[evlatests] Time taken to change bands

[Rick Perley]

    In preparation for a calibrator run this weekend, I've carefully 
checked the time it takes to change bands and correlator modes.  In 
general, it's less than you might (and the OPT does) think.  It's also 
very repeatable amongst the 7 sources in the script. 

    The sequence of operations is as follows.  Note that the sequence is 
selected to minimize subreflector rotation time. 

    1) Move to source:  This was done at L-band.  The preceding band was 
'P'.  Correlator mode change required.
    2) L-band to X-band referenced pointing.   Correlator mode change.   
Large subreflector focus motion. 
    3) X-band referenced pointing to X-band observation.
    4) X to C        Correlator mode change. 
    5) C to S
    6) S to Ku        Correlator mode change
    7) Ku to K        Correlator mode change
    8) K to Q
    9) Q to Ka    
    10) Ka to P      Correlator mode change.   Large subreflector motion 
in focus. 
    11) P to L -- move to the next source.  Correlator mode change.  

    For all observations, the integration/dump time was 1 second.  All 
observations were taken with the 8-bit samplers. 
    For all observations *except P*, I requested 35 seconds On Source 
time. 
    For 'P' band, 40 seconds of On Source was requested. 

    In all cases, the OPT simply adds 20 seconds to the OnSource request 
time. 

    Below is a table giving the time taken to actually effect the 
change.   In this table, 'Time to Change' is the duration between the 
last good data of the originating band, and the first good data in the 
new band. 

    Important Conclusions:

    1) The OPT's built-in addition of 20 seconds to change bands is very 
conservative, and is presumably based on a worst-case motion change.  It 
would be nice if the OPT were made smarter, taking note of the actual 
pair of bands involved, (and whether the correlator changes mode) since 
the change time between high frequency bands is no more than 10 seconds. 

    2) A more careful look at the L to X band, and Ka to P band setup 
times reveals a large variation between antennas. 

        a) L to Xband:   The time taken is 12 to 24 seconds.  The same 
antennas are always the slowest:  ea 10, 12, 16, 20, and 22.  Of these, 
ea20 is always the last.  Either it has the slowest focus motion, or has 
the longest travel distance (or both).   Perhaps up to 10 seconds is 
taken up the the rotation, the remaining variation must be due to focus 
change.

        b) Ka to Pband:  The time taken is between 21 and 39 seconds.   
It is clear from looking at the variation in visibilities during this 
period that the antenna gain change is a very weak function of the 
subreflector position!  (This is good news for VLITE ...).  The same 
antennas always take the longest time:  ea16, 19, and 22. 

    Very unfortunately, ea21 (with the new FRM system) was *not* in the 
array, since it had been moved earlier in the day. 

    3) It appears the correlator mode change times are not an important 
factor here.  The C to S move -- which does not involve a correlator 
change, took 15 seconds, about the same as the S to Ku move, which does 
require a correlator change.  The later move is slightly shorter in 
subreflector motion, and took only 2 more seconds, presumably due to the 
correlator change. 

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