1. 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 (256mm 2, 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. 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