The Research School of Astronomy and

Astrophysics of the Australian National

University at Mt Stromlo Observatory is

developing a wide-field Cassegrain Imager

(Figures 1 & 2) for the new 1.3m SkyMapper

Survey Telescope under construction by

Electro Optics Systems Pty Ltd (Canberra) for

the ANU’s Siding Spring Observatory in NSW,

Australia (Figure 9). The SkyMapper Survey

Telescope is designed as a replacement for

the 50 Inch Great Melbourne Telescope

destroyed in the bushfires at Mt Stromlo

Observatory in 2003.

A. Granlund, P. G. Conroy, S. C.

Keller, A. P. Oates, B. Schmidt,

M. F. Waterson, E. Kowald, M. I.

Dawson

The design of the SkyMapper Imager focal plane is

based on E2V 44-82 deep depletion CCDs. These

devices have 2048 x 4096 15 micron pixels, and

provide a 91% filling factor in our mosaic

configuration of 4 x 8 chips (256mm2, Figure 2). In

addition, the devices have excellent quantum

efficiency over the Survey wavelengths (Figure 12),

near perfect cosmetics, and low-read noise, making

them well suited to the all-sky ultraviolet through

near-IR Southern Sky Survey to be conducted by the

telescope.

The SkyMapper Cassegrain Imager includes an

integrated six-filter changer utilising independent

belt drives (Figure 5). Two banks of pneumatic

cylinders lock the centred filter in place to ensure

photometric repeatability. An additional slide in the

filter housing carries fold mirrors for the

permanently mounted Autoguider and Shack-

Hartmann (S-H) cameras. Data from the S-H system

will be used for remote alignment of the telescope’s

hexapod-mounted secondary mirror.

The array will be controlled using modified versions of the

new IOTA controllers being developed for Pan-STARRS by

Onaka and Tonry et al at the University of Hawaii. These

controllers provide a cost effective, low-volume, high

speed solution for our detector read-out requirements.

The controllers are to be mounted in a custom designed

housing, attached to either side of the CCD Detector

Vacuum Enclosure (Figure 2). Prototype epoxy-filled 640-

pin hermetic feedthroughs (Figures 4 and 8) are being

developed at Mt Stromlo to enable efficient detector

connectivity.

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0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1Wavelength (um)

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RSAA Data

Typical E2VData

Construction of the Imager is proceeding

successfully at Mt Stromlo Observatory, with the

major sub-assemblies undergoing initial fit-

checks (Figures 6 & 7). Characterisation of the

CCD detectors has begun (Figure 12) and

integration with the new controller hardware will

take place later in 2006. The completed Imager

and telescope are planned to begin the Stromlo

Southern Sky Survey in 2007.

Figure 9. Artist’s impression of the SkyMapper Survey Telescope at Siding Spring Observatory.

Figure 10. Custom shutter (280mm square aperture) supplied by the University of Bonn.

Figure 7. The manufactured Imager filter housing prior to chromate finishing.

Figure 5. Imager filter housing, including 7 independent belt drive systems and two banks of pneumatic locks.

Figure 4. An internal view of the back of the CCD mosaic, including cold straps and flex circuits.

Figure 2. A partial section view of the CCD Detector Vacuum Enclosure flanked by customised detector controllers supplied by the University of Hawaii.

Figure 3. Design view of the Imager from below. A tether torque arm carries the instrument cable drape and rotates on a dual-ported helium coupling on the base of the instrument.

Figure 1. Design view of the Imager from above. Its overall dimensions are 1300 x 1250 x 670mm with total mass of less than 300kg.

Figure 11. Two E2V44-82 Detectors being inserted into a test-machined Invar carrier plate.

Figure 6. The partially assembled Imager CCD Detector Vacuum Enclosure prior to chromate finishing.

Research School of Astronomy & Astrophysics, Australian National University, Mt. Stromlo Observatory, Weston, ACT, 2611, Australia.

Figure 12. E2V44-82 detector quantum efficiency measurements from E2V and RSAA.

Figure 8. Epoxy-filled 640-pin hermetic feedthrough prototype developed at Mt Stromlo.

May 2006

The primary scientific aim of the Cassegrain Imager is to conduct the Stromlo Southern Sky Survey: a multi-colour photometric survey of the southern sky. The combined SkyMapper Telescope and Cassegrain Imager has a 5.7 square degree field of view, with good sensitivity from the 340nm-1micron wavelengths. It will automatically acquire data, and possesses an automated data reduction pipeline.

www.mso.anu.edu.au

A large-format imager for the SkyMapper

Survey Telescope