• Use of an on-board inertial reference to maintain pointing stability over a bandwidth from 1 to 300 Hz;

  • The ability to track stars, as faint as 19th magnitude, through the full aperture of the telescope to maintain pointing stability against disturbances at frequencies of less than 10 Hz; and

  • A design optimized for agility and rapid slewing from one point on the sky to another.

The layout of the proposed telescope and details of its optics are shown in Figure 2.1 and Figure 2.2, respectively. Its basic design is that of an afocal, three-mirror anastigmat. The use of three mirrors permits good imaging performance over a relatively large field of view. The 4-meter-diameter, f/1.25 (a design with a focal ratio of f/1.7 is also possible) parabolic primary mirror is 17 mm thick and is equipped with some 260 actuators to control its shape. The convex secondary and concave tertiary mirrors are both hyperboloids. The optical parameters of all three mirrors are given in Table 2.1.

The telescope relays an image of the entrance pupil from the primary mirror to a flat image-stabilization or fast steering mirror (FSM) with a demagnification of 40 (i.e., with an exit pupil diameter of 10 cm). (A demagnification of 20 is another possibility.) The FSM, located at the relayed pupil, is used to stabilize the image at frequencies up to 300 Hz over an area of sky 0.2 degrees across.

The FSM is followed by a dichroic beam splitter that divides the light into infrared and visible branches at a wavelength near 1 micron. These separate beams are subsequently directed to a number of different focal planes by additional beam dividers and reimaging optics. The visible branch contains a wavefront sensor used to assess the shape of the primary mirror and generate correcting signals for the primary ’s figure control actuators. The assumed parameters of the completed telescope are summarized in Boxes 2.1 and 2.2These numbers are used in subsequent sections that address the ATD/NTOT ’s optical performance.

EXPECTED POINT-SPREAD FUNCTION

Knowledge of the observatory-level point-spread function (PSF) and encircled energy is useful for defining the kinds of science and the spectral region to which an optical system such as the ATD/NTOT is best applied. In practice, of course, this determination is a complex systems engineering issue requiring many kinds of input and much analysis. However, the PSF can be estimated by considering two factors known about the ATD/NTOT: its line-of-sight stability and its figure error.

Line-of-Sight Stability

The telescope’s line of sight is maintained at frequencies greater than 1 Hz by illuminating a quad cell with a gyroscopically stabilized light beam from a so-called Inertial Pseudo-Stellar Reference Unit (IPSRU). The resulting error signal is sampled at 300 Hz and used to drive the FSM so that it keeps the telescope’s image fixed relative to the focal plane. Although the IPSRU’s gyros drift at a rate of the order of 5 milliarc sec (mas) per second, they are corrected at frequencies of less than 10 Hz by determining the centroid of the image of a guide star viewed with the fine-guidance CCDs. The error-budget allotment for image stability is quoted as 4.5 mas per axis; thus, the two-dimensional image stability is 6.5 mas. The task group convolved this figure with a Gaussian to broaden expected point-spread functions.

Figure Error

For this discussion, the ATD/NTOT telescope is assumed to consist of five mirrors. The first three are the primary, secondary, and tertiary mirrors of the afocal telescope assembly. The fourth is the FSM at the reimaged telescope pupil, and the fifth is a hypothetical mirror required to bring light to a focus. Bringing the telescope to focus over a reasonable field of view actually requires at least two optics, but assuming only one makes only a small difference in predicted performance. Since astronomical telescopes generally feed a focused image to their instruments, the inclusion of the FSM and a focusing optic allows a direct comparison with performance at the focal plane of other telescopes.



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