Henning Poschmann

OmegaCAM: wide-field imaging with fine spatial resolution

OmegaCAM is the wide-field camera for the VLT Survey Telescope being completed for ESOs Paranal observatory. The instrument, as well as the telescope, have been designed for very good, natural seeing-limited image quality over a 1 degree field. At the heart of the project are a square-foot photometric shutter, a 12-filter storage/exchange mechanism, a 16k x 16k CCD detector mosaic, and plenty of software for instrument control and data handling, analysis and archiving.

OmegaCAM: the 16k×16k CCD camera for the VLT survey telescope

OmegaCAM, a 16k×16k-pixel wide field optical camera, and the VLT Survey Telescope (VST) that is to host it, will constitute a major sky surveying machine that becomes operational in 2004 at ESO’s Paranal Observatory. It maps one square degree of sky with 0.21 arcsec sized pixels. Both individual programs, including monitoring programs, and large sky survey programs are planned. Here we present the integrated design of the VST-OmegaCAM survey machine, including the hardware (large filters and shutter, cf(4836-34)), the VLT compliant control software (cf(4848-10)) and the strongly procedurized observing and calibration strategies. The strict data taking procedures facilitate pipeline data reduction procedures both for the calibration and the science data. In turn, the strongly procedurized data handling allows European-wide federations of data-products. The ASTRO-WISE consortium aims to provide a survey system that makes this possible. On-the-fly re-processing of archival data on the request of individual users with their own plug-ins or newly derived calibrations sets are facilitated in an internationally distributed system. Compared to the classical more static wide-field image archives the newly designed system is characterized by a much more dynamical type of archiving.

The Omegacam Shutter

The shutter for OmegaCam — the 16K×16K CCD mosaic camera for the 2.6-m ESO VLT Survey Telescope (VST) — is described. The OmegaCam is a high precision photometric slit type shutter with an aperture of 370 mm×292 mm. The device was designed and built at the University of Bonn. Performance measurements show that the shortest exposure time is less than 1 msec and that 100 msec exposures are homogeneous to within ± 0.3%.

BUSCA: A simultaneous 4 colour camera with 4K×4K CCDs

BUSCA (Bonn University Simultaneous Camera) is a facility instrument for the 2.2 m telescope of the Calar Alto Observatory. It is equipped with a set of dichroic filters to separate four wavelength bands and with four individual CCD systems, one for each colour channel. Three 4K×4K 15-μm pixel CCD485S (Lockheed Martin Fairchild Systems) are now in operation and fill three of the four focal planes of 12×12arcmin (60×60 mm). A thinned device has been ordered for the UV-channel. The two main advantages over traditional imaging photometry are: i) precious observing time can be saved ii) atmospheric transmission fluctuations (non-photometric conditions) have almost no effect on colour index determination as colour bands are observed simultaneously. The instrument has been tested at the 1 m telescope of Bonn University’s “Hoher List” Observatory and will have its first Calar Alto run in Spring 2000.

Bonn Shutters for the Largest Mosaic Cameras

Bonn shutters have been developed and implemented in various CCD camera systems with apertures ranging from 110 mm×110 mm (BUSCA, Calar Alto Observatory) to 370 mm×292 mm (OmegaCam, ESO/Paranal). Larger shutters with apertures of up to 480 mm×480 mm (for Pan-STARRS) are now in development. The principles of operation are basically contiguous. The key performance parameter is high timing precision which reduces exposure non-homogeneities to less than 1 msec, i.e. <0.1% at 1 sec exposure time. In this paper we report the current status of this technology, as well as new requirements and challenges.

Performance tests of a DIVA-CCD: before and after proton irradiation

The E2V CCD42-20 NIMO type CCD was tested in view of its use for the german astrometric satellite mission DIVA. As in other astrometric missions (FAME, GAIA) the CCDs will be operated in TDI mode synchronous with the stars drifting across the detectors. At the expected operating temperature, around -30C to -50C, the dark current performance is an important parameter. Radiation induced degradations with respect to dark current and CTE are of particular concern, too. We find that TDI operation reduces the dark current by a factor of ≈30 near the DIVA satellite TDI clock rate (1.4 msec). The detector was irradiated with soft protons, in a first run, with rather weak doses of up to an equivalent 10 MeV fluence of 1.6×108 protons/cm2. The increase in dark current is quite small (4% to 5% maximum at -40C) but seems to vary with temperature (e.g. 2% at -60C). The CTE degradation shows a linear dependance on the radiation dose and the CTE gets worse if the detector temperature gets lower (e.g. for the highest dose: 0.999 98 at -60C and 0.999 95 at -100C). Vertical and horizontal deferred charge patterns show a significant difference. The total mission dose will be about 10x higher and the dark current and CTE values are tentatively extrapolated. The results of this study shall serve as a basis for further irradiation experiments combined with laboratory simulations and numerical modelling. During the course of this study the DIVA project had to be stopped due to lack of funding. But our results are applicable equally well to the proposed SMEX mission AMEX which is based on the DIVA concept.