TMS Meeting Minutes, November 5, 2020

Attendees: Andrew, Breann, Heejoo, John, Trenton, Matthieu, Yang (Zoom Meeting)

Meeting Notes:

  • America is a soap opera - Andrew
  • Andrew looked into the astigmatism problem
  • Ch3 length vs time: lengths changing by ~1 um while 5000 nm astig commanded
  • Andrew calculated correct clocking angle for all collimators on mirror perimeter, and aims to fit a sinusoid to the length change, which should relate directly to a change in Z
  • Should see Z5 or Z6 with sinusoid fitting (45 deg. or 0 deg.)
  • Used projection with unit vec along Z multiplied by the change predicted to compute expected change in ch. length
  • Reminder: coefficients are in wavefront RMS (for astig, factor of about root 5 to go from RMS to PV)
  • One thing that became clear, Z changes when spherical aberration is driven (not because of shape, but because of R^2 equiv piston)
  • Heejoo to dig up PSF data and upload to Google Drive for Andrew's point of comparison with his fitted values
  • Driving in 10 um, +/- 5000 nm, the data doesn't seem to match up
  • Andrew not too sure about difference in deltas, but Z5/Z6 where applied and then un-applied, but we see some substantial deviation from expected
  • Observed 5 um chop, likely caused by shape changes, though we only anticipate small shape changes in this dimension
  • Andrew feels the data shows reliability to the 1-2 um level, not the 100-200 nm level (coarse correction)
  • Note, collimator position changes along the direction of the surface normal on an Invar plate

  • SPIE deadline shifted to the 30th
  • Andrew adding structure to the paper by next meeting

  • Largest uncertainty for GMT mirror is the compensation for shape and positioning on the test setup
  • If you obtain an absolute measurement of shape and pair this with the positioning measurement from TMS, improvement by factor of 10

  • Heejoo had issues loading Yang's correction data
  • Heejoo requests individual channel length change with timestamps
  • tms_raw_... is the raw pose output from John's script

  • 3:30-4:30 PM, Yang noted 132 corrections sent from TMS for dX, dY, dZ
  • If no anomalies, as you continue dropping channels while time progresses, correction starts to become unstable
  • Yang wonders about the case were we start with a 9 channel reference if we could just discard one channel from the reference
  • Also wondering if we could start with 12 channels and treat 9 channels as the dropped case
  • In a perfect world, 6, 7, 8, 9 ch. measurements should all provide the same correction (min. 6 ch.)
  • However, this is not what we observe; noise goes as 1/root(N)
  • In the real system, we see that the system becomes unstable over time with 7 ch.

  • Upcoming snowstorm means we may have some closed-dome time to run a two hour test with ch. dropping
  • Yang suggests a second test with dropping 1-2 ch. over time, Andrew suggests ch. 11, ch. 23 (dropping 10%, 2%, respectively)

  • Trenton and Heejoo working on ordering the EO collimators
  • Confirmed that the best model for a retro is a BK7 1 inch aperture
  • For LBT, designed universal pointability of collimators, increasing range while reducing stability
  • GMT is using fixed-pointing mounts, with fine tip-tilt adjustments
  • Andrew suggests that more stability could be introduced at LBT over the current cantilever method

  • Andrew presented solid-Maksutov (object at inf) / solid-Offner (object at pupil) wide-angle retro
  • Critical tolerance at the interal focus on the buried surface
  • Able to use a radius bench with return from both the rear and internal surface to dial the rear surface radius in to within a micron
  • However, not yet practical for our setup because of only 14% return until we get stronger lasers for the eTalon

-- TrentonBrendel - 19 Nov 2020
Topic revision: r4 - 04 Jan 2021, TrentonBrendel
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