TMS Meeting Minutes, April 28, 2022

Attendees: Andrew, Breann, Christian, Heejoo, John, Olga, Trenton, William, Yang (Zoom Meeting)

Pupil Images with Thermal Drift (from Olga's emails):
  • Thermally induced coma on the primary mirror could produce the pupil drift observed
  • Andrew is almost certain that a thermal drift inducing coma will also have an associated tip/tilt on the mirror
  • Though this drift takes place over very long time periods, it is still very important for us to understand this
  • Heejoo is thinking of some sort of non-sequential optimization to attempt to recreate this thermal drift in an optical model
  • Andrew suggests that we should model this, but he also thinks that we should use engineering time to test this
  • Sitting on one field, then keeping the mirrors actively corrected with TMS and driving in temperature differences on the primary with ventilation would be very nice
  • Olga states that there may be some time on May 4th to run this sort of a test
  • Andrew suggests just sitting on a target taking pupil images the whole time with VFAST Extra and then decompose the terms in post; requiring near 1 arcmin seeing
  • From this analysis, we get what mirror thermal does to pointing, and what mirror thermal does to focus, spherical, etc. all from one set of data
  • Olga has the positions of all the pupils from these images in a file, and she suggests comparing one file with the next to determine del theta, del x, del y, then putting it into the mirror reference frame; these changes are of particular interest because Olga has seen these changes historically
  • Andrew suggests plotting the drift in pointing in X and Y, and then alongside this plot the associated change in coma over time; on the order of 10 arcsec RMS error expected
  • John notes that there is mechanical hysteresis on the order of a few arcseconds, not 10's of arcseconds
  • It appears that there is a field-dependent coma which varies across the focal plane
  • There were some strange illumination patterns in certain pupils, particularly the top right pupil in the 2x3 plot from chip 2
  • LBC guiding works by exposure per exposure, meaning that each exposure might have different guiding from the previous
  • These plots do not have guiding turned on
  • This system is not like an off-axis guider that constantly tracks on an off-axis guide star
  • John notes that this telescope mount is constrained by the pointing model, which is expected to be accurate to a few arcsec per hour
  • Christian confirms that, for very slow motion, you won't see much on a few minute exposure
  • The motion of the pupils is around 100 arcseconds, as the pupils are approximately 61 pixels across, or 25 arcsec
  • The drift over time is almost the same across all field points
  • John notes that thermal must be the same for all the pupils
  • We need to find a way to separate the thermal drifting from the pointing drift
  • A thermal pointing change would have dependence on the rotator angle
  • Andrew remarks that we should see some dependence off-axis on the mount tracking, as we have to consider the lever arm effects
  • Olga states that the field got up to around 80 degrees elevation, when we know that near Zenith, there could be more uncertainty in positioning; remarks that it may be useful to go to circumpolar field
  • Near circumpolar points have airmass ~1.5, would could significantly impact seeing, so a higher elevation target was chosen
  • We may need a complex graph like the ones John produces to show all of the effects we wish to analyze here

Three Active Mode Nights:
2022.04.22:
  • Poor seeing on the DIMM
  • Heejoo looked at the data around the time when seeing was worst
  • The group Y, group Rx has very dramatic change from 3 to 5 UT, but FWHM was well above seeing
  • It's possible there was a bad FPIA result at the beginning of the night
  • With the same trend of corrections after this period, the FWHM shot up rapidly and IQ degraded
  • The mode3RX and mode3Y corrections over this 3 to 5 UT period were stepping upwards in a very choppy fashion
  • There seems to be some sort of oscillatory behavior happening over this period of time
  • Christian notes that it could be a case where the field is a bit strange with an odd distribution of stars, where blue is working smoothly but red appears very choppy
  • The same field is observed from 3 - 6 UT, and the TMS Rx and Y corrections where fairly smooth over this period, but the IQ plot may give a bogus result
  • John suggests that it makes a lot of sense to collimate at high elevation with FPIA, then hold this with TMS active while slewing to the observing point at low elevations
  • Andrew suggests that we may consider suggesting this as standard practice, starting the night with collimation at high elevation and moving on from there
  • If we can prove this works, we may be able to say the TMS works better than FPIA at the start of the night
  • Olga notes that the observer's notes state that FPIA had issues with spherical and the first few images were bad and required re-collimation
2022.04.24/2022.04.25:
  • Olga reported 3 image jumps
  • John answered with details, including the fact that we could not see a corresponding jump on TMS for one of the jumps
  • The first and last of the jumps were observed and commanded on TMS, while the middle was not observed on TMS
  • Andrew remarks that it's possible for there to be a PCS error which the TMS interacted with and caused the optics to move
  • John reponds that if TMS had not been running, there would have been no motion commanded to impart this jump
  • TMS imparts a minus correction in groupX and then imparts a plus correction
  • It may be the the Etalon system that caused a fault on both sides
  • Channel 6 was dropped on Dx after this correction, and Channel 28 was dropped on Sx; these channels were recovered after three TMS cycles
  • We do seem to be able to rule out a channel noise problem
  • Andrew plans to process the channel lengths through his Mathematica notebook, though he's not expecting to see any strangeness
  • He proposes working out the probably of a measurement change that could happen through the random chance of errors on channels
Channel Dropping Test 4/29:
  • LBTO all hands meeting at 10 AM
  • Test to begin at 11 AM when the meeting ends and go until 2 PM
  • Andrew requests WhatsApp message when getting ready to start
  • Bo intends to join as well
  • The test will take place on the TMS Zoom channel
  • Yang wonders if, when the channel dropping is enabled, that we want it to be agnostic to whether the channel was dropped manually or via std. dev. filtering
  • At the moment, when we drop a channel, we don't send any pose corrections
  • Now, with the new dropping algorithm, regardless of why we drop a channel, we use the algorithm to continue applying pose corrections with the necessary correction factor, so long as we have the minimum of 6 channels
  • In a certain sense, the new set of channels with the dropped channels excluded becomes the new reference, which is tied to the original reference
  • It is necessary for us to more clearly define our terminology, so that we aren't conflating reference channel lengths with reference pose
  • Yang has added a utility to add and drop channels on the fly during TMS operation, which will be tested tomorrow
  • There should be a smooth drift in pose as we drop or add channels to the Jacobian matrix
  • We should record what our error correction term is, so that we can see how unsmooth our pose corrections would be as we are adjusting the channel set
  • Suggest 1 hour of channel drop testing, including dropping to the point of <6 channels for no solution, then consider changing filter offsets and such while checking for smooth performance
TMS Opportunity:
  • There is a possiblity to run TMS this weekend during OSURC observations, as they have a lot of LBC operations
  • One of the reasons Olga wanted to look into the jumps was b/c she wants to offer to run TMS in active mode during these observations
  • John and Christian are in agreement that you still win while running TMS during the night, with only one jump imparted by the TMS + PCS argument
  • Olga considering proving out the capability of TMS to hold collimation well over big elevation changes
  • Christian remarks that its worth checking things closely with FPIA at the beginning of the night and then holding onto that with TMS
-- TrentonBrendel - 28 Apr 2022
Topic revision: r1 - 29 Apr 2022, TrentonBrendel
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