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Thirty Meter Telescope

The Project Manager's Corner: Does a Thirty Meter Telescope Use an Eyepiece?
Gary Sanders, TMT Project Manager

March 2006

Many of us own a small backyard telescope, or have a friend or neighbor who has one. Perhaps a gift of a small telescope is a fond memory of our childhood.

No matter what size mirror or type of mount is used, these instruments have an eyepiece. A little black cylindrical lens that you look through to see the image from the telescope. Eyepieces are also found on binoculars, and on those long tubular nautical telescopes that use refracting lenses, and on those classy spherical red hobbyist scopes we use in our backyards and on vacation trips to the mountains. These are real reflecting telescopes that ordinary people can afford.

Some sophisticated amateurs mount cameras on their scopes and are able to photograph much fainter heavenly bodies. Telescope and binocular supplier catalogs abound with all manner of neat cameras and camera mounts for these purposes. Amateur astronomers eventually gave up their eyepieces for pretty sophisticated film cameras (and now digital imagers.) Check the ads and product reviews in Sky & Telescope or Astronomy magazine, for example.

Professional astronomers followed a similar path. Galileo looked through his reflector with an eyepiece. Centuries later, Hubble sat on his platform on Mount. Wilson and recorded the fleeing universe, rushing away from the Big Bang, using a kind of camera to permanently record images and measures of the color (wavelength) of the light. But he used a simple eyepiece as well in preliminary work.

The personal experience of the astronomer's eyeball receiving the full power of the telescope directly is vanishing. The eyepiece has given way to cameras, and the film in the cameras has been replaced by dense arrays of charge-coupled devices (CCDs) that populate the focal plane. The image examined by the astronomer is likely displayed on a TV monitor or computer screen, the bits of the image shifted out of the CCDs like pearls on a string enumerating the bright and dark pixels of light that form the image.

The light is also spread into spectra by prisms and lenslets and slicers and all manner of clever devices, laying out the spectra, the dynamic color signatures of the chemical and molecular turmoil in the heavens. And these spectra, once visible in an eyepiece and then recorded on film, are also captured now by electronic sensor arrays.

Telescopes no longer have eyepieces. They have marvelously complex "instruments." Cameras record images. Those incredible Hubble Space Telescope pictures come from cameras. Spectrographs dissect the luminous tell tales of excited states giving way to ground states in the quantum mechanical choreography of stars. And they tie the red shifted color to the age and distance to the object. Photometers carefully measure brightness. Polarimeters decipher the vibrational state and orientation of the fields in the arriving radiation. And very clever combinations of these devices measure wobbles and slight changes in speed, and divide a large object into little pieces, each piece with its own interest and story to reveal.

These amazing instruments are housed in black and silver and bronze cabinets that bolt onto the business end of the telescope. Sometimes they hang under the main telescope, gathered around what might once have been the location of an eyepiece. Sometimes they sit on large platforms off to the side, and the telescope beam is brought over to them. (This is the plan for TMT; see the picture in the corner of our Web pages.)

Often a great new telescope is designed almost simultaneously with one or two "first light" instruments to be delivered early in its life. These instruments represent the early science hopes of the astronomical community gathered around the great machine. Through the life of an observatory, new science and new ambitions and new blood and new money yield new instruments to be added to the arsenal available to the advancing astronomer. Sometimes instruments are sent on temporary visits from one observatory to another.

Most instruments are developed by astronomers themselves, seeking a way to get a better answer to an astronomy question. The resourceful professor, not content to visit observatories and their available instruments to slake his research thirst, is driven to understand just how the instruments function and what their limitations are, and how a new technology or breakthrough optical trick or just plain good design may extend his reach into the universe. These professors build up a group of instrument experts and develop instruments that they provide to observatories, for the promise of improved access to the observatory, and a better chance to answers those questions.

The great telescopes that hunt the night sky now each have a set of powerful instruments. These were built mostly in the manner described. Senior astronomers and professors, engineers and technical wizards gathered together to design, deliver, and commission the tools. Each of the modern generation is a technical accomplishment at the cutting edge. Some had problems as they were realized. Some arrived years late or required more resources or provided only some of the capabilities. These are the outcomes that sometimes result even when the best and brightest try to push the envelope of what is possible.

The new telescopes on the drawing boards need new instruments matched to the breadth of these new programs. These new instruments not only push technology hard, but they, like their parent telescopes, take a decade to conceive and build and they require the planning and management that might be expected of the observatory project itself. As in all big science, we strive to bring the impossible and sublime into the routine and predictable as we build them.

But routine and predictable must come after the most amazing science is imagined and the clever means to reach it is envisioned. The TMT science goals are written down in our Science Requirements Document (SRD). Authored by our Science Advisory Committee, the SRD represents more than a year's worth of debate and discussion and estimating and agreement. In my January Project Manager's Corner column, I described the SRD and told you how we used it to set into motion an instrument design program open to the entire North American astronomy community. (Remember that our partnership is of US and Canadian astronomers.)

The SRD envisioned astronomy that was done in "seeing-limited" mode. This means that the telescope and its instrument are used to observe within the optical limits set by the atmosphere above the observatory. The shimmering of the atmosphere and the resulting twinkling of the stars sets limits and TMT is seeking a mountaintop site that offers very good seeing (see my story My Summer Vacation). A great deal of science can be done with TMT in this manner. The large diameter of TMT collects a great deal of light. Objects that are faint or very far away can be observed. The faintest and oldest galaxies in the universe can be studied by TMT using optical spectra recorded by powerful seeing-limited instruments.

The SRD also described "diffraction-limited" observing. The TMT aperture, capable of collecting a great deal of light, also results in unprecedented sharp point resolution. This is a result of the aperture and the optical phenomenon of diffraction. The light collection grows as the square of the telescope diameter. The diffraction resolution also grows the telescope sensitivity as the square of the diameter.

The TMT diameter of three times that of the Keck telescope provides an advantage that grows as the fourth power of the aperture. This is extraordinary and it is the real reason why a monster scope like TMT is so powerful. The TMT SRD explores the uses of this great advantage. However, using the diffraction-limited power of TMT requires an adaptive optics (AO) system capable of measuring the shimmering of Earth’s atmosphere and correcting it away in the image by using sets of computer controlled deformable mirrors. This is a very challenging technology, but it is at the heart of TMT and the more complex TMT science instruments.

Early in 2005, guided by the SRD recommendation that TMT be instrumented with eight instrument capabilities in its first decade of operation, the project offered all instrument groups in North America the opportunity to carry out feasibility studies of possible instruments. The groups were given the science cases described in the SRD, and the basic parameters of envisioned instruments and AO systems, but were invited to form their own science teams to develop their own views on the science, and to carry out independent—and hopefully clever—design studies of the instruments and how they would be used in the observing program. TMT did not want to assume that the project had gotten everything 100% right. The broader community was invited to steer the program.

The response to this invitation was gratifying: 41 institutions expressed interest and these groups formed collaborative teams resulting in proposals for six of the eight instruments and for the facility AO system intended to support three of the instruments. After a broad peer-review involving astronomy community members, 11 awards were made of funds to carry out studies of the six instruments and the AO systems, with several instruments involved in parallel competitive studies.

The design teams were charged with delivering a description of the science justification, a design for the observing program, a statement of the detailed functional requirements that the instrument had to meet and a description of the design concept with a budget for building the instrument. These studies were due to be delivered by February 15, 2006 with reviews of all designs in March.

All of the teams succeeded in delivering impressive studies. Some carried out designs as expected. Some clever new ideas emerged involving technologies that extended the science reach or reduced costs, or both. The science cases themselves were developed and these will now challenge our Science Advisory Committee to reconsider the SRD.

During ten days in March, 35 external reviewers from around the world gathered in Pasadena with the TMT project and the instrument design teams. The studies were posted in advance on a Web site and the reviewers began their reviews before traveling to Pasadena. The reviewers posted questions in “chat” groups for the design teams so that the face-to-face reviews would grapple with the answers to these questions.

The 35 reviewers left valuable reports on each of the instrument and AO designs. The project will spend the next several months working with the Science Advisory Committee to tune up the science cases, reconsidering the lineup of instruments and which ones to plan for first-light use, and planning the next phase of instrument development.

Instrument development must proceed in parallel with the telescope design in order to assure that the combined systems function well together. They are not add-ons to an existing telescope design, but complex elements of a combined system.

Imagine how hard it would be to see a planet orbiting a far-away star when that star is a billion times brighter than the planet, especially if the telescope is shaking in the wind, blurring the two images. Finding extrasolar planets is a key goal for TMT. The TMT observatory must be designed end-to-end. Our first-round instrument program has already succeeded in this challenging choreography of system design.

In a future column, I’ll describe some of the specific instruments and the science behind them. For now, the March 2006 reviews have paved the way for TMT to move beyond needing an eyepiece.

—Gary Sanders

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TMT ambassadors Jean-Rene Roy and Jim Kennedy (Gemini Observatory), Gary Sanders (TMT Project Manager), Rolf-Peter Kudritzki (University of Hawaii) and Ray Carlberg (University of Toronto) in Honolulu, HI, on February 1, 2006, to discuss placement of TMT.

The TMT project is a collaboration of Caltech, University of California (UC) and the Association of Canadian Universities for Research in Astronomy (ACURA). © Thirty Meter Telescope


Association of Canadian Universities for Research in Astronomy