TMT NFIRAOS Review
  Glen Herriot

The TMT NFIRAOS team was pleased by the interest and attendance at the first interim review of NFIRAOS held all day on December 5, 2007, at the one-third point of the Preliminary Design phase for NFIRAOS, the facility Adaptive Optics system for the TMT. Members of the TMT AO group together with the NFIRAOS design team from the Herzberg Institute of Astrophysics in Victoria, BC, Canada, presented and discussed the design and its status. The Preliminary Design phase of NFIRAOS, and this first interim review in particular, concentrates on performance modeling, and refining NIFRAOS' specifications and interfaces, especially those that affect instruments and the telescope structure. This was an informal review without an invited review panel, and we welcomed all members of the TMT community together with guests from the national optical observatories of the US and Japan. Interest was high, as was apparent by the seven video connections to the conference room at the TMT project office in Pasadena. The audience provided valuable suggestions, and we did not identify any show-stoppers.

We will test NFIRAOS on the sky right after all segments of TMT's primary mirror have been installed and accurately aligned. NFIRAOS uses six laser guide stars and two deformable mirrors to correct image blurring caused by atmospheric turbulence. NFIRAOS will deliver diffraction – limited images to three client instruments working in the near infrared wavelength range of 0.8 to 2.5 micrometers, allowing astronomers to see objects that are fainter, tinier, further and in more crowded locations than ever before.

It would be a shame to wash out such details with a foggy image. The problem is that everything, including mirrors, glows in the infrared, even when on a chilly mountain top. So all of NFIRAOS' optics will be in a freezer box cooled to -30 Celsius; otherwise some important scientific observations would take two and a half times longer, wasting precious telescope time.

While supporting NFIRAOS' cold optics, allowing for shrinkage due to the change in temperature, and carrying the weight of three instruments seven metres above TMT's Nasmyth platform, our design keeps the cold air inside NFIRAOS and permits the instruments to rotate during astronomical observations. As shown in figure 1, the early light configuration will have an integral field spectrograph, IRIS, on the bottom of NFIRAOS and wider-field multi-slit spectrograph, IRMS, on the side port. A future top port is earmarked for WIRC, a wide-field imaging camera, and the side port will eventually be the home for NIRES-b, a high spectral-resolution instrument. The yellow portion hanging below NFIRAOS holds the laser wavefront sensors that do not have to be as cold as the critical science path optics.

Figure 1.  NFIRAOS on the Nasmyth platform with IRMS (cyan octagon) and IRIS (white cylinder)

You can see NFIRAOS’ electronics cabinet on the lower right among the support legs. These three sets of tripod legs are not part of NFIRAOS’ responsibilities, but were presented to the interim review as an existence proof that NFIRAOS and its instruments may be supported from the Nasmyth platform, while allowing adequate access for servicing and replacing instruments.  The interface from NFIRAOS to the telescope structure is at the top of the tripods.  We expect that telescope structure group will carry the tripod design (or its equivalent) forward now.

During the review we also presented the operating concept and work flow showing how the telescope, instruments and AO system will work together to meet the TMT requirement to change targets on the sky and start doing astronomy in 5 minutes.   We also discussed our performance models, including showing that NFIRAOS can adequately correct telescope rigid body misalignments and low order M1 figure distortions. Finally we described other ongoing work and future plans during the next segment of the Preliminary Design of NFIRAOS. For example, in work to begin soon, we will examine NFIRAOS ability to correct M1 segment vibration caused by the wind.