[evlatests] Antenna/ACU/servo behavior during OTF-mosaic tests.

[Vivek Dhawan]

      Antenna/ACU/servo behavior during OTF-mosaic tests.

There will be a discussion of this and related issues at Rick's
meeting, room 317, 10AM Thursday. Plus a follow-up session at 11 in
room 220.

SUMMARY: The VLASS Sky Survey machinery is being tested - Recent
tests on scan patterns showed some antenna tracking issues, notably
a 1.7 (and or 2.2Hz) oscillation on the antennas with NEW ACUs.
Details below. Plots will be displayed at the meeting.

New Features:

1. ACU MIBS: o-stream monitoring is enabled, at a rate as short as
    0.1s, Commanded and Encoder positions are multicast for AZ, EL.

2. Data go into PointingTable.xml in the latest (test) version of
    MCAF,  if: The Intent is ‘OTF’ or ‘Calibrate_Pointing’.
          and: The o-stream is on.

TEST #1:

     Lorant looking at acceleration times, OPT setup of rasters, etc.

     There were several setups, in sequence:

     a. 0.45s integration with 1.8s scans (=phase center changes)
     b. 0.25s integration with 0.75s scans?
     c. 0.15s integration with 0.75s scans.

     Antenna motion consisted of tracks at +3 arcmin/sec, 0’/sec, and
     -3’/sec, for 45sec each. i.e. a fast forward, on-source, reverse.

     Motion was mostly in AZ, near transit,  EL=72deg (3C454.3).

     This test showed that:

     1. Near transit (encoder AZ around 360deg, true AZ=South) the
        interpolation of commands sent to the ACU MIBS is faulty.

        Ken has a fix in the executor, not yet deployed.

     2. NEW ACUs (antennas 1, 14, 17, 21) showed AZ oscillations, to
        be discussed in TEST #2 below. Servo and/or antenna resonances
        are suspected.

     3. OLD ACU antennas were polling values at 0.104s, a vestige of
        the waveguide cycle, not at 0.1s. The beating of the 2 rates
        made the az/el values a bit confusing.

        Modified ACU MIB software was installed temporarily, to log
        at 0.1s rate, for the next test. The precision on the time
        stamps was also enhanced by 2 digits.


TEST #2: A repeat of part a.) from above, after transit, EL ~60deg.

         The slew rates in encoder coordinates were roughly:
          +22”/sec AZ,  -10”/sec EL,  during sidereal tracking.
         +360”/sec AZ,  -320”/sec AZ,  +_160”/sec EL  rasters.

         There were 3 flavors of antenna in this test:

         1. ea01 = NEW ACU, no MIB modification, poll at 0.1s

         Overshoot 5’. Settling time 2-3s. AZ oscillations 1.7, 2.2Hz,
         of amplitude up to 1’ during fast tracks, 3” at sidereal rate.
         Tracking errors (offsets) during fast track ~30”, sidereal 3”.

         2. ea02,3,4,5 = OLD ACU, MIB modified to poll at 0.1s.
            Still not completely clean from data logging effects.

         Overshoot: 5’. Settling time 6-7s.  Oscillations at about the
         same period, 1.7 Hz, amplitudes generally less than ea01.
         Larger amplitude osc seen when DEceleration from fast +AZ to
         sidereal, not in the other direction.

         Both old and new ACUs showed EL variations of 10-20" of a
         random nature, rather than clear oscillations.

         3. ea06,7,9,10 = OLD ACU, no MIB mod, poll at 0.104s.
            Behaved the same as in TEST#1 - No further interest.

Questions:

1. Servo system tuning and other antenna fixes: What is possible?

2. Specs for VLASS? are 20-30" time-variable tracking offsets
    tolerable? (If accurately recorded in the pointing table)?

3. Cleanup of az/el monitoring? logging time latency, can it be
    made more predictable?

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