TMS Meeting Minutes, November 29, 2020

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

Meeting Notes:

  • There was great interest in the TMS system at the Wyant College Industrial Affiliates event this past week
  • Many inquiries about the eTalon system and how we are leveraging it on the LBT
  • There is a growing interest in the optics community in laser-truss metrology

  • Heejoo gave a short update on the astigmatism test
  • Telescope at zenith, with astigmatism induced
  • No motion commanded on the mirror, while TMS observed some mirror motion
  • With +/- 5 um astigmatism commanded, 5000 nm Z11
  • Andrew suggests assigning an azimuth value to each channel based on the rotation of the collimator on the primary
  • Then work out predicted delta Z from the driven aberration, and proceed to work out the predicted channel length change from the delta Z projected onto channel length direction
  • Then plot the changes of length for each azimuth point; this will help to determine how well we can measure astigmatism and spherical aberration
  • Can we measure the shape change reliably? Can we distinguish deviations from rigid body motion?
  • Need to establish the level of accuracy with which we can extract shape change

  • TMS test planned for tomorrow
  • Primarily intended to test dropping of various different channels
  • All of the schemes we have tried come down to the choice of a vector, where, if you don't have enough info, you look back in history to find a vector with enough info
  • The big question is whether this method will give stable, reliable results
  • Yang suggests that we allow a missing channel to persist for a while and see how the system performs
  • Also, there are cases where the missing channel occurs after you move the mirror (by more than gravity influence)
  • Yang expects it to work well for most cases, as you are able to look back a few steps to fill in the gaps in info
  • Andrew suggests that we should consider what else might give changes over time
  • One such effect could be the imperfect measurement of OPD, introducing a geometric scaling, caused by changes in temp, humidity, etc.
  • At the moment, if we have an 8 ch vector and then a 9 ch vector appears after running FPIA, we don't necessarily have a good way of dealing with the jump
  • Andrew proposes that we should come up with reasonable contingencies for the less-likely cases
  • Combo of missing analytical understanding and some errors in the model
  • Yang: Is there a way to distinguish a true correction from an impossible correction?
  • The need for a correction vector when we drop a channel makes sense
  • When we drop channels, we obtain a residual in our Jacobian calculation (offset)
  • Distribution of residual errors now different over time as the number of channels changes
  • The pose acceleration vector method seems to give the same sort of correction, up to the choice of the correction vector

  • For now, we should discuss with John and Christian to qualify the TMS for on-sky use
  • John: TMS just has to be well-behaved when not on-sky.
  • TMS able to make measurements and send corrections
  • Andrew proposes that it would be a good idea to have a clear list of objectives which must be passed to go on-sky
  • Perhaps a page of formal requirements which must be met to go on-sky
  • Yang suggests the software should be further along to go on-sky (needs to be a little more user friendly)
  • A requirement for on-sky use would be that the TO has to have a certain minimal amount of required user-input
  • Heejoo to take ownership of a requirements list, borrowing from other commissioning documents

  • Matthieu in agreement with Yang: we need to get to a point where we go from the experimental stages to a working software
  • Want something ready to go on-sky with a predefined minimum threshold to reach this point
  • Matthieu wants to ensure that we take ownership of the tasks necessary to take it from point A to point B
  • Transforming a great idea into something practical is the name of the game

  • eTalon PC Issues
  • Currently running Windows 7 (no longer supported), propose upgrading to Windows 10
  • Andrew to reach out to eTalon again about upgrading
  • Heejoo would very much like to avoid a situation where we have to stop TMS measurement for multiple days to upgrade to Windows 10, though he would also like to upgrade
  • Do we really have to upgrade our version of Windows every few years?
  • Matthieu notes that many people do not like Windows because it keeps changing; telescope systems run on firmware that is not upgraded, sits in a frozen box and just works
  • Andrew notes that we have talked to eTalon about a Linux version in the past
  • DOS still runs aluminizing at LBT (hard to find machines which still have working capacitors to run it)

  • Still good to use the current truss geometry for tomorrow's test
Topic revision: r1 - 04 Jan 2021, TrentonBrendel
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