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.
Note: Steps in blue are
for the new renishaw
encoders. Steps in green are for the
old unencoded mounts.
- Move telescope to either the N or S zenith boresite position.
- Switch from Mirror to 2nd Dichroic. Make sure that the alignment
pinhole behind the 2nd Dichroic is completely open.
- Move 1st Dichroic, 2nd Dichroic, WFSM and TTCube to
zero. Reset all Cums to Zero.
- [Optional] Close loops (NGS mode) on
white light source and check strehl.
- Change software to LGS mode (both AO and Telco software)
- Change to AO w/ APD guide matrix in Telco.
- Push LGS button in PowerUp Checklist window.
!!!Don't do the next two steps w/new lgs7 APD code: leave both in NGS mode!!!
- Turn off TT Offload relay.
- Turn off T/T Drive relay.
- [Change lgs3 software to configlgs.] As of 6/03, enter "gsmode 1" into CentDiag entry box.
- Start APD Tip/Tilt GUI from AO Loops menu.
- Put in red laser source (RLS).
- Turn on APDs.
- Make sure APD TT sensor sees the RLS. If you can't find the
source, remove all ND and use the mover command to search for it.
- Close APD TT loop. Note the Mirror Position and set mover to those
values.
- Take short exposure on IRCAL to be sure the source is near the
center of the image (within a few arcseconds). Often the source will
be a few arcseconds south if you are in the 5 degree south of zenith
telescope position, this is OK. If it is not in the right place, open
the TT loop, put the mirror in E-Null and see where the RLS is. If it
is still in the wrong place, move the fiber stage to correct pointing.
If it is centered in E-Null mode, then close TT and field steer the
APDs to put the source in the right place. Note that you shouldn't
have to field steer the APD if you properly did the LGS afternoon
alignment.
- Adjust WFS Pointing and Centering (P&C) while APD TT loop is closed
until properly pointed and centered. You will probably need to iterate
between pointing and centering positions until tip and tilt are within
+- 0.05 and the pattern is evenly illuminated.
- Call all motor positions zero for the Dichroics, TT Cube and WFSM. Reset
all the cummulative values in the APD P&C and WFS P&C.
- Open APD TT loop. Move RLS out of the way.
The following two steps are optional. In most
cases the white light source is too bright to close the TT loop on, even
with the ND2 in front of the APD TT sensor.
- [Optional] Put in the white light source (WLS).
- [Optional] Check internal strehl with APD
TT and AO loops closed. Should be comparable or slightly less than the
internal strehl in NGS mode.
- Change to LGS control matrix. Occasionally there will be different
reference centroids and hartmannModes for LGS mode, so those should be
changed over at this point as well.
- Make sure radar is on and visual observers are ready.
- Propagate laser.
- [Optional]Record NGS telescope focus.
- Focus telescope on laser guide star.
- [Optional]Record LGS telescope focus.
- Focus laser spot using laser launch telescope optics. Make use
of the slic video frame grabber program to measure FWHM of the laser spot
during this procedure. The goal is to get the smallest, roundest laser spot
possible.
- Have laser operator steer spot to the boresight position on the guide TV.
- 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.
- Close iris to 5000 or 6000 counts.
- Steer iris to clip Rayleigh scattered light. Often 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).
- Tune laser off wavelength. Take a cflat on the WFS.
- Tune back on wavelength.
- 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 0.5, though we also have run
at 1.0).
- 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
0.2 less than the NGS AO gain for a bright star.
- Open loops and shutter laser.
- Move telescope to a 9th magnitude SAO star within 1.5 degrees (closer is
better) of the first science target. 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.
- 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.
- 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.
- 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.
- Tune APD TT gain. This step may not be necessary, test show that
TT gain doesn't usually have a huge effect on strehl and a typical
value is 0.001 (values usually range between 0.01 to 0.0005).
- Take a reference BWFS image. Typical exposure is 10 or 30 seconds.
- Mover to where APD TT wants mirror to be after telescope offloading is
done.
- Open loops, mark boresight position on guide TV (bin by 1 mode). Record
boresight pixel postion using Slic and enter into Autoguider Excel spreadsheet
on lgs13.
- Move WFS back to LGS focus using airmass lookup table (someday this will
be on-line, but currently is a printout).
- Propagate laser and boresight on guide TV.
- Send light to WFS, close APD TT loop on TT star,
and steer laser so centered. You will probably have to field steer the 9th
magnitude star off the WFS (typical move is 5 arcsec N and 5 arcsec) so that
it isn't visible on the WFS.
- Move iris to clip Rayleigh scatter, if necessasry. This usually isn't
necessary because the iris position set a zenith should still apply.
- Tune off wavelength and take a new WFS cflat.
- Tune back on wavelength.
- Close all loops, adjust WFS Centering, and check strehl on IRCAL.
- Use BWFS measurements to adjust focus position of the WFS.
- Check strehl on IRCAL.
- Open all loops.
The following few steps are optional but highly recommended.
- [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.
- [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.
- [Optional]Propagate laser.
- [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.
- [Optional]Open loops.
- Move telescope to science target guide star.
- Close TT loop on guide star.
- Propagate laser, close uplink TT and AO.
- 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 use
the Autoguider Excel program to position the TT star on the proper pixel
to get it into the APDs.
- Close uplink TT and AO. Adjust WFS centering as necessary.
- Take science data.