Francisco Diego

Confocal image slicer

A confocal image slicer for use in high-resolution spectrography in astrophysics is presented. The deviceimproves the light transmission of a high-resolution spectrograph by an (unprecedented) order of magnitude. The production of a prototype is described, and the first astronomical results obtained with the Anglo-Australian Telescope are presented. The confocal image slicer is being patented and may find useful applications in existing high-resolution spectrographs and in similar instruments planned for the new generation of very large astronomical telescopes.

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.

Total solar eclipses: magic, science and wonder

Imagine that we are in the middle of a field, thousands of years ago. It is a beautiful summers day: blue sky, no clouds, no wind. At noon, it feels quite hot. Then we sense the temperature falling, and the light fading, but it is still a beautiful day – blue sky, no clouds, no wind. Soon the air turns much cooler and the sky much darker. There are still no clouds, but a gentle, cool breeze has picked up. The Sun is still too bright to look at, but it is fading away, and near to it we can see bright planets, as we would at night. The western horizon turns a dark shade of blue-grey, and then, in just a few short seconds, darkness invades the landscape. We look up in terror and see that the Sun has been replaced by a black disc, surrounded by what seems like a ghostly veil of white smoke or steam. The Sun – the most important thing in the sky, our source of light and heat, our clock and calendar – is no longer there.

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.

Echelle spectrographs for 8-m class telescopes

The modifications required to construct an echelle spectrograph similar to the UCLES (currently in use on the 3.9-m AAT and described by Walker et al., 1986) for an 8-m-class telescope are discussed in detail and illustrated with diagrams. The design and operational parameters of the UCLES are reviewed, and a direct factor-of-two linear scaling of the UCLES for an 8-m telescope is outlined, with particular attention to a mosaicked CCD detector array and echelle configuration, the relationship between the field rotation problem and the telescope secondary focus ratio, the feasibility of a Littrow layout with prisms in double pass immediately ahead of the echelle, prism mosaics as a low-cost approach to prismatic cross-dispersion, the characteristics and application of the 79 g/mm R4 echelle, and the possible use of white-light pupil imaging, as proposed by Baranne (1972).

High Resolution Echelle Spectrographs For The Ang Lo-Australian Telescope Coude Focus And William Herschel Telescope Nasmyth Focus

The echelle spectrographs for the Anglo-Australian Telescope (AAT) and for the William Herschel Telescope (WHT) are being developed following the same basic design, which in many respects, differs from previous echelle instruments. The purpose of this paper is to document some of the more innovative concepts for the benefit of future designers and prospective observers. The instruments were originally conceived around the idea of matching the spectrum format to the features of a particular detector - the University College London Image Photon Counting System (UCL-IPCS). Since construction started, very large CCDs have appeared on the horizon and this has called for certain modifications which are also discussed. Attention is drawn to various tactics for maximising throughput, including the new concept of a variable beam size. The arguments in favor of very large fused silica prisms for cross dispersion are summarised. The echellogram will be displaced in both dispersion directions on the detector and the method of accomplishing this is outlined. A novel automatic check of collimation is also described, together with an overview of important spectrograph functions such as those in the pre-slit area.

Toward total transmission: the Confocal Image Slicer

This paper describes an image slicer based on a modified version of the Bowen-Walraven slicer. The new design is called Confocal Image Slicer, and delivers a large number of slices, all in focus. This allows the slicing of a wider area of the seeing disk, to the benefit of very high resolution spectrographs like the Ultra-High-Resolution Facility, where the slicer gives access to stars up to 3 magnitudes fainter than the limit imposed by a narrow slit for the same resolution and signal to noise ratio. The basic principle and design philosophy of the confocal image slicer are presented, together with a description of the development of a prototype and a final version now fully commissioned at the Anglo-Australian telescope. Some astrophysical results are also included.

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.

The Sun in eclipse

To fully appreciate the spectacle which is an eclipse the observer needs to know a little about the Sun, its structure and what is expected to occur on 11 August. This article links the structure of the Sun to the magic of a total solar eclipse.

Ultra-stable high resolution spectrographs for large telescopes

This short contribution presents the demanding requirements of scientific cases for ultra-high stability spectrography, from the study of subtle radial velocity changes induced by asteroseismology to the ones produced by extra-solar planetary companions. The analysis of physical conditions in cool interstellar clouds is presented as a typical application of ultra-high resolution spectrography. The main technical challenges associated with such instruments are outlined, including focal stations and light feeding methods. As a possible way to combine both modes in a single instrument, we describe a design case for the ultra-stable high resolution spectrograph that we are proposing for the Gemini south telescope, specified to detect radial velocity variations down to one meter per second and also to achieve near diffraction- limited spectral resolution approaching one million. This versatile instrument would occupy an insulated room in the pier of the telescope and would be fed initially by an optical fiber coming from the Cassegrain focal station.