Highlights of the SPIE Optical Engineering + Applications Conference at San Diego, CA, 2011

Most of interesting oral presentations was on Sunday, where the astronomical adaptive optics was discussed. Here are some remarks on them from the section Astronomical Adaptive Optics Systems and Applications V.


Integration and test of the Gemini Planet Imager . . . . . . .[8149-01]
For the extreme AO, they plan to achieve 2-4 arcseconds of angular resolution. Since it is a Cassegrain focus, the instruments must be located under the focus and move with the telescope.
There are some interesting lessons they learned from the WFS:

• small subapertures make it hard to align;
• mount of the camera is hard to align;

The lenslet size is 63 micrometers that introduces a lot of diffraction effects.

The controller they use is commercial closed black-box (Fourier, predictive controller).
The WFS used is Shack-Hartmann quadcell. WFS noise is 4-5 e- at 1 KHz speed.



The TMTracer: a modeling tool for the TMT alignment and phasing system, Piotr K. Piatrou, Gary A. Chanan, Univ. of California, Irvine (United States) . . . .[8149-03]

This is about the simulator of the TMT parts written by Piotr K. Piatrou on FORTRAN 95. No diffraction effects, only ray tracing.

This is for alignment and phase sensing of the telescope mirrors. The control of the wavefront is LS tomography - filtration of commands directly to DM. This is due to huge amount of data.
Observability must be computed theoretically, then the SVD decomposition is used.



Athermal design of the optical tube assemblies for the ESO VLT Four Laser Guide Star Facility, Rens Henselmans, David Nijkerk, Martin Lemmen, Fred Kamphues, TNO Science and Industry (Netherlands) . . . .[8149-04]

Interesting speech about the design of LGS tube, they actually use it for VLT with 4 GS for lambda=589 nm and power 25 W.
Optical design of the LGS tube is classical Gallilean 20x beamer expander. Athermal design was a primary goal. Laser absorption is estimated as 0.1 ... 1% for 25 W which is pretty much. They are making a big tube from invar (steel alloy) to expand a light beam - not from caron since it is very expensive. L2 Lense in their design is 38 cm in diameter.


Overview of the control strategies for the TMT alignment and phasing system, Piotr K. Piatrou, Gary A. Chanan, Univ. of California, Irvine (United States) . . . . . . . . .[8149-05]

The main goal here is to automatically control alignment of AO parts on the TMT. The multidirectional tomography is the mainstream approach for TMT alignment.The PAS (alignment and phasing system) is n open-loop system without accounting for the dynamics.

This is for the alignment only. The control is brute-force LS tomography because of huge amount of data. They have 33GB for SVD, and therefore use complexity reduction methods. For instance, projection mehtod like Oa = s -> \[ P\dagger O a = P\dagger a \]

In the case of TMT, I think, it is possbile to nglect the dynamics of the system completely and just assume that the system is static. For low and medium frequencies it will probbly work well. That cruel algorithm (just throw the command to DM) explains the 2*opd coefficient 2.

The quasi-continuous part for the control.