Observational cosmology employing optical surveys often require precise flux calibration. In this context we present SNIFS Calibration Apparatus (SCALA), a flux calibration system developed for the SuperNova Integral Field Spectrograph (SNIFS), operating at the University of Hawaii 2.2 m telescope. SCALA consists of a hexagonal array of 18 small parabolic mirrors distributed over the face of, and feeding parallel light to, the telescope entrance pupil. The mirrors are illuminated by integrating spheres and a wavelength-tunable (from UV to IR) light source, generating light beams with opening angles of 1°. These nearly parallel beams are flat and flux-calibrated at a subpercent level, enabling us to calibrate our “telescope + SNIFS system” at the required precision.
OmegaCAM is the wide-field camera for the VLT Survey Telescope being
completed for ESO's 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.
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.
The 256-Mega-Pixel imager OmegaCAM will become the wide-field camera at the VLT-Survey-Telescope of the ESO Paranal Observatory. The camera will cover 1 square-degree field of view at the 2.6-metre VST telescope with 16k×16k pixel resolution. The opto- and electro-mechanical design is the responsibility of a Dutch-German-Italian consortium whereas the cryogenic detector system is built by ESO. The design phase had been finalized with a successful Final-Design-Review in autumn 2001. Procurement and manufacturing is ongoing till the end of the year 2002 followed by an extensive testing period before Preliminary-Acceptance-in-Europe. The paper will present the camera design including the results of design analyses and performance assessments of which optical and finite-element-analyses will be emphasized. The actual design of large-format optical filters will be addressed as well. Their procurement turned out as a challenging issue.
The 'Bonn University Simultaneous CAmera (BUSCA) is a CCD camera system which allows simultaneous direct imaging of the same sky area in four colors. The optics are designed for an f/8 gem and four 4K X 4K CCDs with 15(mu) pixels covering a field of view of 12 arcmin X 12 arcmin at a 2m class telescope. In September 1998 BUSCA has seen 'First Light'. The instrument is based on three dichroic beam splitters which separate optical wavelength bands such that standard astronomical intermediate-band filter systems can be used. The dichroics are made of plane-parallel glass plates mounted at an angle of 45 degrees. Astigmatism in the transmitted beams is completely canceled by identical plane- parallel glass plates of suitable orientation. BUSCA offers new perspectives in astronomical multicolor photometry: i) The broadband spectral properties of astronomical objects in the optical can be determined with high reliability even in non-photometric atmospheric conditions. ii) Precious observing time is used very efficiently. iii) With the large field of view, extended objects like globular and open star clusters or galaxies are covered in a single exposure. iv) Each exposure results in a complete data set.
In an aplanatic two mirror telescope, spherical and coma aberrations may be introduced if a misalignment of the secondary mirror is present. That misalignment may be intrinsic to telescope, due to small errors in optical design or manufacturing; however it will always be present during the life of the telescope, due to maintenance errors, thermal distortions of the mirror and the structure, mechanical distortions of the mirror mount, mechanical arrangement of the structure and other. An active control of the secondary mirror, allowing free positioning of the secondary, is important to correct such unwanted effects. The Italian Galileo National Telescope is equipped with a secondary mirror supported by an 'hexapod' structure, allowing a complete positioning control. In this paper a strategy for handling the positioning and movement of the exapod support of a secondary mirror will be investigated from two point of view: an analytical and a neural network approach.
We describe the performance of a 2k X 2k, 15 (mu) pixel, buried channel MPP-CCD (Loral FA2048) using different operating modes and the implementation of an anti-blooming clocking technique. The CCD is part of a camera system developed at Bonn University for astronomical wide field photometry and polarimetry. Besides two basic operating modes (partially and non-inverted mode) the multi pinned phase (MPP) design additionally allows a totally inverted mode providing the strongest reduction of dark current. The disadvantage is a low full well capacity of 120000 electrons/pixel which only depends on the small implanted potential offset. As an example for optimization by choosing adequate voltages we show how this original full well capacity can be raised almost by a factor of 2 without decreasing the quality of the read-out. Finally we discuss the physical understanding and technical implementation of anti-blooming and its future application in astronomical photometry. Using clocking rates up to 2 kHz we achieve a minimum anti-blooming efficiency of 400 electrons/sec/pixel/Hz and a low spurious charge generation further reduced by using ramps or intermediate steps in the anti-blooming clocking waveforms.