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has given way to cameras, and the film in the cameras has been
replaced by dense arrays of charge-coupled devices (CCDs) that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 |
