The Coude Echelle Spectrographs

1.3 Hardware Specifications for UHRF

Overview | Calibration lamps | TV Viewing | Slit / Image Slicer  | Slit Filters | Shutters | Collimators & Hartman Masks | Gratings | Camera

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UHRF 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 UHRF, roughly in order along the light path from telescope to detector.

See Appendix C6 for a complete list of commands.

Summary of the light path: Coude 5 or calibration lamps - slit assembly / slicer - focal modifier - slit filters - slit shutter - collimator & echelle - secondary collimator - cross-dispersing gratings - camera lens - focal reducers - camera shutter - detector

Figure 1.3.1 The optical light path of UHRF

UHRF Optical Light Path

Description of UCLES & UHRF
Coude stability
UHRF fact sheet
UHRF Cookbook

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 during setup and to measure the spectral resolution. The UHRF ThAr and quartz lamps are different units from the ones used for UCLES, and are the default lamps for UHRF selected by ARC QUARTZ/THAR/LASER.

Light from the quartz lamp and laser pass through a telescope pupil simulator, and can be accessed via the slicer or the slit. The UHRF ThAr arc intensity was found to be too low when passing through the calibration lamp optics, and prohibitively long exposure times were required. The ThAr lamp now illuminates the image slicer directly. If the slit is being used instead of the slicer, you can select the UCLES lamps by ARC UQUARTZ/UTHAR.


TV Viewing

The TV is set up in a different position for observations through the UHRF slicer than for UCLES or slit observations with UHRF.

With the slicer, only light reflecting off the face plate of the image slicer is viewed. Ensure that the Beam Rotator is out (BR OUT) or it will hit the TV.

With the slit, 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 filter is used in front of the TV to correct for atmospheric dispersion. Filters need to be manually placed on the TV by the technician - B (4400), V (5500), R (7000) and I (9000) filters are available. The night assistant should run LAMBDA on the telescope to provide an offset to the observed wavelength.

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

Slit / Image Slicer

The image scale at the coudé focus is 1.40" arcsec mm-1, or 712 um to 1 arcsec. The slit is demagnified onto the detector by the factors given in Table 1.3.1; 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 1.3.1 also shows the spatial scale in arcsec per pixel for the Tek (24 um pixels) and the MITLL (15 um pixels). UHRF is normally observed with heavy binning (4x or 8x).

The UCLES slit and the UHRF slicer are located on a moving assembly which allows the user to select either one. UHRF usually employs a confocal image slicer (Diego, F., 1993, Applied Optics, 32, 6284). The entrance aperture is a square hole 1.5" on each side. The slicer cuts the image into 35 slices extending over 20 mm, matching the minimum order separation provided by the cross-dispersing gratings. The output of the slicer is directed to a focal modifier which is automatically selected when UHRF is configured.

To provide an interorder region on the detector, the output from the slicer has been masked down to about 26 slices, so less than the full 1.5" x 1.5" entrance hole is transmitted. The transmission of the slicer and the intensity distribution within each slice depend both on the seeing and the exact pointing of the telescope within the 1.5" aperture, with a maximum efficiency of 15% for the slicer unit.

Because of the illumination pattern of the image slicer, UHRF observers may find it helpful to take flat field exposures through both a long and wide slit and through the slicer, using the former to determine pixel-to-pixel variations, and the latter to identify the geometrical aspects of slicer transmission. Twilight sky exposures are also likely to be helpful.

The UCLES slit is continuously adjustable from 0.1 to 10 mm in width and from 2.5 to 40 mm in length. 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.
Although the slit may be used with UHRF, a width of 40 um = 0.0588 arcsec is required to give a resolution of 1e6.  A mode which has gained popularity, however, is to observe at 3e5 resolution with a 0.6 arcsec slit, resulting in resolution of 100,000 with 4 to 5 times better efficiency than the slicer. A special setup is required, so the intended use of this mode needs to be specified in observing proposals. Note that the slit is heavily vignetted in the spatial direction, with only ~5 arcsec unvignetted.

UHRF has a focal modifier; it is fundamental to the UHRF optical path and is automatically selected when UHRF is configured.

Table 1.3.1 Spatial scales for Tek and MITLL detectors (unbinned) and slit projection factors (SPF).
Setup Scale (Tek) Scale (MITLL) SPF (Spatial) SPF (Spectral)
UHRF 1.0M 0.030"/pixel 0.019"/pixel 0.903 0.891
UHRF 0.6M 0.051"/pixel 0.032"/pixel 1.505 1.485
UHRF 0.3M 0.101"/pixel 0.063"/pixel 3.010 2.970


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 1.3.2 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.

Colour filters are often useful to minimise scattered red light when observing in the blue (e.g. UG11 or BG12) or scattered blue light and order overlap when observing in the red (e.g. RG630). Note that the bluepass filters have strong red leaks and poor transmission.

Because these filters are positioned after the slit they will affect the calibration light as well as the starlight.

Table 1.3.2 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 coudé east anteroom.

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

Collimators & Hartmann Masks

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

The UHRF collimator is a 300 mm diameter fused-silica single element lens. The UHRF echelle grating is mounted inside the collimator housing. Focus of UHRF is achieved by moving the entire assembly. Positioning is to within 50 um over its 450 mm travel. It has two separate focus motions, one coarse (UFC) and one fine (UFF). Both are read off the same encoder, so the requested value is the same independently of which you use to drive the collimator carriage.

Note that UFF has a limited travel of approximately 150 ADU (encoder units) so you generally need to use UFC to approach focus and then use UFF to achieve the desired encoder value. UFF may fail to complete if it is near the end of its travel, or position poorly if a motion less than 15 ADU is requested; in the latter case move UFF away and back by 50 ADU or so.

UHRF has Hartmann masks to aid focussing which consists of two barn doors which can be opened or closed as desired.

Main commands:  UFC n; UFF n; HP UP/DOWN/OTHER/SHUT/OPEN


Cross Dispersing Gratings

Once dispersed by the echelle, light in UHRF passes back through the collimator again and converges as it returns towards the slicer, with a beam separation of 2o. A secondary collimator recollimates the beam at a smaller diameter, onto grating cross dispersers. There is a choice of four gratings blazed for different wavebands: U (2400 lines/mm), B (1800 lines/mm), V (1200 lines/mm), and R (1200 lines/mm). There are also two mirrors called MB and MR which can be used to give the full spectrum.

Echelle Gratings

The resolving element in UHRF is a 31.6 lines/mm R2 204 x 408 mm echelle grating, blazed at 64.95 degrees (measured). See Appendix A3 for tabulations of order separations.


UHRF has a fixed camera lens located close to its cross-dispersing gratings, which focuses the spectrum about 4 m away for a resolving power of 1,000,000. The camera focal length can be reduced by inserting one of two focal reducers; the detector must then be moved to one of two additional focal stations. These combinations yield resolving powers of 300,000 and 600,000.

It is planned to be able to move the UHRF detector position and focal reducing lenses from software, with a command URES. However, at present only the focal reducing lens can be changed in software, with the command UFR, and the detector must be moved by the support staff from the coudé room. Note that therefore changing resolving power during the night is not feasible.

Main commands: UFR 13(1.0M) /15(0.6M) /11(0.3M)

Overview | Calibration lamps | TV Viewing | Slit / Image Slicer  | Slit Filters | Shutters | Collimators & Hartman Masks | Gratings | Camera

Next:  Detector Options | Contents | Home

Ray Stathakis (
Last Update: 10 October 2000