Andrew Charalambous

Final tests and commissioning of the UCL echelle spectrograph

The optical design and performance of the UCLES echelle spectrograph, installed at the f/37.7 coude focus of the 3.9-m AAT in June 1988, are described and illustrated with extensive diagrams, drawings, photographs, and sample spectra. The UCLES operates at 300-1100 nm with resolution 30,000-115,000 and adjustable collimated beam size; it employs either 31.6-g/mm or 79-g/mm echelle gratings and a train of UV-transmitting fused-silica prisms for cross-dispersion. Also discussed are the focal modifier lenses; the Bowen-Walraven image slicer; the commissioning procedures; and preliminary observations of Zeta Oph, planetary nebulae, and Seyfert galaxies.

Conceptual design of the high-resolution optical spectrograph for the Gemini South Telescope

The high resolution optical spectrograph (HROS) for Gemini is currently within its conceptual design phase. The science requirements for this instrument demand spectral resolutions of 50,000 and 120,000 with entrance slits of 0.57 and 0.24 arcsec respectively. Amongst the current large telescope projects, HROS will be the only instrument of its class to be mounted at a Cassegrain station and this will pose considerable technical challenges which are described in this paper: HROS will be a spectrograph with unique characteristics, like prismatic cross-dispersion, immersed echelle grating and active compensation of flexure. HROS is expected to perform better than any other high resolution spectrograph with respect to throughput, resolution and simultaneous spectral coverage.

OSCA - an optimized stellar coronagraph for adaptive optics: description and first light

We describe a coronagraph facility built for use with the 4.2 metre William Herschel Telescope (WHT) and its adaptive optics system (NAOMI). The use of the NAOMI adaptive optics system gives an improved image resolution of ~0.15 arcsec at a wavelength of 2.2 microns. This enables our Optimised Stellar Coronagraph for Adaptive optics (OSCA) to null stellar light with smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA is a fully deployable instrument and when in use leaves the focus of the NAOMI beam unchanged. This enables OSCA to be used in conjunction with a number of instruments already commissioned at the WHT. The main imaging camera to be used with OSCA will be INGRID; a 1024×1024 HgCdTe cooled SWIR detector at the NAOMI focus. OSCA can also be used in conjunction with an integral field spectrograph for imaging at visible wavelengths. OSCA provides a selection of 10 different occulting mask sizes from 0.25 - 2.0 arcsec and some with a novel gaussian profile. There is also a choice of 2 different Lyot stops (pupil plane masks). A dichroic placed before the AO system can give us improved nulling when occulting masks larger than the seeing disk are used. We also present results from initial testing and commissioning at the William Herschel Telescope.

Results from the adaptive optics coronagraph at the William Herschel Telescope

Described here is the design and commissioning of a coronagraph facility for the 4.2 metre William Herschel Telescope (WHT) and its Nasmyth Adaptive Optics system for Multi-purpose Instrumentation (NAOMI). The use of the NAOMI system gives an improved image resolution of � 0.15 arcsecs at a wavelength of 2.2� m. This enables the Optimised Stellar Coronagraph for Adaptive optics (OSCA) to suppress stellar light using smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA provides a selection of 10 different occulting masks with sizes of 0.25 - 2.0 arcsecs in diameter, including two with full greyscale Gaussian profiles. There is also a choice of different sized and shaped Lyot stops (pupil plane masks). Computer simulations of the different coronagraphic options with the NAOMI segmented mirror have relevance for the next generation of highly segmented extremely large telescopes.

Active Flexure Compensation for the HROS Spectrograph

The Cassegrain location of the high resolution optical spectrograph (HROS) for the 8-meter Gemini telescope presents a difficult challenge in controlling mechanical flexure. This is especially the case for a high-resolution spectrography, which requires large and heavy optical components. In HROS, to achieve the required spectrum stability of 2.0 micrometers /hr, we developed a closed-loop monitoring and correction system capable of measuring the spectrograph flexure in as it happens and actively compensating for image motion.

Design of the high-resolution optical spectrograph (HROS) for the Gemini telescope

HROS will be the key instrument for high resolution spectroscopy for UV to near-IR wavelengths at eh Gemini South telescope. The instrument is unique in providing a resolving power of R equals 50,000 at the Cassegrain focus of an 8-meter telescope. Taking advantage of this location, the spectrograph is optimized for high throughput, particularly for the UV region, and high efficiency. Here we present the final opto-mechanical design of the spectrograph, together with its predicted performance. In particular, we show how our design delivers an overall peak throughput of almost 30 percent and coverage of wavelengths between 325 and 885 nm in a single CCD exposure. We also discus the development of the design form the science requirements and some of the mechanical issues that drove it to this solution. Finally we report on the current status of optics procurement and testing.

Designing the Gemini high-resolution optical spectrograph structure to meet the flexural performance required at a Cassegrain focus

The HROS is a Cassegrain focus instrument for use on the Gemini South telescope at Cero Pachon, Chile. It is of novel design, using exceptionally large optical components, and subject to the normal high flexural and stability requirements of optical wavelength instrumentation. To meet these requirements while subjecting HROS to the infinite number of gravity vectors found at the Cassegrain focus has resulted in a very difficult design and analysis of the support structure. This paper describes the mechanical design approach to meting these requirements and presents the flexural performance predictions for the structure as given by Finite Element Analysis.

Engineering solution for the High-Resolution Optical Spectrograph on Gemini South

The High Resolution Optical Spectrograph (HROS) will operate at the Cassegrain focus of the Gemini South telescope, at a resolving power of R equals 50,000. It will use an immersed echelle for dispersion, and fused-silica prisms for cross- dispersion. The instrument will weigh two tonnes and will need to meet stringent engineering performance requirements. This paper describes these requirements and the engineering approach used in the HROS design.

Reexamination of high-resolution spectrographs for large telescopes: the Cassegrain solution

We define the stability requirements for a high-resolution spectrograph, then show how these can be met at Cassegrain by modern materials, mechanism design, thermal control, and passive and active compensation for structural flexure. We consider the optimization of the information throughput of the spectrograph, in terms of slit-throughput, with the superb imaging performance of modern large telescopes and sites, new developments in image slicers, the prospects for adaptive-optics feeds for spectrographs, and the internal transmission of the optics. We consider in detail the requirements of, and solutions for, high resolution spectrographs for two large telescope projects - the 6.5 m MMT conversion and the two Gemini 8 m telescopes.

General Philosophy For The Control And Data Reduction For The Anglo-Australian And William Herschel Telescopes' Echelle Spectrographs

Two almost identical echelle spectrographs are under construction: for the 3.9m Anglo-Australian (AAT) and the 4.2m William Herschel (WHT) telescopes. The versatility of the optical design requires the simultaneous control of many mechanical functions, so a high level of automation under computer control is essential. The control and data reduction software packages will be highly interactive and will be implemented at the observatories and at home institutions, allowing prospective users to test the feasibility of proposed observing programs. Then, at the observatory, the accumulating data can be preliminarily reduced allowing real-time decisions to be made. The data reduction package should find applications to echelle data generally and the control system may easily be adapted to other instruments.