GMTIFS: Designing an instrument at RSAA

23 June 2017

A first-light instrument for the GMT is being developed at the Advanced Instrumentation and Technology Centre (AITC) at the Research School of Astronomy & Astrophysics at the Australian National University. GMTIFS – the GMT Integral Field Spectrograph – is a near-infrared integral field spectrograph and camera that is specially designed to look at fine detail in objects such as the centers of galaxies and disks associated with young stars.

External and internal design of GMTIFS. The whole unit is approximately 2.7m x 2.4m x 1.8m.

GMTIFS will work with GMT’s laser tomography adaptive optics system which uses 6 sodium lasers, plus one off-axis natural guide star, to deliver a diffraction limited field approximately 1/60th the size of the full moon. In the AITC lab this month, the GMTIFS team is testing one small, but very important, part of the adaptive optics system – the “on-instrument wavefront sensor” (OIWFS) for the natural guide star.

This system is designed to make the off-axis natural guide star sharper so it is a better reference for the laser guide star system. Controlled by the main adaptive optics system, the OIWFS corrects the distortion introduced by the atmosphere in the direction of the off-axis natural guide star. It concentrates the star’s light and permits an accurate measurement of its position. Knowing the position of the natural guide star helps stabilize the laser-corrected data that are sent to the spectrograph or camera.

The unique challenge with the OIWFS is that it needs to be located deep within the instrument, and in the case of GMTIFS, and many other instruments, means it is in a cryogenically cooled environment.

The team has been testing a key component of any adaptive optics system: the deformable mirror. Deformable mirrors can change their surface shape thanks to actuators pushing or pulling on their back surface. Using this kind of mirror in a cold environment has the potential to affect how the front surface and rear actuators function. No “Commercial Off The Shelf” deformable mirror for astronomy has been used in a cryogenic environment before, so these tests are groundbreaking.

Dr. James Gilbert (left) and Ian Price (right) with the deformable mirror test bench. The cold chamber for the mirror is inside the pink bubble wrap.

Thanks to the work of John Hart, Senior Optomechanical Engineer at ANU, the team opted to place the mirror inside its own “warmer” chamber within the cryogenic environment of the instrument. This makes the environment slightly less challenging for the mirror. Temperatures are relative however, and “warm” in this case means -40C (-40F).

The deformable mirror technology being tested now was one of two types that underwent initial trials. This mirror, made by Boston Micromachines, is 9.5mm on a side and has 492 actuators. The actuators can move the surface of the mirror up to 400 microns. The team is testing the response of a representative fraction of the actuators to see if they function in a precisely consistent way over time in the cold environment. The mirror is tested at room temperature, then stepped down in 5C increments until it reaches -40C. Each test takes about 3 hours and tests will be conducted once a week for three months.

This cooled deformable mirror technology has the potential to be used inside any GMT instrument that uses a natural guide star. Testing the deformable mirror in the cold environment is just one small part of the GMTIFS project that will ultimately take years to design, build and test. With highly skilled engineers at ANU and other GMT founder institutions working on the instruments for the project, we can be sure the science they deliver will transform our understanding of the universe.

Technical details of GMTIFS are on the GMTO website and on the ANU website.