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UCLES notes


UCLES Light Path Summary

A summary of the optical light path for UCLES is as follows:

Coude 5 or calibration lamps - beam rotator - slit assembly - focal modifier - slit filters - slit shutter - collimator - cross-dispersion-prisms - echelle - camera+shutter - CCD

Some Notes:

Note that the Camera Shutter is located immediately in front of the CCD, and is open all the time unless tripped by one of the interlocks on the coude room door, slit-head access door, etc. The Slit shutter is located in the slit-head wall, and only opens during an exposure. Thus, a dark frame, in which the Camera shutter is open and the Slit shutter closed, will include the dark current contributions from the coude room walls, etc.

Two echelle gratings are available @ 31 lines/mm and 79 lines/mm. The 79 grating gives better order separation and improved sky subtraction. The 31 lines/mm grating gives increased wavelength coverage.

The focal modifier may optionally be used. This increases the throughput in poor seeing by reducing the f/ratio. The focal modifier can be used with a longer slit and so provides more pixels for sky subtraction. Users should consider using the focal modifier with the 31.6 lines/mm grating when the seeing is poorer than around 2 arcsec.

The beam rotator compensates for field rotation at Coude focus, i.e. it minimizes spatial dispersion (shift of image position on slit with wavelength). It may be used to track a given position angle or to track at the parallactic angle. The beam rotator decreases the overall throughput by around 5 per cent.

Two calibration lamps are used - a quartz lamp which is used for flat fields, and the ThAr lamp which is used to take arc spectra.

UCLES has two lamp filter wheels (LF1 and LF2) and two slit filter wheels (SF1 and SF2). Filters are not normally needed with UCLES + CCDs, so these are usually used in the clear position.


UCLES Setup Procedures

UCLES is usually set up by afternoon staff or a support astronomer at the AAT. Visiting astronomers are not expected to know how to setup the instrument.

The UCLES setup is normally done using the 31.6 lines/mm grating. The configuration files can then be used with either grating. The setup procedures require:

(i) rotation of the CCD,
(ii) focussing the collimator
(iii) adjusting the echelle theta, gamma and slit offset angles so that the arc lines near the centre of the detector are sharp and parallel to the frame axis.

CCD Rotation

The CCD is adjusted so that the echelle orders are parallel to the vertical axes of the detector. To find out how much the dewar needs to be rotated by, configure UCLES to the wavelength region of interest, then execute the Rotate.tcl sequence, which takes a quartz frame, and measures the tilt of the orders.

To adjust the CCD, go to the coude ancillary room on the 4th floor and put on protective clothing (hat, boots and coat). Then close the camera shutter and go into the coude east room. Turn the lights on as low as possible or use a torch. At the CCD, wind the micrometer screw until it is at the stop and read the current value. Then loosen the four inner screws on the CCD with a 4 mm Allen key. Back off the CCD a bit. Set the micrometer to new reading, where one full turn = 0.5 mm. For the MITLL3 or EEV2 CCD, a positive value from ROTATION corresponds to a clockwise rotation of the screw and vice-versa. Then rotate the CCD until against stop and retighten the screws. When tight, back off micrometer again from stop. Then turn off room lights, exit the coude east room, and open the camera shutter.

Rerun the Rotate.tcl sequence to check that the CCD is now correctly rotated. The CCD rotation should be accurate to better than 0.1 mm. If necessary, repeat the procedure.

Collimator Focus

The collimator focus varies only slightly over its full range which is approximately from -10.0 mm to +5.8 mm. To determine the optimal focus, four frames are taken with the collimator at its maximum and minimum positions.

Execute the Focus.tcl sequence, which will then report the measured shift between images in a dialog box, as well as the recommended collimator focus. If this is within the allowable range, then enter it in the Collimator Focus field of the UCLES Control GUI. If not, have the afternoon shift technician insert or remove the necessary amount of shims, and repeat the Focus.tcl procedure.

Adjust the echelle angles

In order to place a particular wavelength at the actual centre of the CCD and make it horizontal, it is necessary to identify a prominent arc line in the echellogram. For the MITLL3 CCD, the strong line at 8115 Å is good, while for the EEV2 the line at 4198 Å is better. Edit the sequence file Offsets.tcl to select the appropriate wavelength, then execute it.

The dialog box which appears afterward with the results of the analysis will indicate in which pixel the arc line is thought to be centred. Check that it corresponds to the line marked by the green circle as shown here for the MITLL3 and 8115 Å with the 31.6 l/mm grating:

MITLL3
or here for the EEV2 and 4198 Å with the 31.6 l/mm grating:


EEV2

If not, you may need to tweak the echelle offsets yourself (e.g. a theta offset of +0.1 will move the spectra up by ~150 EEV2 pixels; a gamma offset of +0.1 will move the spectra left by ~150 EEV2 pixels) then repeat the Offsets.tcl until it picks up the right line. Or select a different line which is less-confused but for which you are certain of its identity. Otherwise, click "Accept" and the derived offsets will be entered into the UCLES Control GUI, and applied immediately to the current configuration. You can always adjust them later by right-clicking on the field and entering a new value.


Stuart Ryder, sdr -@- aao.gov.au