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How will TMT make its 492 primary mirror segments behave as a single mirror of 30 meter diameter? TMT’s Alignment and Phasing System (APS) team has come up with the solution that will keep all of TMT’s mirrors aligned to a high precision of about 10 nm root-mean-square (RMS).

 

The APS team met in Pasadena, California on May 11 to conduct the first phase of the Preliminary Design Review of TMT’s mirrors alignment and phasing system. The review of the APS Procedure Executive and Analysis Software  and the system wide APS requirements, hazard analysis, failure mode analysis, compliance, and verification took place at the TIO offices in Pasadena, with the participation of external reviewers from the Jet Propulsion Lab, W.M. Keck Observatory, Las Cumbres Observatory, and the National Solar Observatory.

 

TMT’s APS is responsible for positioning individual segments of the primary mirror as well as the secondary and tertiary mirrors. One of the critical tasks associated with the large telescope is the phasing of their segmented surfaces. Each primary mirror segment must work together to act as a single 30 meter mirror.  Without proper phasing, some light from some mirrors would cancel out some light from other mirrors and the telescope would not realize the full benefit of its 30 meter primary mirror. To achieve the resolution commensurable with the full 30 meter mirror telescope, the segments must be phased with a high precision.

 

APS will use starlight to measure  wavefront errors and  determine the appropriate commands to send to align the individual optics. Once the optics are aligned, its various control systems will record the set points for later use. In particular, APS will measure and align TMT’s optics by adjusting the following parameters:

• Primary mirror segments in piston and tip/tilt

• Primary mirror segment surface figure

• Secondary mirror piston and tip/tilt (or x/y-decenter)

• Tertiary mirror tip and rotation

 

The APS will align the telescope optics at various elevation angles. Using the reference points for the three mirror control systems, calibration coefficients will be generated to correct for gravity-induced deformations. In a similar fashion, data will be collected at various temperatures over time, and calibrated configurations will be developed and implemented as a function of temperature.

 

Approximately every two weeks, 10 primary segments will be exchanged in TMT to refresh the coating and ensure overall high reflectivitiy. APS will then be used to realign the re-installed segments in piston, tip, and tilt, and to correct the segment figures. At the same time, APS will adjust the secondary mirror position and tip/tilt, and the tertiary tip and rotation. The APS measurements and associated adjustments to the mirrors will take no more than two hours of observation time. In addition, the Primary Mirror Control System (M1CS) calibration of the sensors on the re-installed segments will require re-alignments at two additional elevation angles and each of those re-alignments will take no more than 30 minutes.

 

The accumulated experience obtained with the Keck mirror segments alignment shows that APS should be run at least once a month, even if there are no segment exchanges. This procedure, which will take no more than 30 minutes of observation time every month, will ensure that the telescope remains properly aligned and capable of realizing its potential.

 

Initially, during the early operations of APS, the system will be located on the elevation axis of the telescope. However, after this phase of early operations the APS might be moved to a position off the elevation axis.

 

The design of TMT’s APS is based upon that of the Keck Phasing Camera System (PCS), which performs a similar role for the Keck 1 and 2 telescopes. In particular, PCS is responsible for aligning the Keck segments in piston, tip, and tilt; for aligning the secondary mirror in piston, tip, and tilt; and for providing segment figure measurements (for the purpose of adjusting warping harnesses). The original team of Gary Chanan, Mitchell Troy, and Scott Michaels, who designed, built, installed, conducted initial operations, maintained, and eventually upgraded PCS at Keck, is involved in designing TMT’s APS. The technologies that were developed and optimized for Keck alignment and phasing are directly applicable to TMT. The segment piston, tip, and tilt alignment functions at Keck were carried out on all 36 segments in parallel and, in this respect, can be scaled up to the larger number of segments for TMT with only minor complications.

 

The APS alignment is decomposed into a set of reusable alignment procedures such as Broadband Phasing, Narrowband Phasing, Rigid Body and Segment Figure Correction, Coarse Tilt Alignment, M3 Alignment, and Off-Axis Measurements, each of which may be executed multiple times to converge on a best aligned configuration.

 

Jason Weiss, Observatory Software Developer at TMT, acknowledged that “The [APS] software had a very successful design review. The work leading to this review has really been a team effort. TMT’s APS features a nice heritage and on-going collaboration with Keck Observatories. Everyone has been working hard on an excellent preparation with a lot of prototyping work done at Keck Observatories that presented a highly valued and mature design for this stage. In fact, TMT’s APS is well underway to the final design phase.”

 

Jimmy Johnson, TMT Telescope Controls Group Leader, added, “The review committee found that APS not only fulfilled the review's scope and criteria but did so impressively. In addition, it was noted that the collaboration between APS and Keck has yielded remarkable results in the design of the TMT APS software. Not only did it meet the preliminary design requirements, but it also went above and beyond by enhancing Keck's capabilities.”