Field Aberration measurements with AGw and IRTC



Two separate tests for field aberration measurement described below


*Prerequisites: stable telescope, good seeing, with active optics routinely converging to ~ 200 nm.

Test 1

In the PSF Left GUI open the "arbitrator" and select "send corrections to primary" (default is "split")

Copy field data table below into current night's commissioning wiki notes.

Persson's Star = BS9152
Start time = 12:15

Guide Star wfscimage000*
0 48-50
17 51-53
0 54-55
18 56-58
0 59-63
7 64-66
0 67-68
End time = 1:15

Set up IDL for active optics following instructions in

Details for collecting field aberration data

(note: % is shell prompt IDL> is IDL prompt and IRTC> is IRAF prompt):

Login to workstation as LBTO

  • % cd /Repository/AGW_Data
  • % mkdir 100328_Field UT Date
  • % chmod ugo+rwx 100328 Allow anyone to read, write, execute
  • % cd 100328
  • % idl Start IDL

Select suitable field and preset in guide mode to GS0 (BS9152 or later in the night BS 9177)

Preset the telescope to the on-axis star in ACTIVE mode and allow GCS to collimate the telescope so that the reported Z4-Z11 rms aberration is less than 300 nm.

With stable guiding on GS 0 (and make sure pointto is sent with Telmode="GUIDE") , type "agw_collimate", and at the prompt select "yes" to send Zernikes to primary.

Repeat until reported Z4-Z11 rms aberration is less than 300 nm.

Record the guide star number (0 in this case) and the range of wfsc file numbers acquired via the agw_collimate command to get the total rms error below 300 nm in the table.

Once a measurement returns with acceptable wavefront immediately preset to first off-axis guide star, with TelMode = "Guide" and PA set to "INDEF".

Time is off the essence here, we want to minimize the gap between collimation on-axis and measurement off-axis.

While preset is acquiring the off-axis guide star type "agw_wfsc_multiple, 3" in IDL terminal and as soon as guiding has locked, enter the command.

Record the guide star number and the range of wfsc file numbers for the three off-axis image collect via the agw_wfsc_multiple command in the table.

Have a preset ready in IRAF so that as soon as the third measurement is made and IDL starts processing the image, preset to GS 0 with the PA set to that of the off-axis star just measured in the previous step.

Again, this preset is sent in "GUIDE" mode.

If at any point between the last iteration of initial collimation on-axis, taking the three measurements off-axis, and getting the first collimation back on axis, there is any delay lasting more than a minute or so, discard that measurement and start again.

Repeat for a number of stars at different field angles, a good data set is 15+ measurements (20 is plenty).

If a set of data is thus acquired, email Doug Miller and ask him to process the data into an ASCII file for Andrew.

Field data collection example

1) Go to on-axis star and collimate

  • IRTC> pointto BS9152 TelMode="ACTIVE" gstar=0 PA=INDEF

One the rms wfe is < 300 nm (displayed on GCS GUI) then it's time to start collecting data.

  • IRTC> pointto BS9152 TelMode="GUIDE" gstar=0 PA=INDEF

  • IDL> agw_collimate (default 30 second exposure for wfsc) When done, four (three) plots will be displayed on upper left window. Check the WFSC image looks "OK". Window asking "Apply to primary?" will appear on upper left window. "Yes" sends zernikes to PSF. "No" makes no corrections.

  • IDL> agw_collimate "Yes" apply...

Collimate 2-5 times until all corrections sent to primary (last list of output zernikes 2-11) are 200 or below. Sometimes focus is jumpy, so 300 or below is OK.

***Record in log=> gstar:0, wfsc:4-10 (for each iteration data is saved in wfscimage00003.fits, *3_dd.fits etc) only need to record number, eg 3

2) Measure off-axis field aberrations for guide star 17

  • IRTC> pointto BS9152 TelMode="GUIDE" gstar=17 PA=INDEF

  • IDL> agw_wfsc_multiple, 3 (acquires three WFSC images, saves and process, but does not send corrections to primary; eg no collimation)

***Record in log=> gstar:17, wfsc:11-13 (Remember to look at PA output by IRAF pointto routine)

3) Collimate on-axis

  • IRTC> pointto BS9152 TelMode="GUIDE" gstar=0 PA=124.77 (PA is set to last PA so rotator doesn't have to move; quicker)

  • IDL> agw_collimate "Yes" (one or two collimates is usually enough except at low and high elevations)

***Record in log=> gstar:0, wfsc:14-15

4) Measure off-axis field aberrations for guide star 18

  • IRTC> pointto BS9152 TelMode="GUIDE" gstar=18 PA=INDEF

  • IDL> agw_wfsc_multiple, 3

***Record in log=> gstar:18, wfsc:16-18

5) Collimate on-axis

  • IRTC> pointto BS9152 TelMode="GUIDE" gstar=0 PA=65.44

  • IDL> agw_collimate (1 or 2 times, "Yes" )

***Record in log=> gstar:0, wfsc:19-20 etc, etc, etc.....

Test 2

Another test of collimation that will be useful to do while the IRTC is mounted is as follows:

In a suitable field, as above, or M67 say, send "ACTIVE" presets in to a variety of stars, sampling many different field angles and aiming for stars >4' off-axis.

Note that GCS is collimating here, not IDL.

For each preset, when GCS is reporting stable collimation, take a cube of data on IRTC, arranging exposure time and sequence number such that we get ~ 30s or more of images to integrate. For example, at an IRAF prompt

  • IRTC> takeseq 1.0 30

where 1.0 is the exposure time in seconds and 30 is the number of frames. Note, if the IRTC Gui is set to have continuous exposures, an acquisition will fail. Click the button with an image of a camera on the IRTC Gui to stop the continuous acquisition. You can see the progress of the cube acquisition on the bottom display bar of the IRTC Gui.

Record Guide star number, AGw stage coordinates associated with each IRTC cube.

Arrange presets such that for each guide star the first preset is sent with PA="INDEF" and an IRTC cube taken once collimation is stable.

For an immediately subsequent preset on the same off-axis GS, send PA = the PA of the previous measurement + 20 degrees, and re-collimate, take another measurement.

For the next preset repeat but this time subtract 20 degrees from the original PA.

In this way, for each off-axis guide star we get three IRTC cubes, one taken when collimation has converged with the x axis of the AGw stage near zero, the second with a large x value of one sign, and the last with a large x value of the opposite sign.

Repeat for approximately 30 measurements (10 guide stars 3 measuremetns on each one). Try to get a good sampling around the field. If running on BS9152 follow the order suggested above, by the time the cycle repeats on GS17 the rotator angle will have changed by >30 degrees).

-- AndrewRakich - 29 Apr 2009

-- DougMiller - 30 Apr 2009
Topic revision: r12 - 21 Sep 2010, JohnHill
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