Telescope oscillations

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This page has been created by CK on 9-Nov-2015.

During the NIKA2 runs since October 2015, the telescope occasionally showed strong and fast oscillations. Here, we collect all available information.

Recommendations (HU to operators on 27-Nov)

Unfortunately I do not have a solution --only-- some comments/suggestions
which may help to characterize the problem:

1. for each scan, when this occurs, can you check and report:
   -- has  there been a message "scan# loaded" ?
   -- has  there been a message "scan# started" ?
   -- have there been any messages "started" for any subscan(s),
      e.g. "subscanID onthefly started" ?
   (messages in antennaDataStream viewer and/or on https://mrt-lx1.iram.es/mrt/ncs/monitor/scanInfo.html,
    subscanID is something like 258.1 for scan 258 subscan 1)

2. when this occurs, try to repeat the same observing mode
   with modified parameters to see if this is related
   with position changes of a particular size.

   In particular:   


   in focusliss.pako try:

   let xAmplitude     90 instead of

   let xAmplitude     45


   for beammaps:

   try NIKA's

   beammap_12x12.pako
   or
   beammap_15x15.pako
   
   instead of beammap.pako



NOTE:   I do NOT recommend to try any Lissajous with slowRate 1
        (this is likely to provoke other problems,
         because generally it is too slow to calculate a halfway smooth curve 
         of commanded positions in the Lissajous curve)

Summary of findings so far

• Oscillations *only* occur with NIKA-2 and its 8Hz slow loop. With EMIR/HERA we did not notice such problems. However, such oscillations had not occured with NIKA1 or with GISMO although they too used 8Hz.
• They manifest themselves with strong Azimuth oscillations of 3arcmin peak-to-peak in Azimuth, while Elevation at the same time shows much weaker oscillations (see below). (AS)
• Oscillations show-up only when doing fast OTF maps with scanning speeds between 40"s and 55"/s. Oscillations also showed-up in lissajous scans (see below).
• These OTF maps are done with very short subscan lengths of between 7.5sec and 11sec, only.
• The period of the oscillations is about 1sec. (AS)
• Motor encoder readings are OK. (JP 9-Nov-2015)
• Often the wind came from the North, the wind meter may have been partially blocked. For one scan, the measured wind velocity was 14m/s.
• Oscillations occur after one or more successful OTF scans on the same source.
• It seems that the oscillations are not repeatable, i.e. they don't occur always for the same script. On the contrary, the same otf script has been in use for several days, without problems.
• Elevations are normal, 24 to 41deg.
• It doesn't make a difference whether a track scan was done before the flaky scan, or not.

Encoder plots

• 

• The Figure below shows in white the commanded, in red the encoder azimuth in antenna files 20151105t190827 till 20151105t192627. The commanded azimut shows as expected the zig-zag during the map 20151105s155 (start UT 05-NOV-2015 19:09:40) but is a straight line during the following map, i.e. during the oscillations. Why ? (RZ, 9-Nov-2015)

Scans showing oscillations

16-Jan-2016

• Scans: 183, 184, 185. Time: ~14:20 UT. Wind Speed: ~7 m/s. Az: ~113º. El: ~36º. Observing: Lissajous.
• Notes: The oscillations appeared once the scan started (second or third subscan).
• After changing the source, the oscillations dissapeared.

25th to 27th November 2015 (CK)

• 27-Nov-2015: With NIKA2 we had one oscillation event: Scan 90.4, UT 18:16, wind 14m.s-1, EL 42 Using focusLiss script. Once oscillating going to slowRate 1 did not help. Later on the wind went down and we had no problem anymore. (Operator report by FD)

• 26-Nov-2015: At the end of the shift, we had again backlashes and problems with the high currents (>50%) in azimut. Scan 86, standard lissajous for focus @ Uranus. (Az,el) = (111º, 34º), windy conditions, around 13 m/s from the East. We canceled before checking changing the slowRate. Scan 87 ok, and happened again with scan 88 (same observation mode). To change to slewRate 1 did not helped, we had to cancel. Now we are going to resume observations. (Operator report by JS)

• 25-Nov-2015: Problems in the last observing hour for NIKA: the telescope suddenly went to the previous target observed in the middle of an scan. Several goOnce restarts and vac1 reboot did not helped. Detailed information send to WB. At the end of the run, scan 57, lissajous on Mars @ (141º,30º), wind from NE > 15 m/s, we started to have strong oscillations in azimuth (and the Az currents went > 57%). I cancelled before checking if to setSlowRate 1 helped (as suggested by JP in a previous mail), but when we resumed the observations the problem did not happened again. (Operator report by JS)

5th and 8th November 2015 (CM)

scan          UT start  Az/El   wind(max)    observing mode/script
              OTF parameters: x/ystart/x/yend/x/ystep/totf/notf/scan speed

20151105s152  18:53:19  110/34  3.9m/s       otf (beammap.pako 2nd part)
              -300"/-112"/300"/-112"/0"/3.5"/11s/65/54.5" per s

20151105s156  19:22:58  116/39  5.8          otf (beammap.pako 2nd part)
              -300/-112/300/-112/0/3.5/11s/65/54.5" per s

20151105s158  19:35:19  118/41  5.2          otf (beammap.pako 2nd part)
              -300/-112/300/-112/0/3.5/11s/65/54.5" per s

20151108s43   01:36:53  338/24  14.2         otf (nkotf.pako 5 5 90 0 20 40 radec)
              150/150/150/-150/-20/0/7.5s/16/40" per s