TMT Construction Proposal - Thirty Meter Telescope

(a)
(b)
Fig 1-1: The telescope design (a), and the entire observatory system (b).
TMT will couple unprecedented light collection area (almost 10 times more than one of the Keck telescopes)
with diffraction-limited spatial resolution that exceeds by Keck by a factor of 3. Relative to the Hubble Space
Telescope (arguable the most revolutionary astronomical instrument of our generation), TMT will have 144
times the collecting area and more than a factor of 10 better spatial resolution at near-infrared and longer
wavelengths.
1.2.1 Aperture Size
Telescope primary mirror (M1) diameter (aperture size) is a key design choice – it drives the size and
complexity of almost every other observatory system. From a science perspective, we know that the
observations limited by atmospheric turbulence improve in sensitivity as diameter squared (D 2 ). Furthermore,
AO systems allow us to remove much of the effects of turbulence, achieve diffraction-limited imaging, and gain
an improvement of D 4 . Hence, science gain motivates the largest diameter enabled by technology and cost.
Technologically, large M1 diameters are enabled by using the precision-controlled, segmented mirror
techniques pioneered with great success by the W. M. Keck Observatory. The TMT M1 design team includes
many of the key developers of the Keck system. In principle, arbitrarily large mirrors are possible. However, our
analysis of AO technology development over the next ten years with respect to our detailed science goals (as
summarized in Chapter 2 of this proposal) suggests that D = 30 meters occupies an attractive and achievable
scientific “sweet spot” at near-infrared wavelengths [2].
Obviously, cost also limits M1 diameter – how does financial pressure affect choice of aperture size Prior to
the design of the Keck telescopes, ground-based optical-IR telescope costs scaled roughly as D 2.7 , reflecting
the nearly volumetric dependence of observatory cost on aperture [3,4]. The advent of thinner primary mirrors
with shorter focal lengths, as in the Kecek design, reduced this relationship to an estimated D 2 ; i.e., varying as
the area of the aperture. A detailed cost study of key TMT design parameters (aperture, segment count,
segment size, segment thickness, and enclosure diameter) for several hundred estimated elements in TMT
concluded that TMT costs should scale as approximately D 1.15 relative to the Keck capital investment. Thus, we
conclude that the TMT design represents an improved cost scaling and that the proposed 30 meter diameter of
TMT represents a good value relative to the extraordinary science reach.
1.2.2 Adaptive Optics
Like all existing ground-based observatories, TMT will be capable of seeing-limited observations, i.e.
observations with spatial resolution limited by the natural turbulence in the Earth’s atmosphere. Since such
observations improve as D 2 , TMT will be able to observe objects nine times fainter than Keck in an equal
amount of time.
TMT Construction Proposal 2