Introduction

The Coude Echelle Spectrographs

1.1 Introduction

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UCLES & UHRF

The University College London Echelle Spectrograph (UCLES) is located in the east coudé room on the 4th floor of the AAT building. UCLES can record spectra of astronomical sources as faint as V = 16-17 with a resolving power (lambda/(delta lambda)) of 50,000-80,000 depending upon the detector used. The spectrograph is operated entirely under computer control from within the ADAM software environment.

UHRF is a cross-dispersed echelle which provides resolving powers of 300000, 600000 and 1000000, intended primarily for interstellar line studies. UHRF shares pre-slit optics with UCLES, and is controlled from the same software.

The User Manual

The UCL Coude Spectrographs Manual describes the components and operation of UCLES and UHRF on the AAT. This version differs from its predecessor in removing all mention of the IPCS detector and adding more recent CCD information. It has also been streamlined to become more web-friendly.

Those working with archival data may wish to refer to AAO UM 25.2, by Sean Ryan and Adrian Fish which was produced in 1995 following the unification of UCLES and UHRF under a common control system. It draws heavily on the two earlier manuals ``The UCL Echelle Spectrograph" by R. D. Robinson, F. Diego, A. C. Fish, W. F. Lupton, M. Pettini, & D. D. Walker, and ``Ultra High Resolution Facility Operating Manual" by J. Spyromilio.

Development of the Spectrographs

The UCL Coudé Echelle Spectrographs form the high resolution optical spectroscopic facility at the AAT. The Anglo-Australian Telescope (AAT) was originally designed to include a classical photographic coudé spectrograph. The mechanical support structure for a horizontal layout was installed with the telescope, but the spectrograph didn't materialize due to financial pressures. Around the early 1980s, AAT users became increasingly aware of the need for a high resolving power facility and in 1983 the Advisory Committee for Instrumentation at the AAT (ACIAAT) gave a high priority to the construction of an echelle spectrograph.

The f/36 coudé focus was preferred over Cassegrain for several reasons:

Mechanical and thermal stability is guaranteed in this environment and simple mechanical structures and mechanisms can be employed.
The large space available allows a large collimated beam size which can partially overcome the throughput disadvantage due to light losses in the five mirror coudé train.
Optical components can be remotely interchangeable and duplicated with coatings optimised for different wavelength bands.
The coudé environment lends itself to an open, optical bench layout which can accommodate future and unexpected enhancements.
The coudé system can be permanently operational, permitting fast changeover from other foci during the night due to instrument failure, changing observing conditions, flexible scheduling etc.

As a result of ACIAAT's recommendations, UCLES was built by David Walker and his team at the Department of Physics and Astronomy, University College London under contract to the AAO.

In early 1990, the Ultra High Resolution Facility (UHRF) was added to the coudé spectrograph. UHRF was built by the Optical Sciences Laboratory of University College London, funded by a grant from SERC and the AAO.

Echelle Spectrographs

An echelle operates at high blaze angle (~63 deg) and order numbers (50 - 150).
The blaze angle is the angle between the surface of the grating and the reflective groove faces.
The high blaze angle permits a comparatively wide spectrograph entrance slit and therefore better light throughput at a given resolution.
The high order numbers result in many orders overlapping, but these can be separated by cross-dispersing, thus giving large spectral coverage while maintaining high spectral resolution.

Wavelengths which overlap in the undispersed echelle spectrum are related by

n₁W₁ = n₂W₂ = ... = n_iW_i

where n_i are the order numbers. The wavelengths W_i thus occupy similar positions in adjacent orders of the cross-dispersed echelle. A given wavelength will appear in several orders, but each echelle order has its own intensity profile (blaze profile) resembling a sinc function, so the intensity at the same wavelength will differ in each order. In practice, a wavelength is concentrated in only one or two orders.

The Free Spectral Range (FSR)

The FSR is delineated in each order by two wavelengths which appear at equal distances from the peak of the order, and peaks of adjacent orders. Observations restricted to the FSR would provide complete wavelength coverage without any duplication, whereas if the detector is narrower than the FSR, there will be gaps in the spectral coverage. The length of the FSR on the detector increases proportional to the central wavelength of the order, giving the familiar trapezoidal format of an echelle spectrum. Note that the spectrum does not terminate at the end of the free spectral range - it continues at lower intensity away from the blaze peak.

Dispersion and Resolution

The dispersion in Å mm^-1 is proportional to wavelength, but the resolving power (lambda/delta(lambda)) and the velocity resolution is constant. Also, the dispersion at a given wavelength depends only on the angle at which the grating is used, not on its groove-spacing. Thus, the two echelle gratings available in UCLES (with 31.6 and 79 lines mm^-1) give the SAME dispersion because their blaze angles are the same. The primary difference is in FSR and order separation.

Description of UCLES & UHRF

UCLES

UCLES gives images with FWHM < 20 um over the 38.5 x 18.8 mm field of its 70 cm focal length camera. The resolving power is therefore dictated by the slit width and pixel size. As one example, a detector resolution element of 48 um corresponds to R = 50000, and projects to 0.9" (or 1.2" with the focal modifier) at the slit. Existing CCDs yield maximum resolving powers of ~50000 (Tek) and ~80000 (MITLL2A/MITLL3)
There is a choice of two gratings: 31.6 and 79 lines mm^-1. Both produce the same dispersion. The free spectral range (FSR) and order separation are 2.5 times larger for the 79 lines mm^-1 grating than for the 31 lines mm^-1 grating.
The 31 echelle gives more complete spectral coverage but at the expense of good sky coverage. (The permissible slit length can be increased by 37% by using the focal modifiers) With this echelle, it is possible to cover the full FSR of every order using the MITLL CCDs (or all orders blueward of 6000 Å with the Tek), but with orders separated by only a few arcsec.
With the 79 lines mm^-1 grating, even the MITLL CCDs do not cover the full FSR redward of ~5400 Å, but the order separation is greater, allowing a ~9" long slit in the red and greater in the blue.

Links:
Tables for the 31 grating (Appendix A1) and 79 gratings (Appendix A2) giving the wavelength coverage, free spectral range, dispersion and order separation for each grating.

Echellograms for 31 and 79 gratings indicating the relative sizes of detectors.

Fact sheet summarising the basic properties of UCLES.

Cookbook for UCLES with some useful hints.

UHRF

UHRF has three resolving powers of nominally 300000 (0.3M), 600000 (0.6M), and 1000000 (1.0M - actually 940000). The lower resolving powers are obtained by inserting focal reducing lenses into the UHRF camera optical path, which also change the wavelength coverage. Note that the detector must also be moved to a different mounting point for each resolution, so changing over during the night is not practical.

UHRF has a single echelle with a suite of cross dispersing gratings suitable for different wavelength ranges. The very small wavelength coverage of the detector (typically 2 - 15 Å) dictates most setup parameters. Usually only a single order of spectrum is observed, widened in the spatial direction due to the necessity of using an image slicer to obtain adequate throughput with the small slit width. A few orders can be observed simultaneously at the lowest resolving power (300000), but extra orders rarely contain a wavelength of interest.

Note that since observations are normally made with the detector binned by 4x or 8x to minimise the readout noise contribution, the MITLL3 detector (which does not offer binning) is not suitable for use with UHRF.

Links:
Tables (Appendix A3) giving the wavelength coverage, free spectral range, dispersion and order separation.

Fact sheet summarising the basic properties of UHRF.

Cookbook for UHRF with some useful hints.

Coude Stability

Important Note to Observers:

All spectrograph mechanisms are under software control, and user access to the coudé room is neither necessary nor desirable. Pressure changes affect the refractive index of air in the room, and these will be evident in the data. The user is not permitted to enter the main spectrograph room unless prior authorization from the support staff has been obtained, and then clean room clothing must be worn. Do not switch on the fluorescent lights in the coudé room during a run, or even 12 hours before the observations are due to start, as the walls fluoresce and the dark count will be seriously increased. Use only tungsten lamps while in the room.

The coudé location of the spectrograph provides great stability. For UCLES, random shifts over one hour are <0.01 pixels (peak to peak), and slow drifts are consistent with refractive index changes due to pressure variations. Even when grating settings are changed many times during a night, it is possible to return to a previous setting with an RMS accuracy of 4 um (0.2 pixels).

Repeat exposures with UHRF at R = 1.0M using a stabilized laser have shown the instrument to be stable to +/-0.2 pixels over 30 minutes. To maintain resolution on a star, limit integration times to around 30 minutes to avoid smearing by changes in the heliocentric correction of the star. Although the coudé optical bench floats on airbags to vibrationally isolate it from the hammerhead, which is itself separate from the building, arcs should not be taken while the telescope and/or dome are slewing. It is advisable to bracket stellar exposures by arc calibrations.

The CCD dewars can be filled from a 25 litre storage dewar by activating a switch in the coude ante-room, so it is no longer necessary to disturb the coude room to fill the CCDs. The 25 litre dewar must be replaced every three days.