Figure 2.1 shows the basic optical configurations available at the AAT. The primary mirror has a hyperbolic cross-section, a clear aperture of about 3.9m and a focal length of 12.7m, giving a focal ratio of about f/3.3 at prime focus. The primary and the largest of the secondary mirrors together form an f/7.9 Ritchey-Chrétien system; i.e. they give a coma-free field. For the f/15 Cassegrain, f/36 chopping secondary and f/36 coudé, the Ritchey-Chrétien condition cannot be maintained and the secondaries are designed simply to give no aberrations on axis.
Figure 2.1: Optical configurations for the AAT
(Click on the image for a larger version)
Figure 2.2 shows the telescope from two perspectives. The tube has a Serrurier truss design which gives nominally parallel and equal deflections at the primary mirror and at the top end. The optical configurations are selected by interchanging three top ends: one for prime focus, one for f/8, and one carrying the f/15 and coudé f/36 mirrors mounted back to back and exchanged by a flip-over action. The f/36 chopping secondary is mounted inside the prime focus top end (with the camera removed). Changing top ends takes nearly an hour and is not recommended during night-time observing. Changing between f/15 and coudé takes about 15 minutes.
Figure 2.2: The telescope structure, viewed from the east and from the south
(Click on the image for a larger version)
All the AAT mirrors are made of Cervit, a glass-ceramic material with a thermal expansion coefficient less than 10-7/°C, and which is therefore practically immune to thermal distortion. Each mirror is coated with pure evaporated aluminium which immediately forms an oxide layer on contact with the air. Apart from this, there is no overcoating. The primary mirror is aluminized once a year, usually around full moon in midsummer (January or February).
The primary mirror has multiple controlled pressure pads for axial support and counter-balanced push-pull lever radial supports. The secondaries and the two largest coudé flats are supported radially by mercury-filled rubber girdles and axially by controlled vacuum (or pressure) behind the mirrors.
The surface of the primary mirror deviates from the nominal ideal (a hyperboloid with conic constant 1.1717) by no more than 0.06um (from wave-shearing interferometer tests and Hartmann tests at the maker's works). With a perfect secondary, this gives a geometrical image with 80% of the light within 0.3" diameter and 99% within 0.65" diameter. The secondary mirrors are hyperboloids, and the geometrical image sizes derived from the maker's tests (with a modified Hindle sphere and wave-shearing interferometer), assuming a perfect primary mirror, are listed in Table 2.1.
Table 2.1: Geometrical image diameter at the AAT
The tube centre section, from which the Serrurier trusses spring, is carried on roller bearings inside a large horseshoe. The horseshoe is joined to the north journal by two rectangular struts, forming the polar axis assembly which rotates on five main oil pads, two bearing on the outer cylindrical surface of the horseshoe and three on a part-spherical surface on the north journal. Two additional axial thrust pads bear on the rim of the horseshoe to improve the azimuthal stiffness of the polar axis location. The natural frequency of oscillation is about 1.3Hz in hour angle and about 4Hz in declination.
The drive for both hour angle and declination is through straight spur gearing with absolute and incremental optical encoders driven from the main gear wheels. The absolute encoders read position with a bit size of about 1" and the incremental encoders give exactly 20 pulses for each 1" rotation. Drive speeds are controlled by comparing the incremental encoder pulse trains with those from a rate generator, which can be set manually or by the control computer.
A torque motor and tachometer are coupled to the horseshoe through a roller to provide for hour angle damping. Only the tachometer is used, since a damping system feeding this tachometer signal back into the main drive motors has worked very effectively. Whenever short, rapid movements of the telescope are required, the characteristics of the servo loops can be altered to improve transient response at the expense of tracking error.
To maintain focus when the steel framework of the telescope changes length with changes in attitude and temperature, the distance between the primary mirror cell and the fixed top ring is compared against a tensioned invar wire. In response to changes in this distance, computer-derived corrections are made to the focus drive which moves the whole top end structure. The corrections applied are about 0.5mm with changing attitude and about 0.1mm/°C.
To minimise local seeing degradation when the primary mirror is warmer than the air, injection and exhaust fans can be switched on to increase the air flow across the mirror surface and so reduce the air temperature rise above the mirror.
This Page Last updated: Feb 21, 1996, by Chris Tinney.