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Prime focus


A corrector is needed at prime focus even for on-axis work, since without correction the circle of least confusion at prime focus is only about 8" wide. The correction of coma across a wide field is much easier with the hyperboloidal primary than with a paraboloid, and the existing correcting lenses perform well for fields up to 1°  in diameter. A new top end corrector now under construction will allow fibre spectroscopy over a 2° field at prime focus.

Figure 2.3: The optical configurations and field sizes of the three existing prime focus correctors, drawn to the same scale. The figure of the aspheric plate has been exaggerated in this diagram.

Three prime focus correctors are currently available, with fields and scales as shown in Figure 2.3. A fourth has been constructed for the  for the Two-degree Field project. Spot diagrams for the doublet and triplet correctors, computed for various field angles and wavelengths, are shown in Figures 2.4 and 2.5. Chapter 1 of AAO UM 5 ( Photography with the AAT by David F. Malin), from which these diagrams are taken, discusses the correctors in more detail and also gives some information about photography at the f/8 Ritchey-Chrétien Cassegrain focus.

The aspheric plate produces images with diameters less than 0.5"over a flat field of 7' (25mm) diameter and over a curved field 10' (37mm) in diameter (concave to the primary mirror with radius 1.308m). Without some special provision, the images are badly aberrated at the minimum field angle usable by the offset guider without shadowing the 10' field. The image at 15' radius is more than 4" across (largely due to astigmatism) even when the focus is readjusted to an optimum. As a result, the aspheric plate has been virtually unused.

The doublet produces images under 0.5" diameter over its unvignetted field of 25' diameter, and also produces images acceptable for guiding out to 1° diameter. The spot plot at 30'  radius (Figure 2.4) is re-focused to an optimum and includes 54 rays, compared with the 100 transmitted on axis. The performance deteriorates very little over the whole photographic wavelength range.

Figure 2.4: Spot diagrams for the doublet corrector, with the axes labelled in mm. Image quality is good well beyond the spectral range indicated here.

The triplet is designed for a field diameter of 1° and a wavelength range 380 - 600nm (i.e. including the B and V wavebands), but the performance from U through to I is acceptable. A central ghost about 60mm in diameter, corresponding to an image of the primary mirror and formed by reflection from the colour filter and the second last surface of the corrector, appears if the triplet is used in U or I band. The ghost also appears if O or J plates are exposed without a filter. To minimise this effect in the B and V wavebands, the last two surfaces of the corrector have been multilayer anti-reflection coated. Most filters are also anti-reflection coated. In practice, the ghost image is undetectable in the B, V and R passbands.

Figure 2.5: Spot diagrams for the triplet corrector. Images are usable for guiding at all the wavelengths illustrated here, even at the edge of the 1°  field.  (Click here to load a larger version of this image.)

Central obscuration and baffling

The profile of the prime focus cage is a semi-circle with diameter 1450mm centred on the optical axis and joined to a semi-circle of the same size displaced 115mm from the axis. The resulting area obscuration is 15%. Support vanes obstruct about a further 1% of the primary area.

Prime focus offset guider

The guide probes and filters in the prime focus cage are operated by hand, so an observer needs to ride in the cage for all but the simplest operations.

The prime focus camera has an offset guiding microscope adjustable over the ranges shown in Figure 2.6. Also shown is the extent of the probe shadow in the focal plane. Two interchangeable probes are provided to double the total range attainable in Y.

The short probe is usually preferable to the long, since it intrudes less into the camera field. When used with the doublet corrector, the short probe sees an astigmatic, vignetted image of the guide star but the distortion is not a serious problem. The long probe must be used with the aspheric plate, since image quality deteriorates rapidly off axis. An auxiliary relay lens is needed in front of the probe to accommodate the non-standard focus positions of the CCD systems.

A guiding microscope is available for visual guiding, but autoguiding is usually more convenient. It is best to pre-calculate guide star positions before observing, using the utility program GS (Interdata) or AATGS (VAX). There is also a plastic overlay for Schmidt sky survey films which allows an observer to read off rough X and Y positions for guide stars. Guide stars should generally be brighter than 14th magnitude, though stars down to 15th magnitude can be used in good conditions.

Figure 2.6: Prime focus camera: area covered by probes

Observers usually prefer to stay in the cage during observing so as not to lose time by lengthy slews to and from prime focus access. If extremely long exposures are planned, however, it is possible to set the autoguider probes, return to PF access and then have the telescope return to the object without the observer. This process typically takes about 7 minutes. Similarly, if exposures are short enough not to need the autoguider (typically less than 5-10 minutes), and no filter or plate changing is required (for example, when observing with a CCD and a single filter), then no observer is needed in the cage.

For more information on the prime focus camera and autoguider, see Photography at the AAT (AAO UM 5) and the AAT CCD Users' Guide (AAO UM 17).

Figure 2.7: Prime focus camera mounting plate for special instruments
(Click on the image for a larger version of this figure)

Attaching special instruments at prime focus

Instruments weighing up to 50kg can be mounted on a face plate in the camera, which is 5mm above the optimum focal plane. The camera and corrector assembly may be driven axially up to 45mm from optimum focus with consequent loss of image quality. To compensate in part for this, the doublet corrector can be raised 30mm. There are four mounting holes in the face plate (Figure 2.7), which form a rectangle 11 inches in the X direction by 14¼  inches in the Y direction, centered on the optical axis. The threaded holes accept 3/4  inch long, 3/8  inch diameter UNC screws. Clearance below the mounting face is set first by the filter slide at approximately 8mm, then by the roller blind 40mm below the mounting face, then by the guide probe whose top surface is 45mm below it.

Instruments may not protrude from the mounting plate by more than 1.0m along the optical axis (or else they foul the dome structure). In the first 10cm from the mounting face, the instrument will not foul any part of the camera controls if it lies within a cylinder of radius 15cm from the optical axis. Beyond the first 10cm the instrument may swell to a 40cm radius from the optical axis without fouling any part of the prime focus cage. It may then, however, be difficult to access the guide probe and other controls or to fit the observer into the cage. If large instruments are contemplated for the prime focus, intending users should consult the AAO engineering staff.

Finally, it is possible to remove the camera head altogether and mount a heavier and bulkier instrument on the corrector assembly.

A 240volt 50Hz single phase AC outlet is available in the prime focus cage wall. There are also three 75-way and one 50-way twisted-pair cables, and a 75 ohm coaxial cable available between the prime focus cage, the Cassegrain cage and the control room. Additional cabling or piping needs to be at least 20m long to reach from the Cassegrain cage.

next up previous contents index
Next: Cassegrain focus Up: The Anglo-Australian Telescope Previous: General description

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