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Cassegrain focus

Focal ratios

Three Cassegrain focal ratios are available; the f/8 Ritchey-Chrétien, f/15 and the f/36 chopping secondary (which mounts in the prime focus cage once the prime focus camera is removed).

Dimensional data for the Cassegrain mirrors are shown in Figure 2.8, while Figure 2.9 shows spot diagrams for various configurations.

 
Figure 2.8: Nominal optical data for Cassegrain foci

 
Table 2.2: Nominal optical data for Cassegrain and coudé foci

 
Figure 2.9: Spot diagrams for Cassegrain foci. The bottom pair of images shows the effect of moving away from the nominal focus, as discussed in § 2.3.5.  

Central obscuration and baffling

The obscuration with the f/8 top end is 1680mm diameter (19%) and with the f/15-36, 1450mm diameter (14%). Note that for the f/15-36 top end, the central baffle obscuration is greater than that of the top end. Support vanes obstruct about a further 1% of the primary area.

With the exception of the chopping secondary, the baffling is designed to exclude direct paths from the sky to the working fields. For f/15, it is necessary to close down an auxiliary sky baffle within the central sky baffle (operated from the console). This is not advisable for infrared work, since the telescope is brighter than the sky.

Acquisition and guiding assembly

The Cassegrain focus is almost never used without the acquisition unit and the guiding unit in place. These units, which together comprise the A&G assembly, are depicted in Figure 2.10.

 
Figure 2.10: Cassegrain acquisition and guiding assembly

The acquisition unit bolts directly to the primary mirror cell and includes a large bearing and rotation drive. The position angle of the unit is encoded to an accuracy of 0.01°. It can also be read directly from the circular scale to about 0.1° accuracy.

The acquisition unit houses a mirror carriage which can be remotely set to three positions, redirecting the telescope beam to auxiliary focus 1 or 2, or allowing the beam to pass directly to the main Cassegrain focus below the guiding unit.

Auxiliary focus 1 has an unvignetted field of 100mm diameter, and can be used for photography with 5x7 inch plates. Other small instruments such as a high-speed photometer or auxiliary CCD can also be used here, and the RGO spectrograph can be fed from the auxiliary focus using the FOCAP fibre system with specially drilled plates and a 12' field. In some cases, these auxiliary instruments may conflict with using the TV to view instrument apertures (see below) as it is necessary to remove the flip mirror to obtain the full unvignetted field. The flip mirror may be left in if the instrument does not use a field of more than about 20mm. Using auxiliary focus 1 restricts the available area for finding guide stars to that shown in Figure 2.11.

 
Figure 2.11: Cassegrain A&G unit: vignetting of and by probes.
(Click on the image for a larger version)

A high sensitivity television camera is normally mounted at auxiliary focus 2. It is parfocal with the standard instrument aperture position (152.4mm below the instrument attachment plane) but can be adjusted to bring instruments up to 900mm below the attachment plane approximately parfocal.

When the mirror carriage is positioned to direct the starlight to auxiliary focus 2, the TV camera is said to be in `TV direct' mode. A six-position wheel accommodating filters 50.8mm square and driven from the console is mounted ahead of the TV camera. Various colour filters are mounted on this wheel, along with a x5 relay lens (for accurate measurements of seeing) and a knife edge.

The TV direct field is about 2.7' x 3.6' 2 at f/8, and about 1.4' x 1.9' at f/15. With the f/36 chopping secondary, a re-imaging lens is used in front of the TV camera to provide a field about 1.4' x 1.1'. With good seeing and a dark sky, stars of magnitude 21.5 are detectable with about 5 seconds of integration at f/8 and 10 seconds at f/15.

As well as directly observing the Cassegrain field, the TV camera can be used with relay lenses either to observe the field reflected from an instrument aperture, (e.g. from the slit jaws and dekker of a spectrograph), or to observe the field transmitted through an instrument aperture. In both cases the instrument must be at the standard focus position (within about 3mm).

The guiding unit bolts directly to the lower face of the acquisition unit and houses two offset guiding probes. Figure 2.11 shows the ranges over which the probes can be set and the penumbra outline resulting at the focal plane. The convention adopted for encoding X and Y is that the axis of rotation of the A&G assembly corresponds to the point X = 200mm, Y = 600mm. This provides for all-positive, non-overlapping ranges in X and Y. The direction convention is that, referred to the celestial sphere, +Y is north and +X west when the position angle is zero. For position angles increasing from zero, the Y direction moves eastward from north. Thus, with PA = 135° for example, +Y is to the SE and +X to the NE.

The mechanical probe motions in X, Y and Z are encoded with accuracies of 0.01mm, 0.01mm and 0.05mm respectively. The stability and repeatability of any X or Y setting should be better than ±0.02mm for the full range of telescope attitude. The absolute accuracy of an offset should be ±(0.025 + d/2000) mm where d is the amount of the offset from the axis.

Light intercepted by a probe can be brought to a focus either at the photocathode on an image dissector for autoguiding, or at a graticule (observed visually through an optical train which accommodates movements in X and Y and brings the image to an eyepiece fixed on the guiding unit case). By interchanging two prisms (selected remotely) the beam can be directed either wholly to the image dissector (ID) or 90% to the eyepiece and 10% to the ID.

Only probe 2 is used for guiding, probe 1 carries a Pockels cell or waveplate polarimeter.

Chopping secondary

The f/36 chopping secondary, built by the University of Melbourne, is mounted in place of the camera and corrector in the prime focus top end, along with reflecting baffles for infrared work. Table 2.2 gives nominal optical data for the mirror. The secondary is undersized so that the beam is contained within the surface of the primary for deviations of up to 2'  from the rotator axis, but displacements of up to 3'  are possible.

 
Figure 2.12: Cassegrain cage: underside view of the instrument support trusses
(Click on the image for a larger version).

The chopper can operate at frequencies of up to 80Hz and peak-to-peak beam throws of up to 360". The mid-point of throw can be offset from the telescope axis along the throw direction with the constraint that neither beam can deviate from the axis by more than 180". The rise time is 3ms with 10" and 25ms with 360" throws. There are several modes of operation:

  1. At an internally generated frequency.
  2. From an external level which dictates the chopper state as beam A or B.
  3. From an arbitrary waveform such that the axial displacement is less than 3'.

In mode (1) a synchronising signal is output; the waveforms required in (2) and (3) can be supplied by visitors' instrumentation. The chopper can be controlled by hand switches or a keyboard in the control room, or from a hand paddle in the Cassegrain cage. The operating parameters can be displayed on a VDU, and the waveform on an oscilloscope.

 
Figure 2.13: Cassegrain cage: approximate dimensions (in mm)

Attaching special instruments at Cassegrain

Below the A&G unit are four trusses for attaching instruments (see Figure 2.12). These trusses hang from near the top of the guiding unit case and, being relatively flexible in the radial direction, will accommodate differential thermal expansion between the steel guiding unit case and the instrument while maintaining the instrument accurately central. The trusses lend virtually no rigidity to the attached instrument except in the direction parallel to the telescope axis. The total weight of any instrument should not exceed 700kg. The amount of space available in the Cassegrain cage is also shown schematically in Figure 2.13.

The Cassegrain focus falls nominally 152.4mm behind the attachment plane of the instrument support trusses. The secondary mirror focus range provides adjustment of the f/8 focus from 300mm above to 300mm below nominal and the f/15 focus from 400mm above to 1400mm below nominal. (See Figure 2.9 for the effect on aberrations). For optimum use of the guiding unit offset probes the focus should lie within about ±10mm of the nominal plane, but it is possible to design an auxiliary relay lens for the probe to accommodate most of the usable range of focus positions.

Instrument builders should note that there are several adapter plates available to bridge the gap between smaller instruments and the A&G mounting boltholes.



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Next: Coudé focus Up: The Anglo-Australian Telescope Previous: Prime focus



This Page Last updated: Feb 21, 1996, by Chris Tinney.