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Thirty Meter Telescope

Interview with David Crampton

David CramptonDavid Crampton is the instruments group leader for the Thirty Meter Telescope project, and the head of the instrumentation group at the Dominion Astrophysical Observatory/Herzberg Institute of Astrophysics (HIA) in Victoria, Canada. In his capacity for TMT, David is leading the planning and management of the instrumentation activity, in close partnership with Brent Ellerbroek, who is playing a similar role for adaptive optics.

David has been the Principal Investigator or had major involvement in the production of several multi-object spectrographs and adaptive optic systems for the Canada-France-Hawaii Telescope (CFHT) and the Gemini Observatory, and has been a member of the science teams for the James Webb Space Telescope (JWST) NIRCAM and NIRSPEC instruments. His scientific research focuses on the high-redshift universe, principally through studies of extremely high-redshift galaxies, quasars and gravitational lenses. A few decades ago, he was a co-discoverer of the first black hole in a galaxy outside our own, a discovery that helped establish the reality of black holes.

David spoke recently with Doug Isbell of NOAO public affairs about the state of the TMT instrumentation program, and its greatest challenges.

Q: What the general status of the TMT instrument program?

Twelve feasibility and conceptual design studies were completed earlier this year for the instruments and associated adaptive optics (AO) systems that will enable TMT to carry out top-rank science projects in its first decade of operations. These study contracts were awarded to instrument teams as a result of a competitive peer-reviewed process that began in January 2005.

Altogether, an estimated 200 astronomers and engineers from more than 34 institutions in the U.S., 10 in Canada and two in France were involved in the submission of proposals. Several of the successful teams involve major collaborations.

Beyond providing technical concepts for the instruments and their adaptive optic systems, these studies did the hard work of providing scientific and technical performance estimates and requirements. This really helps us establish some basic principles that provide important guidance in the design of the telescope and, really, the entire observatory.

The total estimated cost of all these instruments and AO systems is in excess of $300 million—much more than can be supported by the construction budget. Also, integrating and commissioning a major instrument or AO system typically takes more than a year each, so that only a few instruments are planned for “first light”—when scientific observations begin, circa 2015.

Consequentially, in July, based on the outcome of the feasibility studies, the TMT Science Advisory Committee established priorities for the first-light instruments. The top-ranked instrument is IRIS, an integral-field spectrograph and imager that will analyze diffraction-limited images delivered by the facility adaptive optics system. [Note: see the related “Science Nugget” in this issue of the TMT Newscast].

Q: Why does a telescope that won’t see first light for 8-10 years need such an aggressive program?

The simple answer is that the cycle to design and build state-of-the-art instruments typically takes 8-9 years. Granted, the first few years are taken up with the process of investigating design options, and making hard choices about capabilities and related budgets. The instruments also must be robust, reliable and easy to operate, and so considerable time will be taken to rigorously test them and to ensure that they meet their performance requirements.

Q: Why have you chosen to dedicate so much time to this effort—why is it important to you?

Because it’s such an exciting project both technically and scientifically! It’s also great fun to be involved with a project like this during the early conceptual phase.

TMT is being designed as a complete system to deliver diffraction-limited images—for a 30-meter aperture, the results will be spectacular and will allow us to attack some of the grandest challenges in astronomy, for decades to come.

Q: Tell us about the scale of these instruments—what do they weigh? How big of an incoming beam do they have to deal with from a 30-meter aperture?

The instruments that use the straight beam from the telescope have to be large because the telescope scale is so large (about 2 arcseconds per millimeter) that the full field is 2.6 meters in diameter. The large scale also means that the optics and dispersing elements for these instruments must be massive in order to deliver the required spectral resolution. These instruments will have typical size scales of 8-10 meters and may weigh 50 tons.

The diffraction-limited instruments will be considerably smaller, with scales of 2-5 meters, but they will have to be extremely precise. All of the instruments and AO systems will be located on two huge Nasmyth platforms that are each the size of basketball courts. The telescope beam will be deflected to the instruments by movement of the tertiary mirror, also known as M3. This will allow rapid switching among instruments to take advantage of optimal conditions for a particular type of observation or to observe a "target of opportunity".

Q: How will these instruments be built? Are new approaches needed as compared to instruments for the 8-10 meter-sized telescopes of today?

They will be built by relatively large groups or consortia. Because they are so large and costly, they will be managed more formally than most previous ground-based instruments, using best practices project management procedures.

Q: What are the biggest technical challenges?

The instruments are becoming larger at the same time as the tolerances and requirements become tighter. The size of optics and mechanisms is perhaps the main dominant factor for seeing-limited instruments. TMT will produce very tiny images with adaptive optics (as small 3.5 milli-arcseconds across, or 7 microns in the f/15 telescope focal plane). In order to exploit such images, extremely precise, stable instruments are required—orders of magnitude better than we’re used to having today.

Q: How do the instruments interact with the adaptive optics system?

Actually, the instruments and AO systems are becoming more and more completely integrated, and consequentially I think of them as one.

In some cases, like the Planet Finder instrument, they are one. In other cases, like IRIS, only the wavefront sensors for the natural guide stars are located inside the IRIS cryostat; nevertheless, they must work together completely in concert.

Q: What will the TMT be able to do in the area of exoplanets?

Lots! It is anticipated that astronomers will be able to study and directly image planets around nearby stars, they will be able to study how planets are formed, and perhaps determine which planets are most suitable for life.

Q: How do the TMT instruments compare to what is being planned for the James Webb Space Telescope?

The complementarity of the TMT instrument suite with the capabilities at other planned facilities (including the Atacama Large Millimeter Array) was a significant factor in the TMT Science Advisory Committee’s decisions.

Q: What are the next major milestones in the instrument program?

Conceptual designs for the first few instruments are expected to be developed beginning next year. Currently, we are attempting to gather together the best possible information on the costs of all the instruments, AO systems, and the supporting subsystems and resources, to provide to the comprehensive TMT cost review later this year.

Q: You have been an observing astronomer for a long time. How will astronomers use the instruments at TMT—will observing with TMT be done a different mode than today?

When I first began observing as a professional astronomer, we used 2-meter class telescopes and spent most of the night staring through an eyepiece to guide the telescope, sitting high up on a ladder.

In the early days of CFHT, the only instrument available was a large prime focus camera that used photographic plates. Because it took considerable time to access the prime focus cage, we attempted to remain there all night without stopping. The temperature was typically just below 0 C, and so there were three major things that competed with the excitement of exploration: cold, hunger and bathroom requirements. My personal endurance record was 11 hours, but others did better!

From a technical standpoint, one of my most exciting nights was the first night that we turned on the adaptive optics system on CFHT and immediately saw diffraction rings. In attempting to use that system at its limits for very faint extragalactic objects with faint natural guide stars, I became converted to the concept of queue observing.

It was only possible to achieve my goals with the best-possible conditions of seeing and transparency, and those didn’t occur often during a typical few night run. Since then, with queue observing on Gemini, we have been able to gather observations in ideal conditions to accumulate 100,000-second exposures and we’ve even attempted 100-hour exposures (collecting photons over many nights during the course of a year).

I expect that TMT will operate in the same way, to take advantage of the best conditions for a given scientific goal, although some fraction of the time will undoubtedly be used for classical observations too. The possibility that instruments will be used for very long exposures accumulated over many months, or for projects that require observations over many years, implies that they must be extremely stable, robust, and reliable. These “mundane” requirements can often be equally or more important as raw throughput.

Q: What do you personally look forward to observing with the TMT should you get the chance?

Right now, my answer would be “first-light objects”—objects that are the first visible things that can be seen emerging in the early universe, more than 13 billion years ago (see the “Focus On” article in this Newscast). My suspicion is that these will be so faint that they’ll be challenging even for TMT and that we’ll need boosts from gravitational lensing or gamma-ray bursters to study them. But I can’t wait to find out!


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