There are two coudé focus rooms. The eastern room (room 1) is dedicated to the UCL echelle spectrograph (UCLES - see AAO UM 25), while the western room (room 2) is essentially free for users' equipment. Figure 2.14 shows the available area in room 2.
Figure 2.14: Plan of AAT coudé room 2 (coudé west)
Figure 2.15: Range of attainable azimuth angles for coudé beam
The mirror train (after the secondary) comprises three flats; coudé number 3 (elliptical) to reflect along the declination axis, coudé number 4 reflecting down the western polar axis strut to intersect the polar axis, and coudé number 5 with its centre at this intersection point. Coudé 5 is gimbal-mounted on its own support pedestal to the north of the telescope base frame, and is linked to a mechanism which can be slaved to the telescope hour angle drive to maintain a fixed horizontal path to the coudé focus. The mechanism can be repositioned to direct the beam over a range of azimuths toward either coudé room 1 or 2 as depicted in Figure 2.15. The top-end focus range allows a star to be focused from about 2m before the coudé 5 centre to about 6m beyond it (i.e. about 4m within the coudé instrument room).
The spot diagrams for the f/36 coudé focus are shown in Figure 2.16, and the relevant optical data in Table 2.2. Because of the flip-mirror arrangement used, obscuration and baffling are the same as for the f/15 Cassegrain focus.
Figure 2.16: Spot diagrams for coudé focus
The geometry of the optical path to the coudé focus means that the field rotates linearly with declination and as a non-linear function of hour angle, the latter function depending on the azimuth of the beam from coudé 5 as shown in Figure 2.17. For small instruments, a rotating instrument mount is available to compensate for this rotation. It is slaved to a digital angle demand from the control computer, and can support about 50kg. Any instrument attached to this rotator should be balanced about the rotator axis.
Figure 2.17: Field rotation at coudé
A small mirror (coudé 6) is available to re-image the f/36 beam to f/15. This allows instruments configured for the f/15 Cassegrain mounting to be set up at coudé.
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