Chapter 1 - The Telescope & Optics

This chapter provides basic information on the AAT and its performance - both mechanically and optically. In particular, the optical performance of the AAT in combination with its Prime Focus correctors are descussed. Detailed descriptions of the imaging instruments can be found in §3 - The Imaging Cameras.

  1. General Description
  2. Prime Focus
    1. The Aspheric Plate
    2. The Doublet
    3. The Triplet
    4. Central Obscuration and Baffling
    5. The Autoguider
  3. Cassegrain Focus
  4. The LBL f/1 Focal Reducer
  5. Attaching Visiting Imagers to the AAT Prime Focus


Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters

Back Contents Next

Last Contents Next

1.1 General Description

Figure 1.1 shows the basic optical configurations available at the AAT. The primary mirror has a hyperbolic cross-section, a clear aperture of about 3.9 m and a focal length of 12.7 m, 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.  The primary in such a Ritchey-Chrétien system is hyperbolic, so correction is needed at the f/3.3 prime focus even for on-axis work. Without correction the circle of least confusion would be about 8" in diameter. The following prime focus correctors are available:

  1. an aspheric plate,
  2. a doublet, and
  3. a triplet.

Figure 1.1 - AAT Optical Configurations.

CCD imaging is also possible at the f/7.9 Cassegrain focus, which has a 39´ coma free field without further correction. For CCD imaging, however, the plate scale is quite large, so only small fields of view are available (1.1' for a 1024x1024 24um CCD). One advantage of this configuration, however, is that it is simple to switch to, from other Cassegrain observations (eg. with the RGO Spectrograph), making this mode popular with observers who require near-simulateous spectra and images.

All the AAT mirrors are made of Cervit, a glass-ceramic material with a thermal expansion coefficient less than 10-7/°C, 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).  To maintain focus when the steel framework changes length with changes in attitude and temperature, the distance between the primary mirror cell and the fixed top ring is compared with a tensioned invar wire. In response to changes in this distance, computer derived corrections are made to the whole top-end structure. The corrections applied are up to about 0.1mm/°C.

The primary mirror has multiple controlled pressure pads for axial support and counter-balanced push-pull lever radial supports. The f/8 secondary is supported radially by a mercury-filled rubber girdle and axially by controlled vacuum (or pressure) behind the mirror.

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.

 

1.2 The Prime Focus Optics

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 correcting lenses perform well for fields up to 1° in diameter. The optical configuration of the three correctors available for prime focus imaging are shown in Figure 1.2.

The new 2dF top-end corrector allows fibre spectroscopy over a 2° field at prime focus, and produces sufficiently good images to allow imaging,  though no imaging system for this corrector is forseen in the near future.

Figure 1.2 - Prime Focus Correctors

1.2.1 The Aspheric Plate

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 is almost useless for CCD imaging.

1.2.2 The Doublet

Fig. 1.3  illustrates the performance of the doublet. It produces images under 0.5 arc sec diameter over its 25' diameter unvignetted field. As indicated by the ray diagram spots for the 320 and 1000nm wavelengths, the doublet performs well over a very wide range of colours and has a flat field. The spot plots, apart from those at 30 arc min radius, are set at the optimum focus for the centre of the field at 550nm. The doublet corrector itself imposes no requirement to re-focus at different  wavelengths - though if your filters are of different thinknesses, this will obviously be necessary.

Despite the increasing vignetting beyond 12.5' radius, the doublet produces images suitable for guiding out to a 30' radius. The spot plots at 30' radius are re-focused to an optimum and include 54 rays, compared with the 100 transmitted on axis. Note that the spot diagrams reveal some radial shift in image position with changing colour.

The two uncoated fused silica elements of the doublet give it a transmission of about 86% from below 300nm to beyond 2um.

With the doublet, the effective focal length is slightly less than that of the primary alone. Near the optical axis it is about 12.58m, yielding a scale about 16.4 arc sec per mm. It has pincushion distortion, so that images of stars at the edge of the field are displaced outwards with respect to the undistorted case near the centre of the field. The distortion reaches about 0.2% 12.5 arc min off axis, and has been satisfactorily modelled by a simple cubic function of the radius (Argue and Sullivan, 1980, Observatory, 100, 152).
Table 1.1: Optical Parameters of the Doublet
Curvature
(/m)
Diameter
(m)
Seperation
(m)
Material
Comments
-0.039370 3.893 Primary Mirror b=-1.1717
12.161500 air
1.573622 0.252
0.007620 fused silica
2.610630 0.249
0.017780 air
0.406693 0.249
0.015240 fused silica
-0.684646 0.248
0.490000 air
0 0.254
0.002 filter glass
0
0.013 air
FOCAL SURFACE

Note: this design strictly applies to the photographic mount. For CCD imaging the
filter will be of a different thickness and at a different location between the last
corrector surface and the focal plane.

Currently, in order for the focal plane to reach the AAO's CCD detectors, the doublet must be raised axially 30mm from its standard position in the camera pedestal. (Otherwise, spherical aberration due to the displacement of the required focus position with respect to the primary mirror would cause an image blur of about 1.2 arc sec diameter). With the doublet raised in this way, a supplementary lens must be fitted to the guider probe to bring focus within its range.

Figure 1.3 Spot diagrams for the doublet corrector, with the axes labelled in millimeters. Like the aspheric plate, the doublet is usable well beyond the spectral range indicated here. (A larger version of this image can be found here)

1.2.3 The Triplet

The triplet was designed to give an unvignetted field of 1° diameter over a wavelength range of 380 to 600nm (ie. the B and V wavebands), though the performance from U through I is acceptable, as shown by the ray tracing spot plots in Fig. 1.4. For the B, V and R bands, focus can be left at the V value as normally found by a focus sequence. However, slightly improved performance can be gained for U and I by re-focusing from the V position, as was done for these plots. The optical design of the triplet is given in Table 1.2, and the top end movement required for best focus is given in Table 1.3
Table 1.2: Optical Parameters of the Triplet

Curvature
(/m)

Diameter
(m)

Seperation
(m)

Material

Comments

-0.039370

3.893

Primary Mirror b=-1.1717

11.894600

air

0.900917

0.455

0.027610

UBK7

0.134768

0.453

0.186385

air

0.582268

0.361

0.007366

UBK7

2.520169

0.346

0.424434

air

1.631041

0.304

0.023546

UBK7

0.184635

0.302

0.172147

air

0

0.254

0.002

filter glass

0

0.013

air

0

FOCAL SURFACE

Note: this design strictly applies to the photographic mount. For CCD imaging the
filter will be of a different thickness and at a different location between the last
corrector surface and the focal plane.

It should be noted that until recently, CCD detectors have not been large enough to justify their use with the triplet corrector. This can be expected to change in the near future. Until that happens, it should be noted that the experience below is based on photographic imaging with the triplet corrector.

A central doughnut-shaped ghost, about 60mm in diameter, may appear appear as an enhancement of a few percent above the sky background if the triplet is used in either the U or I bands. Such a ghost has been observed to arise when photographic imaging is attempted in these bands, when it is caused by a reflection from the emulsion and from the colour filter, returned to the focal plane by the second last surface of the corrector. Similar reflections can be expected from a CCD detector or its dewar window.

 

Table 1.3: Triplet Corrector Best Focus

to use at

change from V
(ie 5500Å)

by

3200Å

increase

0.13 mm

U

increase

0.11 mm

B,V,R

no change needed

I

decrease

0.08 mm

1um

decrease

0.11 mm

2um

decrease

0.33 mm

The second last surface and the last surface of the corrector have been multilayer anti-reflection coated to minimise the ghost in the B and V wavebands. In practice, the ghost image is completely undetectable in the B, V and R passbands. The spectral reflectance of the multi-layer coated surface which contributes to the ghost is shown in Fig. 1.6. With its coated surfaces and 58.5mm thickness of UBK7 on axis, the triplet has a spectral transmission approximately as shown in Fig. 1.7.

The triplet appreciably enlarges the image scale, giving an effective focal length near the axis of about 13.54m, corresponding to an image scale of 15.2 arc sec per mm. It also has pincushion distortion, reaching about 1.4%, 30 arc min off axis, which is a little less at a given radius than the doublet. The distortion can be very closely fitted by a simple polynomial. For spot diagrams calculated at wavelength 550nm, the radius, r, to an image centre is given by:-

r = 13.539 tan ß + A (tanß)3 + B (tanß)5

where:-

ß = angle off axis, A = 2.310 x 103   and B = 2.003 x 106, in metres.

Note that the spot diagrams indicate some radial shift in images with changing colour.

Figure 1.4 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. Axes are in millimeters. (A larger version of this image can be found here)

Figure 1.5. Optical properties of the anti-reflection coatings on the triplet corrector.

Figure 1.6 The optical transmittance of the triplet corrector on axis, with and without the anti-reflection coatings.

1.2.4 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.

1.2.5 The Autoguider

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 1.5. Also shown is the extent of the probe shadow in the focal plane.

A guiding microscope is available for visual guiding, but autoguiding is usually more convenient. Guide star positions can be quickly calculated on-line using the HST Guide Star Catalog. The program which queries the catalog will provide approximate X-Y co-ordinates for acquiring guide stars, which can be relayed to the observer riding in the Prime Focus Cage.

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 (7 minutes). 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. Similarly, if exposures are short enough not to need the autoguider (typically less than 5-10 minutes) and no filter 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  § 3.1 Prime Focus f/3.3 Imaging.



Figure 1.5 - The Areas of the Prime Focus Focal Plane covered by the Autoguider Probe.

1.3 The Cassegrain Optics

Imaging may also be carried at the wide field Ritchey-Chrétien Cassegrain focus; the design parameters are given in Table 1.6. Working at f/7.9, a fully-illuminated, coma-free field of 39 arc minutes diameter is obtained over a 14  x 14 inch square area. The plate scale is 6.7 arc sec per millimeter and the focal plane is curved, concave to the sky, with a radius of 4.369m. This means that for CCD imaging (given CCDs can't be easily bent) the field of view is quite small. In practise, this is not a limitation, as the image scale is so large that a CCD can only see a very small area in any case (With the THX CCD the scale is 0.128"  per pixel and the field size 1.31')


Figure 1.6 - Nominal Optical Data for the Cassegrain Foci.
Table 1.4: Optical Parameters of the f/8 Cassegrain focus
Curvature
(/m)
Diameter
(m)
Seperation
(m)
Asphericity Comments
-0.039370 3.893 -1.1717 Primary Mirror
8.4416
0.068975 1.419 -8.3086 Secondary Mirrot
10.2916
0 0.355
0.003 filter glass
0 0.355
0.028
-0.228885 focal surface

Image quality, as shown in Fig. 1.8, degrades to about 1 arc second at the edge of the field, due mainly to astigmatism. Naturally, there is no chromatic aberration in the all-mirror system.

Figure 1.7 - Cassegrain f/8 and f/15 Spot Diagrams. The lower images show the effects of moving away from the nominal focus as descibed in the text.

A simple `no frills' adaptor has been built to mount one of the AAO CCD cameras with a manually-operated filter wheel at the Cassegrain auxiliary focus. Since it is possible to swap between the main and auxiliary foci at the push of a button (in ~20  seconds), this is useful for programs requiring both spectroscopy and `quick look' imaging, and also enables the best use to be made of periods of photometric weather. Since the filter wheel is not remotely controlled, the present system is best suited to programs requiring photometry through only one filter (which can be any of the standard UBVRI 2 inch square filters).

1.4 The LBL f/1 Focal Reducer

1.4.1 The Optics

The f/1 focal reducer for wide-field imaging was custom-built for the AAT by Applied Physics Specialities, Toronto, Canada, in collaboration with Dr. Carl Pennypacker of Lawrence Berkeley Laboratories (LBL). The focal reducer converts the f/3.3 prime focus beam of the AAT to f/1, and consists of a hyperbolic mirror and three BK-7 glass lenses optimised for imaging in the 5000-9000Å region. Large off-axis chromatic aberrations are seen at redder and bluer wavelengths (e.g. 2 arcsec chromatic aberrations at 8 arcmin off-axis for images taken in the B passband). Sample flat fields (which show the vignetting of the system) can be found in Appendix 7.

1.4.2 Image Quality

The mirror-based design means that the CCD dewar, shutter and filter holder sit inverted below the AAT prime focus cage. Autoguiding is not possible with this setup, and exposures should be limited to less than 5-10 minutes to avoid trailing. A filter holder accommodates up to three filters (generally V, R and I), which can be changed remotely from the AAT control room, so there is no need for an observer to ride in the prime focus cage.

1.4.3 Wavelength Range

The focal reducer is used with a 1024x1024 Thomson CSF THX 31156 CCD. The scale on the CCD is 0.98 arcsec/pixel, giving a total field of view of 16.7x 16.7 arcmin. The overall efficiency of the f/1 system (telescope plus focal reducer plus filter) is roughly 50%, while the Thomson CCD has a detective quantum efficiency of around 40% over the wavelength range 5000-8000Å (see Table 2.2).

1.5 Attaching Visiting Instruments to the AAT Prime Focus

Observers proposing to bring a visiting instrument must obtain approval from the Director before submitting a proposal. Please read the document Guidelines for Visitor Instruments to be used on the Anglo Australian Telescope a copy of which can be obtained here.

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 1.8), 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.

Figure 1.8 - The Prime Focus Camera Mounting Plate

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

A 240V 50 Hz 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, or 70m long to reach from the observing floor directly to the Prime Focus cage.


Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters

Back Contents Next

Last Contents Next

This Page maintained by : Chris Tinney (cgt@aaoepp.aao.gov.au)
This Page last updated:  10 Mar 1996, by Chris Tinney