The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) at Mt Stromlo Observatory is developing a wide-field Cassegrain Imager for the new 1.3m SkyMapper Survey Telescope under construction for Siding Spring Observatory, NSW, Australia. The Imager features a fast-readout, low-noise 268 Million pixel CCD mosaic that provides a 5.7 square degree field of view. Given the close relative sizes of the telescope and Imager, the work is proceeding in close collaboration with the telescope's manufacturer, Electro Optics Systems Pty Ltd (Canberra, Australia).
The design of the SkyMapper Imager focal plane is based on E2V (Chelmsford, UK) 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. In addition, the devices have excellent quantum efficiency from 300nm-950nm, 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 array will be controlled using modified versions of the new IOTA controllers being developed for Pan-STARRS by Onaka and Tonry et al. These controllers provide a cost effective, low-volume, high speed solution for our detector read-out requirements. The system will have an integrated 6-filter exchanger, and Shack-Hartmann optics, and will be cooled by closed-cycle helium coolers.
This paper will present the specifications, and opto-mechanical and detector control design of the SkyMapper Imager, including the test results of the detector characterisation and manufacturing progress.
Within the last few years several manufacturers have been producing the 'next generation' of scientific CCDs. These devices have small pixels (approximately 15 micrometers), high UV and broad-band spectral response (greater than 80%), very low readout noise (less than 4 e<SUP>-</SUP> rms), large format (2048*4096) and close butting capability. We present examples of recent data taken on the WHT (at the Roque de los Muchachos Observatory, La Palma) obtained from one such device -- the EEV CCD42 array. The detector has been used for spectroscopy and direct imaging with excellent results. Design and performance details, as well as various special operational modes will be discussed. This device has been adopted for scientific imaging on the Gemini telescopes, as well as several other major observatories -- and so these first operational results should demonstrate the power of these new sensors. Variants of the CCD42 design are now being made to yield slightly different architectures and packaging options. We will compare predicted with actual performance, and discuss characteristics and applications of this new sensor.
The prime focus of the William Herschel telescope (WHT) provides a field of one degree which is to be used for fiber spectroscopy. The WYFFOS spectrograph, based on a Baranne white pupil design, is located on one of the Nasmyth platforms of the telescope and is fed from prime focus by 126, 26 meter long fibers. The system is designed for a wavelength range of 350 nm to 1.1 microns using both transmission and reflection gratings. This paper describes the integration and testing of the spectrograph undertaken in the laboratory. The image quality and spectral resolving power have been measured. The scattered light and amount of cross-talk between adjacent fibers has been assessed. The provision of calibration illumination and facilities for back illumination of the fibers, a requirement needed by the fiber positioner, is discussed.
CCDs are widely used in astronomy for spectroscopy, photometry, astrometry, and many other applications. This array sensor has a periodic structure which defines individual picture elements (pixels). The stability and excellent overall response of this type of detector are well known. Less widely described is the fact that the internal structure of the array gives rise to intrapixel variations in responsivity. These response modulations are particularly relevant when the data is spatially undersampled; a situation that is not entirely uncommon in certain instruments. We have made detailed measurements of the response variations within CCDs, using an experimental arrangement that gave a 2 micron resolution. Optical response has been determined at all sample points within a pixel, at selected wavelengths in the range 400 to 900 nm. Measurements are presented for front-illuminated thick (EEV) devices and backside- illuminated thin (Tek) CCDs. Models of the internal structure have been constructed and used to calculate theoretical response data; these have been compared with the experimental results. An example of an extracted (FOS) spectrum which demonstrates these undersampling effects is discussed.
The evolution, current status, and planned improvements of the FLAIR fiber-optic-link multiobject spectroscopy system used on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory are reviewed and illustrated with diagrams and sample spectra. Consideration is given to the original FLAIR system using photographic film to record spectra, the introduction of a slow-scan cryogenic CCD camera in 1986, the problems of focal-surface coverage and sensitivity in the prototype FLAIR, the improved Panache fiber feed and on-chip pixel binning scheme installed in 1988, and the dispersion options offered by the FLAIR-Panache system. The limitations of the present FLAIR configuration are discussed, along with improvements involving (1) the use of a new spectrograph with Schmidt optics for both collimator and camera and (2) advanced plate-holder and positioner systems making it possible to load FLAIR into the telescope in a few seconds instead of an hour.
The standard CCD used in the camera installed at the Schmidt Telescope has a poor short wavelength response and so all the CCDs are surface coated with a fluorescent dye to partially overcome this problem. In 1988 it was decided that the system's response around 4000 A could be improved further by replacing the FLAIR (fiber linked array image reformatter) fibers with a set of fibers offering superior transmission properties at this wavelength. The introduction of these larger core diameter fibers would have meant, however, accepting a reduction in signal-to-noise as the fibers illuminate more pixels on the CCD. The CCD sequencing was therefore modified to permit pixel binning across the dispersion direction. Recent modifications also provide a detection capability for approximately twice the number of objects, by appending a second CCD detector and correlated double sample processor to the existing sequencer. Both CCDs are operated via a signal controller which can route clocks and video between the detectors and sequencing electronics. Reduction of galaxy data show that FLAIR, combined with a low noise detector, in both single and dual CCD mode, is easily capable of obtaining cross-correlation redshifts in the blue with a high success rate.