[evlatests] J2000 vs B1950

[Walter Brisken]

Bottom line -- it appears B1950 to J2000 coordinate conversions work 
properly, otherwise I would not have seen the X-band amplitudes be 
consistent in the two frames.  Short summary of the test follows.  Times 
in UT on Feb 24, 2006.

13:26 *	TBRISK1: Start observations
 	L-band 4 IF / 15 channel mode; .obs file in J2000 coordinates

14:04 	At source change all IFs of 13, 14, and 16 show periodic amplitude
 	drops (to low, non-zero values) and 180 degree phase jumps

14:09 *	TBRISK2: Start new file, identical format, but in B1950 
coordinates

14:22	Jumping, dropping behavior stops mid scan

14:42	Antenna 17 (vla) drops out for remainder of file:
 	Cryo techs charging compressors.  Antenna stowed.

14:54 *	TBRISK3: Start new file
 	X-band continuum; .obs file in J2000 coordinates
 	EVLA antennas indistinguishable from VLA antennas except
 	notable phase slopes in Ant 13 due to bad baseline.

15:02	14B alone gets flakey (unstable?)

15:05	14B finally dropped out of observation, not to return

15:10	Starting at scan boundary Ant 12 all IFs becomes flakey.
 	No explanation for this!

15:14	Starting at next scan boundary Ant 12 recovers

15:24 *	TBRISK4: Start new file, identical format, but in B1950
 	coordinates EVLA antennas except 13 phase ramping and 14B are
 	indistinguishable from VLA antennas until...

15:32	13, 14, 16 all IFs show amplitude drops (to low values, not zero)
 	and 180 degree phase flips.

15:54 *	Finished.


Barry thinks the 13,14,16 phase flips, amp drops are caused by the EVLA 
and VLA antennas becoming desynchronized by 1 waveguide cycle.  This is 
consistent with what I've seen. EVLA-EVLA baselines are stronger than 
EVLA-VLA baselines during these times.  Ant 16 showed notably lower 
amplitudes during these periods than 13 or 14, probably a consequence of 
the particular walsh codes used.


-W