The Coudé Echelle Spectrograph (UCLES)

This spectrograph was commissioned in June 1988, and allows the spectra of objects as faint as V = 16-17 to be observed with a resolving power () of 60,000 to 115,000 depending on the detector used. The spectrograph uses the f/36 coudé focus and is operated entirely under computer control. Either the IPCS or a CCD can be used as the detector, though in practise the TEK CCD now offers superior performance in all but a few very specialised applications.

Figure 5.2 shows the layout of UCLES, and a full description of the system is given in AAO UM 25.2 : The UCL Echelle Spectrographs. (See the Documentation area of this Manual  or the Documentation area of the AAO WWW pages)

Figure 5.2: UCLES optical path

• The camera is a folded Schmidt design which incorporates a special field flattening and correcting lens. It has a 70cm focal length and a field coverage of 38.5 x  18.8mm.
• Two gratings are available, one with 31.6 and the other with 79 lines/mm, both giving the same dispersion. The main difference between them is that the free spectral range (FSR) and order separation available with the 79 lines/mm grating are 2.5 times those of the 31.6 lines/mm grating. The 31.6 lines/mm grating thus allows full coverage of the free spectral range of an order with the IPCS (standard format) for wavelengths shortward of 7000Å but has the orders pushed closely together, with a minimum order separation of only 3.5" (at around 7500Å). With the 79 lines/mm grating, only a fraction of the free spectral range can be recorded at any time but the minimum order separation is  ~9" (at 7800Å). Thus, in general the 31.6 echelle is to be preferred for observations of bright objects requiring continuous spectral coverage, while the 79 echelle is more suitable whenever accurate sky subtraction is an important consideration.
• The spectrograph has a quartz-halogen continuum source, a deuterium lamp for blue flat fields and a thorium-argon hollow-cathode lamp for wavelength calibration (for an atlas of the arc lamp, see AAO UM 28 and 34: UCLES spectrum of the thorium-argon hollow-cathode lamp (I and II)).
• The optical performance of the spectrograph is remarkably good, giving images with FWHM < 20um over the full field of the IPCS. For this reason it is possible to obtain a (non-standard) high resolution mode by reducing the IPCS pixel size to 10um. This setup yields a resolving power of 115,000. The main disadvantages are the small slit size required (0.35") and the decrease in spectral coverage, since the detector length is effectively reduced by 2/3.
• Because of the large format normally used with the IPCS (2048 x 256), non-linearity effects become evident at counting rates as low as 0.2-0.4 Hz. When observing bright objects it is important that ND filters be inserted to reduce the count rates below this level.
• One of the motivations for building the echelle spectrograph at coudé is the inherent stability of this focal station. Repeated one minute CCD exposures of alpha Centauri taken shortly after the commissioning period showed that random shifts over a period of one hour are no worse than 0.005 pixels (peak to peak), corresponding to velocity shifts of only about 15m/s. Intended improvements in the temperature stabilization of the main coudé east room may further improve this figure.
• The configuring of the spectrograph is also extremely reproducible. The spectrograph can be reset to a specified wavelength to an accuracy of better than 0.3 IPCS pixels. This reduces significantly the frequency with which exposures of the ThAr calibration lamp need to be recorded. For most applications one such exposure every few hours (per wavelength region observed) should be adequate.
• An image rotator is available in front of the slit to compensate for the field rotation at the coudé focus. It can be set to track a specific position on the sky, or to align along the direction of atmospheric dispersion (parallactic angle).