The Coude Echelle Spectrographs

1.2 Hardware Specifications for UCLES

Overview | Beam rotator | Calibration lamps | TV Viewing | Slit | Periscopes | Focal Modifiers | Slit Filters | Shutters | Collimators & Hartman Masks | Cross Dispersing Prisms | Echelle gratings | Camera

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UCLES is located in the Coude East room and pre-slit area. Some optics are shared between UCLES and UHRF.
Normally only the technician has access to these areas (see warning).
The SYNTEL microprocessor system that interfaces the VAX to the instrument is located in the Coude East anteroom together with the spectrograph power supplies, CCD controller rack and shutter overrides.

The following sections give a description and list software commands for the components of UCLES, roughly in order along the light path from telescope to detector.

For a summary of commands see the Cookbook or Appendix C6 for a complete list.

Summary of the light path: Coude 5 or calibration lamps - beam rotator - slit assembly - focal modifiers - slit filters - slit shutter - collimator - cross-dispersion prisms - echelle - camera & shutter - detector.

Figure 1.2.1 The optical path of UCLES

Optical Path UCLES

Useful Links:
Description of UCLES & UHRF
Coude stability
UCLES fact sheet
UCLES Cookbook

Beam Rotator

An image rotator is available in front of the slit to compensate for field rotation at the coudé focus, i.e. it minimises the shift of image position with wavelength due to atmospheric dispersion. It is a fused silica Dove prism with anti-reflection coated silica windows optically bonded to each surface. The throughput of the 1 arcmin field is ~96%. (However, due to slight misalignments in the optics, the telescope pupil is moved in the UCLES camera, causing vignetting losses of up to 30%. Efforts to correct this have been only partially successful.)

Since the beam rotator introduces an optical element upstream of the slit, the night assistant must be kept informed if it is being moved in and out while observing, as the telescope focus has to be offset each time.

Information concerning the RA and Dec of the telescope is fed from the telescope control computer via CAMAC to the UCLES program on the VAX, and commands are available which cause the slit orientation to be maintained at a given position angle on the sky (TRACKPA pa) or to be kept vertical to the horizon, i.e. at the parallactic angle (VERTICAL). Communication via CAMAC requires that the UCLES software be run on the AAT40A.

The most common use of the beam rotator is to track the parallactic angle, so that the UCLES slit is aligned with atmospherically dispersed images. The value of this, especially for bluer wavelengths, may be judged from Figure 2 which shows the difference in refraction for several wavelengths compared to 5500 Å, near the peak in sensitivity of the acquisition TV. For zenith distances exceeding 25o, a stellar image at 4000 Å will lie on the edge of a 1 arcsec wide slit unless it is orientated close to the parallactic angle.

Figure 1.2.2 Atmospheric disperision

Atmospheric Dispersion


Calibration Lamps

Three calibration lamps are mounted in the pre-slit area - a quartz halogen lamp for flat fields, a ThAr lamp for wavelength calibration arcs,and a stabilized He-Ne 6328.160 Å laser with diffuser disk which is used in setup and to measure the spectral resolution. Lamps are selected with ARC QUARTZ/THAR/LASER. The light from all lamps passes through a telescope pupil simulator.

The UCLES calibration lamp system has two 8 position filter wheels containing neutral density and colour filters. The current filter specifications are found by typing HELP FILTERS. (It is not usually necessary to have lamp filters in the beam with any of the CCDs.)

Table 1.2.1 Calibration Lamp Filters for UCLES as at April 2000
LF1   LF2  
Position Filter Position Filter
2 0.39ND 2 0.19ND
3 0.75ND 3 0.56ND
4 0.93ND 4 BG24
5 1.35ND 5 BG38
6 1.58ND 6 UG5
7 2.14ND 7 CLEAR

Main commands: ARC QUARTZ/THAR/LASER; LF1 n, LF2 n

TV Viewing

The TV can view light reflected off the slit (STAR), or a 1 arcmin field accessed using a mirror mounted on the comparison lamp TV mirror assembly (TV WIDE).

A TV autoguider (operated by the night assistant) is used for all but the shortest exposures.


The image scale at the coudé focus is 1.40 arcsec/mm, or 712 um to 1 arcsec. The slit is demagnified onto the detector by the slit projection factors given in Table 1.2.2; these factors are known to the control software, and slit dimensions can be given in a variety of units: mm, arcsec, or detector pixels. Table 1.2.2 also shows the spatial scale in arcsec per pixel for the Tek (24 um pixels) and MITLL (15 um pixels) detectors.

The slit is continuously adjustable from 0.1 to 10 mm in width and from 2.5 to 40 mm in length. A slit width of 1" projects to 54 um on the detector, or 39 um with the focal modifier. Along the slit length 1" will project to 83 um on the detector, or 63 um with the focal modifier.

It is necessary to limit the slit length to prevent orders overlapping. With the 31.6 lines/mm grating the maximum recommended slit lengths range from ~11" in the blue (3100 Å) to 3.5" in the red (7500 Å). The corresponding values with the 79 lines/mm grating are 2.5 times greater. When setting the spectrograph with CONFIG, the keyword SLAUTO will automatically set the slit length to the maximum value which avoids order overlap and leaves one clear interorder pixel.

Table 1.2.2 : Spatial scales for Tek and MITLL detectors (unbinned) and slit projection factors (SPF)
Setup Scale (Tek) Scale (MITLL) SPF (Spatial) SPF (Spectral)
UCLES 0.29"/pixel 0.18"/pixel 8.63 13.25
UCLES+FM 0.38"/pixel 0.24"/pixel 11.25 18.15



The maximum slit length available with the 31.6 lines/mm grating is often insufficient to provide a clear sky spectrum on the same frame as the object. A periscope assembly is available which redirects light from two apertures 1.5" long and separated on the sky by 40". Each echelle order is therefore split into two sections, one containing light from the object and the other from the sky. Because of the narrow aperture size needed to prevent order overlap with the 31.6 lines/mm grating, there will be a loss of light in poor seeing conditions. There are two periscopes available which are mounted at one end of the dekker slide and are optimized for the red and blue ends of the spectrum. The control program will automatically select the correct one to use. The periscopes automatically use the focal modifiers in order to gain adequate order separation.

Main commands: PERI IN/OUT

Focal Modifiers

The focal modifiers may optionally be used to increase throughput in poor seeing. The focal modifiers reduce the f/ratio of the collimator and increase the beam diameter by 37% from 170 mm to 220 mm. The optics is overfilled, but the new slit projection factors allow the use of a 37% wider and longer slit without reducing the spectral resolution. This results in a net gain in throughput when the seeing FWHM exceeds ~2 arcsec. The extra slit length can make the difference between having well sampled sky rows or not, especially for the 31.6 lines/mm grating.

UCLES has two focal modifiers, optimized for light bluer and redder than 5500Å. The control program will automatically select the correct one to use, unless overridden by a specified colour.

Main commands: FM IN/OUT

Slit Filters

There are two 8 position filter wheels located immediately after the slit. The filter types held in these wheels are shown in Table 1.2.3 which gives the set as at April 2000.  Note that they are changed regularly - the current set can be listed using HELP FILTERS. Contact the observatory in advance of run if specific filters are required for your observation.

Because these filters are positioned after the slit they will affect the calibration light as well as the starlight.
(It is not usually necessary to have filters in the beam with any of the CCDs.)

Table 1.2.3: Slit Filter Specifications.
SF1   SF2  
Position Filter Position Filter
2 0.35ND 2 UG11
3 0.80ND 3 RG630
4 0.96ND 4 GG400
5 1.35ND 5 GG435
6 660/11 6 GG475
7 BG12 7 OG530
8 BG13 8 OG590

Main commands: SF1 n; SF2 n; HELP FILTERS


There are two types of shutter on the spectrographs:

The slit shutter, which opens and closes for each exposure, is positioned in the wall separating the main coudé east room from the pre-slit area. It is controlled through OBSERVER or via a "panic button" in the control room. These controls are in series, so both must be open for the shutter to operate. The slit shutter will normally show SHUT in the spectrograph status display, unless the DISPLAY command is issued while an exposure is in progress, in which case it should show OPEN.

Camera shutters are located directly in front of the detectors and are used as dark slides. They are controlled by switches on a panel in the coudé east anteroom, and also by microswitches on the doors to the main coudé room and to the pre-slit area which close the shutters if either of these doors is opened. If this happens, the camera shutter must be re-opened manually from the panel in the coude east anteroom.

The "panic" indicators (a holdover from the days of the IPCS) are red when shutters are open.

Collimators & Hartmann Masks

UCLES is focussed by adjusting the collimator position along the optical path.

There is a choice of two collimators, coated with aluminium and silver respectively. The software selects the aluminium one for central wavelengths shorter than 4000Å and the silver one at longer central wavelengths, unless overridden. Both are off-axis paraboloids.

A half-circular mask which may be driven in front of the collimator and rotated into either of two positions enables focussing via the Hartmann test.

Main commands: CF x; HP UP/DOWN/OTHER

Cross Dispersing Prisms

Cross-dispersion in UCLES is carried out by a series of three large, composite fused silica prisms, each composed of three slices oiled together. All air-glass surfaces are anti-reflection coated. The optimum prism position is determined automatically by the setup programs to avoid vignetting.

Echelle Gratings

UCLES has two gratings, one with 31.6 and the other with 79 lines/mm, both producing the same dispersion. The 79 lines/mm grating has 2.5 times the free spectral range (FSR) and order separation, but less complete wavelength coverage.  See appendices A1 (31) and A2 (79) for tables of order separations.


The UCLES camera is a folded Schmidt design which incorporates a special field flattening and correcting lens. It has a 70 cm focal length and a field coverage of 38.5 x 18.8 mm.

Overview | Beam rotator | Calibration lamps | TV Viewing | Slit | Periscopes | Focal Modifiers | Slit Filters | Shutters | Collimators & Hartman Masks | Cross Dispersing Prisms | Echelle gratings | Camera

Next:   UHRF Hardware | Detector Options | Contents | Home

Ray Stathakis (
Last Update: 04 October 2000