GMT Laser Tomography Adaptive Optics (LTAO) Delta Preliminary Design Review
Published in the RSAA Lunations
Vol1 Issue57 1–30 November 2024
If you were not listening to Arctic Monkeys or Justin Bieber, you may remember some important events that took place in 2013: The death of Nelson Mandela and Margaret Thatcher, the Edward Snowden leaks and obviously, the successful Preliminary Design Review of the Great Magellan Telescope (GMT) Laser Tomography Adaptive Optics (LTAO) system at the AITC. After that, the instrument design took a 10-year hiatus which leaves me plenty of time to explain what LTAO is:
Typically, large observatories equip each of their diffraction limited instruments with adaptive optic systems which consists of a wavefront sensor and a deformable mirror to correct this wavefront before light enters the instrument. In that scenario, the telescope just needs to supply a laser guide star system for the adaptive optics system to target on. GMT’s approach to diffraction limit is rather novel, taking on the responsibility of wavefront correction with its active secondary mirrors and adding not one, but six laser guide stars for a wide-field correction using a technique called “laser tomography”. The responsibility of ANU’s LTAO team was to supply GMT with a design for the laser guide star system and the wavefront sensors that will eventually equip Giant Magellan Telescope Integral-Field Spectrograph (GMTIFS) and Giant Magellan Telescope Near-IR Spectrograph (GMTNIRS).
Now that you are all caught up, it was GMT’s turn to do some catching up. In the last ten years, some of the technologies used in the 2013 design have been supplanted and the design needed to be accordingly updated before progressing. The AITC was therefore tasked to carry out a “delta preliminary design” to review and update the design to consider the technology shifts as well as the evolution of the GMT observatory itself.
This work took most of 2024 and culminated with a review at the end of October. I’ll skip all the juicy details but, in a nutshell, 10 years led to more powerful lasers, larger and faster cameras and an improve supply chain of key components. The limited time to carry out this delta-design meant that the main architecture of the two subsystems had to remain as close as possible to the original design. The first sub-system, the Laser Guide Star System (LGSS) shown in left-hand image, kept the side-launch configuration and the same key components to launch the laser to the desired asterism. The second sub-system, the Laser Tomography Wavefront Sensor (LTWFS) kept its gun-cylinder design despite having a lot of changes to its collimating optics.
So, did the reviewers think we are ready to go to the next steps of the design? According to their report, yes:
“We were impressed with ANU’s Laser Tomography Wavefront Sensor (LTWS) and Laser Guide Star System (LGSS) teams, GMT’s leadership of the review and the GMT-ANU collaboration.”
Besides the praise, the reviewers’ comments were very useful for us to plan the next set of actions. GMT and AITC have agreed to start a prototyping phase to derisk some of the complex components of the wavefront sensors. Over the next 10 months, we will produce a simplified version of the LTWFS to test all the moving parts as well as one of its cameras. This work will be critical to assess the validity of the current design before finalising it for production. So, I am happy to say “See you in 10 months!” for the next part to this story and not “See you in 10 years!”
Tony Travouillon for the LTAO team.
Image: Figure 1 (left) Six lasers make up the constellation needed to cover the large field of view and maintain near diffraction limit performance across that field.
Figure 2 (right) Six cameras mounted on a rotating stage are necessary to cancel the rotation of the telescope with respect to the laser guide stars. The whole assembly can itself be translated backward and forward to keep the laser in focus for different elevations.
