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ING Annual Report 1998
Previous: Chapter 1 - Scientific Highlights | Up: Table of Contents | Next: Chapter 3 - Telescope OperationChapter 2
New Instrumentation and Enhancements
WILLIAM HERSCHEL TELESCOPE
The most important instrument development project for the WHT is the common-user adaptive optics system, NAOMI, which progressed well during 1998. The NAOMI project aims to deliver a focal plane of the telescope in which wavefront distortions introduced mainly by the atmosphere are corrected to a high degree. To this end, first the rapidly varying wavefront distortions are sensed many times per second. Based on that information, subsequently all adjustable elements of a segmented mirror are positioned in such a way that the light reflected off this mirror is corrected for the distortions originally introduced by the atmosphere. The result will be diffraction limited image quality in the near infra-red, and very substantial improvement of image quality at visible wavelengths. The first instrument that will be used to exploit this adaptive-optics corrected focus will be an infra-red imaging camera. First implementation of NAOMI will use natural guide stars to determine the required wavefront corrections, but future developments could include laser guide stars. NAOMI is a collaborative development between the University of Durham, the UK Astronomy Technology Centre in Edinburgh, and the ING.
At the core of the adaptive optics system is the ELECTRA segmented mirror, developed at Durham University. This segmented mirror consists of 76 elements, each of which can be positioned very quickly and accurately to take out the rapidly changing wavefront distortions. The mirror motions are controlled by actuators, and accurate strain-gauges provide hysteresis compensation. The fully functioning segmented mirror has been successfully tested at the telescope, producing images with a width of only 0.15 arcseconds. Other components of the NAOMI system such as the opto-mechanical chassis and the wavefront sensor have continued to make progress. At the telescope the Nasmyth enclosure is being prepared for future deployment of the adaptive optics system.
The ING Red Imaging Device, INGRID, is the new infra-red camera for the WHT, replacing WHIRCAM. This new camera is based around a 1024 ×1024 elements HgCdTe array from Rockwell and is optimized for a wide field of view at relatively short wavelengths, with good performance expected up to a wavelength of 2.2 microns. This camera will primarily be deployed at the Cassegrain focus of the WHT for direct imaging at a pixel scale of 0.25 arcseconds/pixel, and it will be the principal detector for the NAOMI Adaptive Optics system, where it will provide a pixel scale of 0.04 arcseconds/pixel. Work on the camera has progressed well, although difficulties were encountered due to the closure of the Royal Greenwich Observatory where the project was originally carried out. Commissioning of the camera is expected to take place during the first quarter of 2000.
Multi-object and integral field spectroscopy will become a more important aspect of the instrumentation suit of the WHT. The WYFFOS spectrograph, which has been specially designed to be fed by optical fibres, therefore fulfils a central role. The INTEGRAL coherent fibre feed to the WYFFOS spectrograph at the Nasmyth focus of the WHT successfully passed final commissioning. This fibre feed, a collaboration between the Instituto de Astrofísica de Canarias, the Royal Greenwich Observatory and ING, comprises three science bundles which can be inter-changed quickly and remotely. Field sizes range from 10 to 40 arcseconds. The different fibre core sizes allow observers to make the most efficient use of the prevailing seeing conditions. INTEGRAL was used successfully for scheduled science observations.
The multi-object fibre unit for the prime focus of the WHT, AUTOFIB, also feeding the WYFFOS spectrograph, has undergone major modifications to the chassis and fibre-griper to eliminate problems that had plagued the system. The new system was extensively used with very few problems and much improved positional accuracy of the fibres.
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Left: ELECTRA has a deformable mirror consisting of 76 individual segments. Each of the segments is driven by three (hysteresis corrected) piezoactuators, hence giving it 228-degrees-of- freedom. Hysteresis correction is maintained through strain gages on the piezos. Right: Prof Ian Halliday, PPARC Chief Executive, at the instrumentation laboratory in the WHT during his visit to ING in November 1998. The instrument in the background is AUTOFIB-2. In this photo he appears accompanied by Kevin Dee, Head of Mechanical Engineering, and Prof Carlos Frenk, JSC Chairman, (to the left) and Dr René Rutten, Director of ING (to the right).
Commissioning of the WYFFOS echelle mode was successfully completed in August 1998. In this mode, one of 5 orders is selected with an order-sorting filter, giving a range on the current CCD of between 580 and 250 Å, and highest resolution 0.8 Å.The Instituto de Astrofísica de Canarias is developing a cooled near-infrared spectrograph for the Cassegrain focus of the WHT. This instrument, named LIRIS, will use a Rockwell 1024 ×1024 HgCdTe array covering a large spectral range and a wide spatial field of view. The optical design by the UK-Astronomy Technology Centre has been completed. ING and the IAC are collaborating to achieve a high level of commonality between the data acquisition and instrument control systems of INGRID and LIRIS. Ths ensures fast and full integration of LIRIS in the existing infrastructure.
ISAAC NEWTON TELESCOPE
The INT Wide Field Camera was upgraded from the set original Loral CCDs to four thinned, large format 4k × 2k EEV CCDs. These EEV devices combine much improved data quality with excellent quantum efficiency, and at the same time more than double the field of view. Commissioning of the upgraded array took place in April 1998, leaving a fully operational system for scheduled observing, and making the INT Wide Field Camera the largest optical imager using thinned chip technology.
The Cambridge Institute of Astronomy's CIRSI IR panoramic camera (J, H bands) was fully commissioned at the INT and WHT prime foci. The detector comprises a mosaic of 4 Rockwell 1024 ×1024 pixel devices. This is currently the largest-area near-IR camera in the world, which makes it a highly competitive instrument in the near infrared.
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Upper left: CIRSI camera on the INT prime focus in December 1997. Bottom left: CIRSI on the WHT prime focus in January 1998. Right: Three-colour image of the galaxy cluster Abell 2219 based on a 2.5hr H band exposure obtained with CIRSI combined with B and I band optical CCD images. This image was obtained on the 4.2m William Herschel Telescope in June 1998 and it reveals evidence for lensed features. The observation are being used to trace the dark matter distribution within the cluster. The field of view is 4.8' x 4.8'. DETECTOR ENHANCEMENTS
The main line of detector development concentrated on procurement, testing and commissioning of the new generation of 2k × 4k thinned CCDs from EEV. By the end of the year two of these devices were in regular use on the telescopes and had become the detectors of choice for most of the observations. Their excellent quantum efficiency at short wavelengths, combined with good spatial resolution, low read noise and high cosmetic quality renders these detectors the best currently available for nearly all areas of observations. The final stage of delivery of these devices will include a two-chip mosaic to be deployed at the prime focus of the WHT. Future procurement of new detectors will concentrate on devices with high quantum efficiency at longer wavelengths, and with reduced fringing.
A programme was initiated to replace the current generation of CCD controllers with San Diego State University (SDSU) controllers. These controllers have various benefits over the existing controllers, such as much faster readout speeds, lower susceptibility to pickup noise, are easier to configure, and share commonality with many other observatories (e.g. the Gemini telescopes). The first of these controllers will be deployed at the INT during the summer of 1999.
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