UNIVERSITY OF CALIFORNIA OBSERVATORIES / LICK OBSERVATORY


LGS Alignment Procedures

AO LGS Optical Alignment
NGS to LGS on Sky Alignment
Moving to a new LGS science target
Tracking alignment during an LGS exposure
Switching from LGS to NGS mode
Afternoon LGS Alignment Check

AO LGS Optical Alignment

AO Alignment for LGS Mode (Assumes currently reasonably aligned for NGS mode)

  1. Install 600nm dichroic instead of mirror in 2nd dichroic mount.
  2. Put in alignment laser (remove initial turning mirror if it is installed).
  3. Align through all pinholes with alignment laser.

    NOTE: The following 2 steps don't need to be done when mounting AO on the telescope unless you suspect something has drastically changed since the alignment was done in the lab.

  4. Coarse align to IRCAL (hit target on IRCAL dark cover). Note that this should not be necessary if the NGS alignment was good.
  5. Measure where alignment laser lands in IRCAL field. The alignment laser is a large spot on IRCAL, so record center of spot as best you can.
  6. Put in red light point source.
  7. If doing a full alignment in the lab:
    Move FiberStage X and Y so that the red light point source is centered on the same spot as the alignment laser. Aligning within a few pixels is sufficient.
  8. If doing initial optical alignment on the telescope:
    Move FiberStage X and Y so that the red light point source is centered on IRCAL (pixel 128,128). There is a script on ircalbox called rlsPosition.tcl that automates most of this procedure.
  9. Align into slow WFS.
    1. Take exposure with slow WFS.
    2. Move TT cube to adjust centering (or the small turning mirror in front of the slow WFS).

      TTCube Y-axis: -counts moves spots right, +counts moves spots left
      TTCube X-axis: +counts moves spots up, - counts moves spots down typical moves are a few thousand to a few hundred

  10. Put into LGS mode by clicking LGS button in CentDiag GUI.
  11. Align APD TT sensor - Back Propagation. This is done in the lab only, not on the telescope unless something bad has happened to the APD TT sensor (e.g. gotten bumped hard while mounting the AO system).
    1. Turn off red light source.
    2. Install the Oriel white light source and turn brightness all the way up.
    3. Connect fiber from white light source to APD fiber #1 (back propagation).
    4. Put paper target on DM.
    5. Examine centering of back propagation on DM target.
    6. Do steps c-e with the other 3 fibers.
    7. Determine if majority of fibers have the same centering, if so, move the small beam splitter cube to get good centering for that set of fibers. Note that usually the beam splitter cube does not have to be moved and you should investigate other likely causes for the misalignment before moving the cube.
    8. Use small knurled knobs to adjust centering for each APD fiber that doesn't have good centering in the previous step.
    9. Now that all four APD fibers are coaligned, check to see if back propagation hits the face of the red point source fiber (or white light source pinhole if it is at the cassegrain focus).
    10. Steer APD TT stage so that light hits the red point source fiber. Note that the stage is rarely out of alignment and you should only steer it if absolutely necessary.
    11. Recheck back propagation centering for each fiber. If they are all off, use small beam splitter cube to recenter (note: only move the small cube if you are certain that is what should be moved. In most cases you should not have to touch this optic.).
    12. Repeat previous two steps until satisfied with alignment.
  12. Check centering on slow WFS. If not properly centered, fix and repeat previouis step.
  13. Done in lab only, not on telescope. Align into IRCAL (pointing and pupil alignment).(see procedure elsewhere)
  14. Done on telescope initial alignment. Align into IRCAL. Use rlsPosition script in ircalui software to position RLS to center of IRCAL.
  15. Boresight APDs to IRCAL.
    1. See if light hits APDs, if not, use CentDiag TTImage to find it. If all else fails, go back to steps 10 j-k.
    2. Close APD TT loop.
    3. See where light hits IRCAL.
    4. If in lab: Steer APD TT stage (using micrometers) so that red light source is well pointed on the WFS and hits right spot on IRCAL.
    5. If on telescope: Use APD P&C GUI to adjust pointing of APDs so that spot is at pixel 128,128 on IRCAL.
    6. Check centering on Slow WFS camera.
  16. Boresight WFS to APDs and IRCAL by fieldsteering (Fast WFS P&C Control) red light source on WFS. Tip and Tilt readings on CentDiag should be as close to zero as possible (less than +/- 0.1). Centering should have outer subapertures as evenly illuminated as possible.
  17. Change software back to NGS mode.
    1. Turn off APDs
    2. Click NGS button in CentDiag GUI.
  18. Put in White Light Source (WLS).
  19. Focus IRCAL. (see procedure elsewhere)
  20. Image Sharpen IRCAL. (see procedure elsewhere).

NGS to LGS on Sky Alignment

This procedure describes the various necessary alignment step for switching to LGS mode after observing in NGS mode the first part of the night.

This has been updated for the new Renishaw encoded mounts (2004 May 9).

  1. Move telescope to either the N or S zenith boresite position.
  2. Switch from Mirror to 2nd Dichroic. Make sure that the alignment pinhole behind the 2nd Dichroic is completely open.
  3. Field Steer WFS and APDs to zero.
  4. Change software to LGS mode (both AO and Telco software)
    1. Change to AO w/ APD guide matrix in Telco.
    2. Push LGS button in CentDiag window.
  5. Change to LGS control matrix (typically we use CMSep03_00wls3 matrix, though if conditions are poor, use the CMSep03_00laser, laserwls, or dimlaer matrices). There will be different hartmannModes for LGS mode, so those should be changed over at this point as well.
  6. Make sure radar is on and visual observers are ready.
  7. Propagate laser.
  8. Focus telescope on laser guide star.
  9. Focus laser spot using laser launch telescope optics. The goal is to get the smallest, roundest laser spot possible.
  10. Have laser operator steer spot to the boresight position on the guide TV.
  11. Send laser light to WFS (make sure WFS is still at NGS focus position!) and have laser operator steer spot to center of WFS. You may have to open iris to 30000 counts or so to give a wider field of view for finding the laser guide star.
  12. Close iris to about 5000 counts (depending on conditions you may have to have a smaller or larger iris setting).
  13. If necessary, steer iris to clip Rayleigh scattered light. Often the iris doesn't need to be steered at all, or sometimes the iris is already in the correct position if the laser guide star has been used on previous nights. Typical iris position is 1 arcsec North. Once set it should not have to move again for the rest of the night (or run for that matter, unless something bad happens to the system's alignment).
  14. Tune laser off wavelength. Take a cflat on the WFS.
  15. Tune back on wavelength.
  16. Close uplink TT. Confirm that it is working properly. You may want to tune the TT gain (often we run at a TT gain of 0.6, though we also have run with a gain up to 1.0, if conditions warrant it).
  17. Close AO. Confirm that it is working properly. You may want to tune the AO gain for best performance. Usually the best gain is about 0.1 or 0.2 less than the NGS AO gain for a bright star.
  18. Open loops and shutter laser.
  19. Move telescope to a 9th magnitude star near (closer is better) the first science target (make sure it is cleared by space command). We use this bright star to establish a boresight position for this region of the sky, as well as establish a reference BWFS image for use the rest of the night. This star should be bright enough to easily see on the WFS to assist finding the boresight position for the APDs.
  20. Close APD TT loop on the star (you may have to use a some ND in the APD TT sensor). If this is the first night of an LGS run, check to make sure telescope offloading is working correctly.
  21. Refocus telescope by moving secondary mirror position until WFS focus reads within +- 1.0. You may have to adjust WFS P&C so that you are well aligned to get a good focus measurement off the WFS. Note that you will probably have to take a new cflat on the WFS to get a good focus measurement on the 9th magnitude star.
  22. Close AO loop and check strehl (use the Ks filter in IRCAL and a few second exposure). This strehl is likely to be less than in NGS mode because of using the laser control matrix and using APD TT and AO instead of regular NGS mode.
  23. Tune APD TT gain. A typical value is 0.005 (values usually range between 0.01 to 0.001).
  24. Take a reference BWFS image. Typical exposure is 10 or 30 seconds.
  25. Open loops, mark boresight position on guide TV (bin by 1 mode).
  26. Move WFS back to LGS focus using airmass lookup table (using LGSairmass progroam).
  27. Propagate laser and boresight on guide TV.
  28. Send light to WFS, close APD TT loop on TT star, and steer laser so it is centered. Use the narrowband sodium filter to block light from the star if necessary. If the sodium filter is not installed, you may have to field steer the star off the WFS (typical move is 5 arcsec N and 5 arcsec E).
  29. Move iris to clip Rayleigh scatter, if necessasry. This usually isn't necessary because the iris position set a zenith should still apply.
  30. Tune off wavelength and take a new WFS cflat.
  31. Tune back on wavelength.
  32. Close all loops, adjust WFS Centering, and [optional] check strehl on IRCAL.
  33. Use BWFS measurements to adjust focus position of the WFS.
  34. Check strehl on IRCAL.
  35. Open all loops.

    The following few steps are optional but particularly recommended for the first night of an LGS run.

  36. [Optional]Move to an 11-13th magnitude star near the science target. This star is used to check strehl and do on-sky image sharpening. This is difficult to do with the 9th magnitude star because you have to field steer too far off-axis to get the star off the WFS.
  37. [Optional]Close APD TT loop on 11th mag star. You may have to field steer a few arcseconds off-axis to get the 11th mag star off the WFS.
  38. [Optional]Propagate laser.
  39. [Optional]Do on-sky image sharpening. Typical exposure times are 10 to 20 seconds in the Ks filter. First step is to modify mode 3, typically setting it to zero. Often that is all the image sharpening required.
  40. [Optional]Open loops.
  41. Move telescope to science target guide star.
  42. Close TT loop on guide star.
  43. Propagate laser, close uplink TT and AO.
  44. Field steer to science target. Remember to open loops before field steering and to close the APD TT loop promptly after the field steer is completed. If the TT star is faint, you may have to TTImage in CentDiag to locate it.
  45. Close uplink TT and AO. Adjust WFS centering as necessary.
  46. Take science data.

Moving to a new LGS science target

  1. Field steer back to zero arcsecond offsets and centering
  2. Move telescope to 9th to 11th magnitude star near science target (this does need to be a cleared target by Space Command).
  3. Close APD TT loop. Allow telescope to offload.
  4. Move WFS to proper position for current airmass.
  5. Open APD TT loop and mark position of star on guide TV.
  6. Propagate laser and steer to same position as the star.
  7. Put laser on WFS (you will probably have to field steer the TT star 5 arcsec N and E to get it off the WFS).
  8. Have the laser operator steer the laser on to the WFS.
  9. Close all loops. Adjust gains/rates, get new cflat, etc. as necessary.
  10. Take BWFS image to check WFS focus position.
  11. [Optional] Take image on IRCAL to check strehl.
  12. Open all loops.
  13. Field steer APD TT star back on axis.
  14. Shutter laser. Move telescope to science target TT star.
  15. Make sure you see TT star on APDs.
  16. Open APD loop. Field steer to science target.
  17. Close APD TT loop.
  18. Propagate laser.
  19. Close loops.
  20. Take science data.

Tracking alignment during an LGS exposure

Periodically, both WFS pointing and centering need to be checked, as well as the WFS focus position. In practice, I adjust the WFS focus position when the airmass changes by 0.02 to 0.05 (depending on airmass). There is an available chart that shows how quickly the laser moves a depth of focus as airmass changes. WFS pointing and centering should be checked and adjusted between (but not during) exposures. The frequency of these corrections depends upon the frequency of dithers as well as the laser brightness and Rayleigh scatter.

Switching from LGS to NGS mode

  1. Switch back to NGS mode software.
  2. Load in NGS centroids, control matrix and hartmannModes.
  3. Move WFS to NGS focus.
  4. Close loops on white light source.
  5. Focus spot on IRCAL by moving WFSFocus stage (not IRCamFocus!) (either do a regular focus fit run or use BWFS - 1st choice seems to be more accurate)
  6. Go to science target guide star and observe!

Daily Afternoon LGS Alignment Check

Every afternoon some basic alignment should be checked and tweeked so that we get the best performance possible out of the AO system.
  1. Exit ircalui, archive data using the ircalcopy script, and restart ircalui.
  2. Check read noise level on IRCAL.
    • Move IRCAL filters to the dark position.
    • Take 2 exposures of 1000ms x 1 coadd x 16 reads, and subtract them from each other. Standard deviation should be < 3DN.
  3. Turn on red laser source (RLS).
  4. Turn on 100V DM power and 24V Motor power.
  5. Set RLS position.
    • Take a short exposure (1000ms is typical) with IRCAL (all filters in the Open position) and make sure RLS is in the central region of the chip. If it isn't, move Fiber X and Y stages so that it is.
    • Use rlsPosition script in ircalui to place RLS at the center of the array.
    • Call Fiber X and Y positions zero and Go to zero (motor stops servoing when you call the position zero, so you need to move the motors to zero to get them to servo at that position).
  6. Switch to LGS mode in CentDiag GUI.
  7. Turn on APDs and water pump. It takes about a minute for the APDs to fully turn on and be completely functional (you may see one or more APDs give zero counts a couple times in this minute, that is normal). Put in an ND filter if the RLS is too bright.
  8. Close the APD TT loop on the RLS (If the RLS is not hitting the APDs, use TTImage and the APD field steer motors to get the RLS on the APDs so you can close the APD loop).
  9. Take a short exposure on IRCAL and measure pixel position of RLS.
  10. Adjust APD Pointing (with APD loop closed) so that the RLS hits the center of the IRCAL chip (pixel 128,128) within +/- 1 pixel.
  11. Move WFS to the nominal NGS focus position if it isn't already there. (NGS focus is 10,000 stepper motor counts from the reverse limit, call this position zero).
  12. Make sure that you have a current cflat and NGS control matrix for the WFS.
  13. Adjust WFS Pointing and Centering (P&C) until RLS well aligned on WFS (this step is commonly called boresighting the WFS). WFS Tip and Tilt readings should be very small, e.g. +-0.1 or less if possible, with APD TT closed.
  14. Close AO loop and check Back WFS centering. If centering is way off, try to determine what has moved before adjusting anything (e.g. has just the TT Cube or 1st Dichroic position not close to zero, or has the iris behind the 2nd Dichroic gotten partially closed). If you can find no other source of the problem, you can adjust the APD Centering. After adjusting APD Centering, you should back propagate to make sure that you are still properly centered with respect to the APDs. If back propagation shows that everything is off, think long and hard about causes of the misalignment before changing anything that isn't a motorized mount. Odds are the alignment is a motorized mount problem not a manually steered optic. If you decide to move the small beam splitter cube or an individual fiber, be prepared to do all steps of LGS alignment involving the APD TT stage in the main alignment procedure above. This is time consuming and difficult and to be avoided as much as possible during an LGS AO run.
  15. If Back WFS centering is acceptable, call the positions of all the field steer motors zero, go to zero for all field steer motors, and reset all the cumulative moves in the WFS and APD P&C windows.
  16. Switch back to NGS TT mode:
    1. Turn off APDs.
    2. Push NGS button in CentDiag GUI.
  17. Put in White Light Source (WLS). Adjust position of WLS using Fiber X and Y motors so that tip and tilt are < +/- 0.1.
  18. Close loops and check strehl on IRCAL with WLS. Typically this should be done at 500Hz, AO gain of 0.5, and TT gain of 0.5 (or 0.3). Typical exposure time of 100ms with 5 coadds using the Br-Gamma filter in IRCAL.
  19. Focus and Image sharpen IRCAL. This can be done using the imagesharpen2 script in ircalui.

Elinor Gates
Last modified: Tue Jun 14 11:55:49 PDT 2005