[evlatests] Essential Results from C-Band Polarimetry

[Ray Ferraro]

	The problem with 3D mentioned in section 2 was most likely caused by an 
observed VLA sampler connector problem. This should now be fixed.
			Ray

Rick Perley wrote:
>     I used 7 hours of dynamic time last Sunday afternoon/evening to do 
> deep polarimetry at eight frequencies across C-band, using the seven 
> available antennas equipped with wideband OMTs (2, 3, 9, 15, 21, 24, and 
> 28).
> 
>     The source observed was 0217+738 -- a perfect point source with low 
> polarization and 4.1 Jy flux density.  An observation of 3C48 was added 
> at the beginning to check the flux scale.  Observations were made in 50 
> MHz continuum, with 1.67 seconds averaging.   The frequencies used were 
> 4885/4385, 5385/5885, 6385/6885, 7385/7885 MHz.  (AC/BD frequencies). 
> 
>     Essential Results:
> 
>     1) Data quality was excellent.  One or two antennas, as some 
> frequencies, were slow to lock up (not a pointing issue) by a few 
> seconds.  No clear patterns for this were seen -- it's a rare event. 
> 
>     2) No phase jumps were seen.  I loaded the data as correlation 
> coefficients, and most behaved normally.  Two antenna-IFs were peculiar 
> at all frequencies, changing from scan to scan by about 10% (i.e., never 
> within a scan, only between scans):  3D and 28C.  This cannot be a 
> pointing or efficiency effect, so the system temperature is apparently 
> changing -- perhaps from mis-set power levels to the sampler? 
> 
>     3) Antenna sensitivity follows the trends reported by Emmanuel and 
> me in the past:  about 25% poorer between 4 and 6 GHz than between 6 and 
> 8 GHz.  Tests by Bob Hayward and me indicate this is due both to a 
> system temperature increase and an efficiency decrease -- possibly due 
> to illumination effects.  (Further elucidation will require 
> holography).  Overall, sensitivity between 6 and 8 GHz is extremely good 
> on all antennas:  Typical Tsys/effic. = 45.  Antennas 15 and 24 are the 
> best, with Tsys/effic = 39 and 41 K,  respectively.  Conversely, the 
> typical Tsys/effic between 4 and 6 GHz is about 55K, again with 15 and 
> 24 being the best (and 2, 3, and 9 the worst, as they are at the high 
> end of the band). 
> 
>     4) Polarization calibration using AIPS was done, using antenna 28 as 
> a reference.  All antennas have higher cross-polarization ('D'-terms) at 
> the band edges than in the middle -- as expected.  Antennas 9 and 21 are 
> outstandingly good, with cross-polarization of 1 to 2 % across most of 
> the band.  Antenna 3 is typically 3 to 4%.  Antennas 2, 15 and 24 are 
> notably poorer -- especially the last one -- with polarization typically 
> 4 to 8%.  All cross-polarization values are very stable, as the 
> resulting polarization images (after calibration) are nearly noise 
> limited. 
> 
>     5) Closure errors between the EVLA antennas are impressively low -- 
> typically less than 0.5% and 0.2 degrees. 
> 
>     6) Images were made in all Stokes' parameters at all frequencies.  
> All are close, but definitely not at, the expected noise level.   In Q 
> and U, the discrepancy is no more than 30%.  Various explanations can be 
> offered, but are not worth pursuing (IMHO), given the imminent arrival 
> of WIDAR.  
> 
>     The data utilized here will also be analyzed by George (within CASA) 
> and by Bob Sault (within Miriad).  They plan to use a full polarization 
> solution (as opposed to the approximations employed by the AIPS program 
> PCAL).  Time-variable analyses of the cross-polarization will be done -- 
> this is a crucial parameter. 
> 
>     With luck, all results will be shown at the next test meeting on 
> April 23. 
> 
> 
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
> 
> 

-- 
Ray Ferraro
National Radio Astronomy Observatory
Array Operations Center
P.O. Box O
1003 Lopezville Road
Socorro, NM 87801-0387
575-835-7196
fax: 575-835-7027