v254a -- some thoughts

[Leonid Petrov <Leonid.Petrov@xxxxxxxxxxx>]

Dear colleagues,

  Chris Phillips suggested to observe v254a on 2008.02.05_04:00 in the 
same mode as the proposed LBA Calibrator Survey. 

  We need to decide several things:
1) list of candidate sources
2) frequency sequence
3) how many scans per source
4) scan duration.

  There are 5 stations allocated for the experiment: ATCA-104, CEDUNA, 
HOBART26, MOPRA, PARKES. HARTRAO is allocated for IVS experiments around
this date. Although it is possible to negotiate with the IVS participation
of HARTRAO, but since scheduling HARTRAO with LBA is difficult, especially
with PARKES (30.8 deg elevation limit) and requires upgrade of existing
software, I tend to agree with Chris, that we should go ahead with the 
5 station network without HARTRAO.

  I prepared the list of candidate sources from the preliminary AT20G
catalogue. I took all the sources that
1) were not observed with VLBA before
2) declination < -50 
3) single-dish flux density at 8.4 GHz > 200 mJy 
4) the spectral index flatter than  -0.6

  The candidate source list is available here:
  http://lacerta.gsfc.nasa.gov/vlbi/lcs/v254a.spind and their spectra here:
  http://lacerta.gsfc.nasa.gov/vlbi/lcs/v254a_spe.pdf

  There was a national holiday in the US, good time for work, so I generated 
a bunch of trial schedules. A burst of 4 troposphere calibrators was 
inserted every 60 minutes. The scan length for observing the troposphere 
calibrator was set to 90 seconds.

  I varied the scan duration for target sources and the number of scan 
per source.

  Statistics of trial schedules:

scan duration: 300 s, 3 scans per source:  77 sources, efficiency: 0.892
scan duration: 180 s, 3 scans per source: 112 sources, efficiency: 0.806
scan duration: 150 s, 3 scans per source: 127 sources, efficiency: 0.767
scan duration: 120 s, 3 scans per source: 150 sources, efficiency: 0.712

scan duration: 300 s, 2 scans per source: 110 sources, efficiency: 0.878
scan duration: 180 s, 2 scans per source: 168 sources, efficiency: 0.783
scan duration: 120 s, 2 scans per source: 186 sources, efficiency: 0.609

The efficiency is defined as a ratio of "on source time" to the total
time allocated. It is typically 0.4-0.5 for geodetic sessions and 0.6-0.7
for VLBA surveys.

  Personally I am leaning towards 150 sec scan duration, 3 scans per source.
http://lacerta.gsfc.nasa.gov/vlbi/lcs/v254a_t1.vex
http://lacerta.gsfc.nasa.gov/vlbi/lcs/v254a_t1.plan
http://lacerta.gsfc.nasa.gov/vlbi/lcs/v254a_t1.stat

NB: sections BBC, DAS, MODE are bogus. They will be corrected later.

Sincerely,
Leonid
21-JAN-2008 20:42:33