Telescope oscillations

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

During the 1st NIKA-2 run in October/November 2015, the telescope occasionally showed strong and fast oscillations. Here, we collect all available information.

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.
• 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.
• 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 (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