SX NCPA and Wobble

Starting with wobble test.

Loop parameters:
  • bin 2
  • 153 modes
  • 625 Hz
  • 11.5 mag light source

Going to 100 degrees rotator angle.

18:47 LUCI1 parameters
  • 2.5 DIT
  • 7 NDIT
  • Integrated

18:50 Moving rotator/re-rotator

SX NCPA and Wobble

Starting with wobble test.

Going to 100 degrees rotator angle.

18:47 LUCI1 parameters
  • 2.5 DIT
  • 7 NDIT
  • Integrated
18:50 Moving rotator/re-rotator
luci1 image rotator angle Star Position Comment
45 100   dark
46 100 1100, 1063  
47 110 1093, 1068  
48 120 1088, 1074  
49 130 1084, 1080  
50 140 1081, 1087  
51 150 1076, 1094  
52 160   vibration? Double image
53 160 1074, 1099  
01 170 1073, 1111 changed date to 20161201
02 180 1074, 1116  
03 190   vibration? Double image
04 190 1081, 1118  
05 200 1085, 1118  

19:14 06 (circle) T/T gain only of 1.6, a lot to flashing

19:16 07 (elongated) T/T gain 0.8

19:18 08 (elongated but better) T/T gain1.2

19:20 22 degree offset is maximum

19:12 Moving re-rotator only start rot 1
luci1 image rotator angle Star Position Comment
10 215.9 1097, 1062  
11 211.9 1094, 1066  
17 207.9 1090, 1071  
18 203.9 1087, 1075  
19 199.9 1085, 1080  
13 193.9 1084, 1088 Zero postion
20 193.9 1084, 1088  
25 193.9 1084, 1088  
24 191.9 1083, 1091  
23 187.9 1084, 1096  
22 183.9 1085, 1102  
21 179.9 1087, 1107  
16 179.9 1087, 1107  
15 175.9 1089, 1111  
14 171.9 1093, 1115  

Results

We investigated the affect of a misaligned SX WUnit re-rotator during daytime closed dome test using the ARGOS CalUnit. The test performed was the measurement the position of the CalUnit source on LUCI1 as the re-rotator, and only the re-rotator, was moved in Close Loop AO. Any resulting focal plane motion was then only due to the re-rotator motion, not internal LUCI1 flexure or possible SX WUnit flexure.

We confirmed the image motion is seen on LUCI1 when the re-rotator is moved and the amplitude of the resulting image motion measured by LUCI1 N30 camera is the same as the ARGOS team measured closed dome rotating the instrument 360 degrees in Closed Loop AO and measuring spot motion with the N3.75 camera.

Details:

In order to test the affect of rotating the re-rotator on the source position on LUCI1 we were required that the AO loop was closed in only Tip/Tilt. If the loop were closed in higher order modes, the re-rotator could only rotate a fraction of a degree before the high order modes measured by the WUnit would be completely misaligned with the corresponding high order modes on the secondary and the AO loop would become unstable. However, even a fairly large angle of misalignment between the WUnit and AdSec is possible and still have a reasonable stable AO Loop Closed and if only Tip/Tilt corrections are applied.

The procedure we used was to manually close the AO loop with all modes to a get good correction in the focal plane. The high order gains, HO1 and HO2, were both set to 0.0 and the Tip/Tilt gain was decreased (1.6->1.2). On the WUnit Hardware GUI the re-rotator angle was moved in steps until the Tip/Tilt loop became unstable. The maximum angle found was 22 degrees, which is 44 degrees in the focal plane. We found that the image quality of the Tip/Tilt only closed loop remained good enough to measure the centroid of the source on LUCI1 to about 1 pixel in the N30 camera for about 5 minutes.

We closed the full (all modes) loop at a nominal rotator position (194 degrees for the re-rotator), rotated -22 degrees in closed T/T loop, collect a LUCI1 image with the N30 camera. Then rotator +4 degrees in T/T close loop and collect another LUCI1 image. Etc. After about 3 measurements the image quality became poor enough that we had to rotator back to the nominal rotator position and close the full AO loop. Rotating and collecting LUCI1 images then continued. In this way we measure the position of the source in the focal plane at 4 degree intervals from +22 to -22 degrees from the starting nominal re-rotator position. See the attached plot sx_re_rotator.jpg

We turned down the ARGOS CalUnit light source to about 11.5 magnitude so that the LUCI1 images were not saturated in FeII. The LUCI1 images collected were taken with exposure time (DIT) of 2.5 seconds and 7 frames (NDIT) (See filenames above). With the dimmer light source we had to run the FLAO AO loop in bin 2, 625 Hz with 153 modes.

Note, the Instrument, LUCI1, and the AGw were not rotating during this test so there is no image motion on LUCI1 due to internal LUCI1 flexure or possible SX AGw/WUnit flexure. This made data collection and "analysis" easy. When a LUCI1 image was collected we used the LUCI Realtime display (Aladdin) to calculate a gaussian centroid. We estimated that this centroid was accurate to about one pixel.

The results, as seen in the attached plot, is that arc of the measured image motion shows that a full rotation of the WUnit re-rotator will cause about a 75 pixel diameter circle on the LUCI1 N30 camera, which is a circle of 1.125 arc second in diameter. This is, within measurement errors, the same amplitude as the ARGOS Team measured using the LUCI1 N3.75 camera.

We suggest three possible solution to this image motion problem (there may be more): 1) Re-align the re-rotator on the WUnit optical bench. This could be very difficult, if not impossible to the level needed, and very time consuming. 2) Use a lookup table to move the bayside stages as the re-rotator moves to compensate for the re-rotator induced motion on LUCI1. This may require too frequent of updates to the bayside stage positions may or may cause jitter in the position of the source in the LUCI1 focal plane. 3) As discussed in the document 687f002, "W Unit Opto-Mechanical Acceptance Test Specifications", page 13, "we could stabilize [the] PSF on the WFS by adjusting the tip tilt mirror". This would require a a lookup table to apply x,y offset voltages to the tip tilt mirror that feeds light to the pyramid WFS.

Comments or Suggestions?

Next we we have daytime closed dome tests scheduled with the ARGOS CalUnit. Our plan is to perform the same tests described above on the DX AO WUnit to see if there is misalignment of the re-rotator and, if there is, measure the wobble.

In addition, we could also measure the offset voltage as a function of re-rotator angle needed to remove the image wobble due to the WUnit re-rotator misalignment.

Alfio, Enrico, Simone,

To offset the center of the tip/tilt mirror motion we can place an offset in x and y to the tip/tilt mirror voltage on the WUnit Hardware GUI: Tip-Tilt page. What is the range of offset voltages that can be applied to the Tip/Tilt mirror?

Doug

Starting NCPA

Rotator 180

Mode Orig. Value New Value
4 13.2  
5 73.0  
6 -28.0  
7 7.9 -5.
8 9.9 -5.
9 -14.0 -10.
10 19.9  
11 -13.2  

Huge vibration at 20:26

Rotator 120

Mode Orig. Value New Value
4 10.1  
5 -40.0  
6 -15.0  
7 -9.9  
8 4.0  
9 7.0  
10 -10.0 -20
11 -7.6  

Rotator 60

Mode Orig. Value New Value
4 -12.0  
5 0.0 15
6 34.0  
7 -7.9 -2
8 7.9 10
9 -14.9  
10 5.0  
11 -4.8  

20:59 Rotator 30

Mode Orig. Value New Value
4 -13.0  
5 0.0  
6 -30.0  
7 -7.9 0
8 9.9  
9 10.0  
10 10.0  
11 -2.0  

21:10 Rotator 90

Mode Orig. Value New Value
4 0.1  
5 -36.0 -30
6 23.0  
7 -10.9  
8 0.0 10
9 -14.9  
10 -14.9  
11 -6.7  

20:20 Rotator 150

Mode Orig. Value New Value
4 16.2  
5 23.0  
6 -58.0  
7 0.0 -10
8 9.9 0
9 14.9  
10 10.0  
11 -10.4  

21:30 Rotator 210

Mode Orig. Value New Value
4 17.4  
5 50.0  
6 30.0  
7 14.9 10
8 -5.9 -10
9 -10.0  
10 -0.0  
11 -9.4  

21:38 Rotator 270

Mode Orig. Value New Value
4 -5.0  
5 -5.0  
6 15.0  
7 15.0  
8 0  
9 14.9 10
10 5.0  
11 -3.8  

21:47 Rotator 330

Mode Orig. Value New Value
4 -10.0  
5 -15.0  
6 0.0  
7 7.9  
8 9.9  
9 -10.0  
10 -10.0  
11 -3.9  

-- %USERSIG{GregoryTaylor - 2016-11-30}%
Topic revision: r9 - 09 Dec 2016, DougMiller
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