[evlatests] Odd Tcals at X-band

[Rick Perley]

    The Herc A run has the nice feature of first looking at a source at 
high elevation, then looking at another source at very low elevation.  
The latter has a (reasonably well) known atmospheric addition which we 
can use to roughly calibrate our Tcals. 

    I've reviewed the X-band data taken yesterday morning.  Nearly all 
antennas show a reasonable system temperature (from low 20s to high 
30s), and a ~6 degree difference between zenith and low elevation.  
There are some notable exceptions:

    1) Antenna 2 shows a Tsys of 12 K in subband 1 (8.0 GHz)!  (We 
wish).  The increment is only 2K, so the Tcals must be too low.  But 
this little problem is hardly seen in subband 4, so the run of Tcal with 
frequency must be wrong.  (The database shows Tcal = 1.3 in subband 1, 
but 2.9 in subband 4). 

    2) Antenna 21 shows a Tsys of about 200K!!!  This is not real, as 
the increment is also large -- 25K.  To the Tcals are off by a factor of 
at least 3.  The table lists the Tcal as 9.9K!  (How did this happen?)

    3) Antenna 22 also shows a high Tsys and high increment.  But it's 
hard to see what is going on since the power from this antenna is 
jumping all over the place -- the proper change in elevation as the 
target source rises (beautifully seen in all other antennas) is hard to 
discern in this one.  Something bad is going on ...

    4)  Antenna 26 has a Tsys ~ 100, and an atmospheric increment at 
least 2 times to high.  The tabled Tcal is given as 3.8K -- should be 
closer to 2.0.

    BTW -- antenna 24 looks perfectly normal in Tsys and increment.  I 
don't yet know about its sensitivity.