - Scientific Services - lsjouwerman@nrao.edu
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