TMT Construction Proposal - Thirty Meter Telescope

Fig 3-1: Atmospheric transmission for a high altitude site (Mauna Kea).
3.2 Telescope
The telescope itself is the key to achieving our science goals. In this section we will describe the performance
desired for the broad science objectives of this facility. The telescope is foremost an optical system, but it must
also move to point to science targets, and it must provide suitable support of the science instruments that
attach to it.
3.2.1 Optical
The telescope optics limit observations in a variety of ways and we describe the desired performance for the
major categories.
3.2.1.1 Optical Configuration
The circumscribing circle around the primary is 30.0 m, and the useful
collecting area is about 655 m 2 . The primary mirror (M1) is the entrance pupil
of the telescope.
We want a closely filled aperture to produce diffraction-limited images with the
strongest central concentration of the light. When the starlight is faint
compared to the background, the noise under the star image dictates the
sensitivity. The time needed to reach a given S/N is proportional to
b*(equivalent-noise area) where b is the background/unit area and the
equivalent-noise area is the area that multiplies the background b to yield the
variance in the estimate of a star intensity [2]. This is directly proportional to
the needed observing time. The equivalent-noise area is
α = 1/ ∫PSF 2 dA, where the PSF is normalized so ∫PSFdA = 1
where PSF is the point spread function. For a given collecting area but with
different configurations, α is the reciprocal of the observing efficiency. We indicate α for diffraction-limited
observations with two configurations: a filled circular aperture and the segmented, TMT primary mirror (as
shown in Fig. 3-2).
aperture
Filled aperture 1.31e-4 arcsec 2 (1.00)
TMT 1.36e-4 arcsec 2 (1.04)
equivalent-noise area for 1 μm (relative integration time)
Fig 3-2: Pattern of 492
segments of the primary
mirror.
The TMT primary mirror configuration is shown in Figure 3-2. The diffraction pattern profile of this mirror is
shown in Figure 3-3.
Experience with large telescopes has shown us that a single mirror (M1) giving images at prime focus is limiting
for science instruments. A two-mirror telescope (M1+M2) giving a final focus at the Cassegrain focus (typically
behind the primary) is more useful. An even more useful system employs an additional third fold flat (M3) that
allows the Cassegrain focus to be folded over to the Nasmyth platforms. This design lets the telescope
accommodate multiple science instruments with more room.
TMT Construction Proposal 20