[evlatests] Troubles with long slews

[Rick Perley]

    The 'Herc A' demo project was run on Friday.  The SB included four 
observations of the calibrator 3C286 (RA = 13 31, Dec = 30 30), which is 
some distance away from the target source Herc A (RA = 16 51, Dec = 01 
29).  Observations were made at four bands, with five frequency 
tunings.  Numerous tests done for the 'flux densities' run seemed to 
indicate that the OPT and the executor were in agreement on the length 
of time needed to execute the motions. 

    But there is a significant problem.  Upon examination of the data, I 
find that in every case involving a slew requiring a travel time of more 
than a minute or so -- in either azimuth or elevation -- the time 
actually spent on the target source was insufficient -- and usually less 
than zero (meaning, the antennas were still moving when the allotted 
time for that band/source expired.  In some cases, the antennas did not 
reach the target source until nearly two minutes after the expiration of 
the first desired observation! 

    The problem does not appear to lie with the OPT.  I examined the 
OPTs calculations of time moving and 'sitting' -- and these are always 
in excellent agreement with what I expect given the distance to be 
traversed.
    An example should help clarify the problem:
             At one particular moment, we needed to move from Herc A to 
3C286.  The azimuths and elevations were at that time:
             Herc A:  Az = 118   El = 42
             3C286:   Az = 252   El = 79

    With azimuth slew speeds of 40 deg/min, and elevation speeds of 20 
deg/min, it's obvious that more than 3 minutes will be needed to cover 
that distance.  The OPT states the time required would be 3m 26s.  
Adding in my requested 45 seconds 'on source' time, the scan observation 
time for 3C286 in this instance should be a little over 4 minutes.  And 
indeed, this is what the OPT says will be provided. 
    But it's not what happened.  In fact, the length of that scan was 2m 
20s -- nearly 2 minutes short.  So not only was that calibration scan 
missed, but so was the subsequent one (same source, different band). 

    Further evidence of the problem lies with the total length of the 
observation.  The OPT claims the total length of the observation is 8h 
0m.  (Indeed, I constructed the file to fit this time window).  But the 
*real* observation length was 7h 47m -- the 23 minutes differential is 
the sum of the all travel time miscalculations. 

    This is a significant problem, especially to those programs which 
have many long slews -- like the 'flux densities/calibration scale' 
run.  Fortunately, this program was not run this weekend, due to poor 
weather. 

    What is wrong here?