TMS Meeting Minutes, October 15, 2020

Attendees: Andrew, Breann, Heejoo, John, Trenton, Yang (Zoom Meeting)
  • Laser safety reminder
  • All lasers are considered eye-safe (both visible and telecom)
  • Bending modes discussion
  • PSF does a good job correction X translation
  • TMS seems to be overshooting in Y

  • On one hand, moving mirror around on actuators
  • On the other hand, moving the mirror around on a steel structure, where changes are not picked up by PSF
  • TMS should help correct some of these unsensed structural perturbations
  • W/ Y Translation, there is a similar behavior in Rx (Rx about 5X more motion than Ry)
  • Collimation model variation w/ elevation is seen in Y offset + Rx offset, which is corrected with software
  • Need collimation model calculation with change in Y + change in Rx which should be subtracted out

  • PSF already compensates the LUT for the collimation model, but the TMS does NOT incorporate the LUT, yet
  • Andrew suggests using the collimation model with TMS, as well as PSF
  • Blue line is PSF + gravity flexure

  • TMS sees the change that the collimation model was built to correct, but doesn't correct it
  • John: "PSF data is the collimation model."

  • As the hub rotates, we would immediately run out of range in Y as elevation changes
  • Instead, we use a LUT in X and Y to rotate about a coma neutral point, providing for best collimation at all elevation

  • Over range of elevation, the collimation model is correcting something like 5 mm of Y change

  • SEE JOHN'S EMAIL FOR COLLIMATION MODEL SPECIFICS
  • DXPMLBCCollimation.zip

  • Yang's code is not currently applying the LUT Collimation Model to the TMS data readout
  • Code has some comments about applying the correction
  • "could eliminate correction by moving correction from collimation model to the telescope pointing module"

  • We need to accommodate the commanded change from the LUT correction
  • The correction algorithm DOES appear to be applying the LUT correction after the measurement

  • The big up and downs at 4:40 and 5:05 are where the jumps happen (100 um spikes, plus and minus in Y)
  • As we apply SA correction, we change the goal posts, and we aren't fully accommodating the change
  • John reminds us that the SA correction should make a step, not a spike, as observed

  • Between 4:40 and 5:00, during the three superfoc series (see ladder in Z translation), is where the Y spikes are observed
  • No manual inputs were applied on PSF, though these could be at different positions on LBC, but this is irrelevant
  • Seems like a linear increment per superfoc, but it's too big to be thermal

  • This might be caused by moving the mirror on the hardpoints while running TMS
  • As w/ Rx, Ry, we have a Z change of elevation with the LUT
  • We are seeing a difference between the LUT and the uncompensated, raw measurement

  • TMS code only has to take care of pointing problem from running out range for correction
  • From last week, Heejoo showed the different channels changing size at different rates
  • This week's data might explain some of the variable changes in channel length
  • Andrew suggests that this is mostly attributable to not applying the LUT offsets to the TMS data

  • The hub is not only changing focus position, but is also rotating
  • This causes the different legs to change by different amounts
  • Good exercise to apply the LUT offsets to TMS data to see the effect

  • Also, may check the health of the LUT collimation model

  • Yang has been working non-stop for Al's OSCO project, and has not yet had time to finish the TMS software update
  • Has time tomorrow, 10/16 to work on TMS

  • Based on Andrew's testing, the algorithm we have come up with seems to work well in Mathematica
  • Yang set to implement the algorithm and set up a test to see if it works well

  • Andrew working on SPIE paper for TMS to present in December at Astronomical Telescopes and Instrumentation
  • "A laser-truss-based optical alignment system on LBT"
  • Goal is to develop a paper where someone starting from scratch could have a good start

  • Recall, the system works by contrast from usable return from the fiber tip and the return from the retro
  • eTalon is developing a new generation of hardware with a more powerful laser and Maksutov-type retro allowing for 10 mm beam walk
  • New Maksutov-type retro has low return, so won't work unless used with a more powerful laser
  • Andrew suggests talking with Mark at eTalon about this issue to talk technical details

-- TrentonBrendel - 16 Oct 2020
Topic revision: r1 - 16 Oct 2020, TrentonBrendel
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