From VVV Survey
VISTA in its enclosure (Credit: ESO)
|Location||Paranal Observatory, Atacama desert, Chile|
|Wavelength||0.85 – 2.3 μm (infrared)|
|Secondary dia.||1.24 m|
|Focal length||12.1 m, f/3.26|
The VISTA (Visible and Infrared Survey Telescope for Astronomy) is a 4-metre class wide-field telescope at the Paranal Observatory in Chile. Built by a consortium of United Kingdom universities, it was handed over to the European Southern Observatory in December 2009.
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VISTA is to carry out surveys of the southern sky at near infrared and potentially also visible wavelengths. Such surveys should both return direct scientific results and help select objects for further studies with larger telescopes. There are two related projects: The Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope in Hawaii carries out infrared surveys of the northern sky, and the VLT Survey Telescope in Chile will carry out surveys of the southern sky in visible light. The initial plan was to build two cameras for VISTA, an infrared camera and a camera for visible light. Funding constraints have led to only the infrared camera being built so far.
The project was initiated in 1999 by the VISTA Consortium of 18 universities in the United Kingdom (UK), which obtained funding from a joint infrastructure fund of the UK government and further funding from the Particle Physics and Astronomy Research Council. The project is valued at €46M (£36M).
After considering several sites in Chile, the consortium chose the Paranal Observatory of the European Southern Observatory (ESO), namely a secondary peak 1,500 m from the Very Large Telescope (VLT). The consortium selected the UK Astronomy Technology Centre to take technical responsibility for design and construction of the telescope. Two years later – in 2002 – the UK joined ESO, and VISTA became an in-kind component of the joining fee. The consortium then completed the construction and commissioning of the telescope, and the Science and Technology Facilities Council – on behalf of the UK – hand over the telescope to ESO, for the benefit of astronomers in all its member countries.
The objective to repeatedly image large areas of sky at seeing-limited resolution led to a unique optical design. The primary mirror is a concave hyperboloid with 4.1 m diameter and about f/1 focal ratio. The mirror has a meniscus shape of 17 cm thickness with a central 1.2 m hole to accommodate the camera at the Cassegrain focus. It was cast from Zerodur by Schott in Germany and subsequently polished and figured by LZOS, Moscow. It is the largest mirror of this shape and of such short focal ratio; polishing it took 2 years, which was longer than anticipated. The mirror is supported by a number of actuators (81 on the back and 24 around the edge), which allow its shape to be controlled by computer.
The secondary mirror is a convex hyperboloid of 1.24 m diameter. The combination of the two hyperbolic mirrors makes this a quasi-Ritchey-Chrétien design. The combined focal ratio is about f/3, but the image quality of the two mirrors alone would be poor. The secondary mirror is mounted on a hexapod support so that its position, tip, and tilt are also computer-controlled.
The infrared camera was built by a consortium composed of the Rutherford Appleton Laboratory, the UK Astronomy Technology Centre, and Durham University, and is the world's largest at 2.9 tonnes. Telescope and camera form a single optical design, as the 3 field correction lenses in the camera are essential for the projection of a focussed image of the sky on the detectors. For an infrared camera, it is also vital to block heat radiation from the telescope and dome. This is accomplished by a sequence of cooled baffles in front of the field corrector lenses. Also, the secondary mirror is undersized to avoid edge detectors viewing warm structure outside the edge of the primary; this means the aperture seen by any point in the image plane is 3.7 m. This design requires the camera's vacuum cryostat – which cools the detectors as well as the baffles – to be more than 2 m long, with a front window of 95 cm diameter. A filter wheel just in front of the detectors allows the selection of a particular infrared wavelength range.
Over an area corresponding to 1.65° diameter on the sky, the image plane has 16 arrays of infrared detectors, each array with 2048x2048 pixels of 20 μm size, corresponding on average to 0.34" on the sky. The focal length of 12.1 m combines with the baffled aperture of 3.7 m to a focal ratio of 3.26. The arrays are separated by 90% of their width in one direction and by just under 50% their width in the other direction. A single exposure therefore corresponds to a patchy "paw print" on the sky. To fill the gaps and obtain a conventional image at least six shifted paw prints have to be combined into a "tile", which then is 1.5° by 1.0°.
The image plane of the camera also has wave front detectors used to control the shape of the primary mirror and the position and tip/tilt of the secondary mirror (active optics). This compensates for flexure and ensures a focussed image at all altitudes.
The hilltop where VISTA is located was flattened to erect the enclosure building and an auxiliary building. The auxiliary building includes facilities to wash, strip, and coat the primary mirror. The coating can be in aluminium, or normally in protected silver for better infrared performance. The fixed base of the enclosure supports the rotating steel dome. Two sliding doors form the dome slit. Further dome panels can be opened to increase ventilation, and a wind shield can be deployed to close parts of the slit. During the day, the dome is kept at night-time temperature.
Operation and data flow
On completion, the telescope was handed over to ESO, which has selected six public surveys for VISTA, taking up 75% of the available observing time. Proprietary surveys to occupy the remaining time are proposed to ESO, which will schedule approved proposals for observation. The observations are carried out by operators at the Paranal Observatory, remotely from the VLT control building.
The combination of the large detector array and the short and frequent exposures necessary at infrared wavelengths results in a high data rate of 200-300 GB per night. A quick-look reduction at the Paranal Observatory will be used for daily quality control, but the principal data flow is to transfer the raw data on physical media to ESO headquarters in Garching near Munich, Germany, for ingestion into the data archive. Users can extract paw prints (see above) and pass them through a calibration pipeline to remove instrumental artefacts and to calibrate the astrometry and photometry. The archive data will also be copied to the VISTA Data Flow System in the UK, where the paw prints will be combined into tiles (see above) and where source catalogues will be prepared from these.
Notes and references
- ↑ The VISTA consortium consists of Queen Mary, University of London as consortium leader, Queen's University Belfast, University of Birmingham, University of Cambridge, Cardiff University, University of Central Lancashire, Durham University, University of Edinburgh, University of Hertfordshire, Keele University, University of Leicester, Liverpool John Moores University, University of Nottingham, University of Oxford, University of St Andrews, University of Southampton, University of Sussex, and University College London.
- ↑ "Big red eye is ready - VISTA camera shipped to Paranal". ESO Press Release 04/07, 17 January 2007. Retrieved 23 September 2009.
- ↑ 3.0 3.1 "Survey telescope nears completion". Press Release, Royal Observatory Edinburgh, 17 April 2008. Retrieved 23 September 2009.
- ↑ "United Kingdom to join ESO on July 1, 2002". ESO Press Release 27/01, 5 December 2001. Retrieved 23 September 2009.
- ↑ "VISTA: Pioneering new survey telescope starts work". ESO Press Release 49/09, 11 December 2009. Retrieved 11 December 2009.
- ↑ 6.0 6.1 Emerson, J., McPherson, A., Sutherland, W. (2006). "Visible and Infrared Survey Telescope for Astronomy: Progress report". The Messenger, 126. p.41.
- ↑ 7.0 7.1 7.2 Craig, S.C., McPherson, A. (2003). "VISTA project overview". UK Astronomy Technology Centre. Retrieved 23 September 2009.
- ↑ 8.0 8.1 VISTA web site
- Emerson, J.P., Sutherland, W.J., McPherson, A.M., Craig, S.C., Dalton, G.B., Ward, A.K. (2005). "The Visible & Infrared Survey Telescope for Astronomy". The Messenger, 117. p.27.
- "The ESO survey telescopes". European Southern Observatory. Retrieved 23 September 2009.
- "VISTA - Visible and Infrared Survey Telescope for Astronomy". European Southern Observatory. Retrieved 23 September 2009.
- "VISTA: Pioneering new survey telescope starts work". ESO Press Release 49/09, 11 December 2009. Retrieved 11 December 2009. Includes a number of pictures of or taken by the telescope.