Chapter 3. Description of the Spectrograph

Sections: The Spectrograph | Grating Changes | Calibration Lamps | RGO Console (subsections)| Acquisition Light Paths
Previous: Planning the Observing Run | Next: At the Telescope | CONTENTS

3.1 The Spectrograph

The spectrograph is shown from two views in Figure 3.1 as it is seen in the Cassegrain cage. Note that it may be at any rotation angle. The angle shown is 90°.

Figure 3.1a. View of the RGO spectrograph mounted in the Cassegrain, taken from the door of the cage, with the RGO at 90°.
View of the RGO

Figure 3.1b. View of the RGO spectrograph showing the reverse side to 3.1a.
View of RGO, 2

The controls for the dark slide for the 82cm camera is indicated. To close the 82 cm dark slide, just push in. The 25cm camera dark slide control is a rod that extends under the grating box. The echellette control is nearby and looks similar, but is shorter. To close the 25 cm dark slide, make sure that the grating is at 40°, and push up slowly and firmly, then twist a short distance clockwise (it is easier from under the spectrograph). When opening the dark slide, it should twist slightly anti-clockwise at the bottom of the travel.

All hatches are held by spring knobs which are rotated clockwise and pushed in to close. The local RGO control is shown. This is normally disabled during observing, and should have flick switches set to REMOTE, and DISPLAY OFF. The orientation of the slit on the sky is completely adjustable, and with the new A&G unit, there is now little overhead.

Note that whenever possible all changes in the Cassegrain cage should be made by the technical staff.

3.2 Grating Changes

Grating changes take about 5 minutes, plus the time needed to move the telescope to zenith, so grating changes during the night are feasible. However, all configurations should be set up during the afternoon as the focus required can vary significantly.

Changing of gratings is done by technical or support staff. The gratings are stored in the dome, on shelves on your right as you enter the dome towards the Cassegrain cage. There may also be in the three storage positions in the RGO spectrograph. However, they are shared by 2DF, SPIRAL and by 6dF at the Schmidt, so it is wise to let the day staff know in advance which gratings will be required. This is particularly important for the 316R grating, which is frequently in use for the 6dF Galaxy Survey. To change gratings, first move the grating angle to 40°. Figue 3.2 shows the grating mounted in the blaze-to-collimator position for the 25cm camera. Note that the grating is not shown at 40°, at which angle the grating is parallel with RGO cabinet.

Figure 3.2 A grating mounted within the RGO.
Grating in RGO

3.3 Calibration Lamps

Flatfield Lamps

A tungsten lamp is located within the RGO spectrograph to provide flatfield illumination. It also can be used with a narrow dekker to create a synthetic star for rotation calibration. This lamp does not illuminate the outer edges of the slit, or travel through the Polarisation Waveplate Module. A quartz lamp is available in the chimney (Figure 3.3), which illuminates a diffuse screen and provides a more uniform response over the field of view than the internal tungsten lamp. However the longer pathlength means that the incoming light is far weaker for the chimney lamp, so a considerably longer exposure time is required. Also the dome must be dark, or the telescope parked at zenith and the chimney lid in place. A third method of obtaining a flatfield is to use a quartz lamp to illuminate the windshield. This provides possibly the best long slit illumination, but requires the dome to be dark and the telescope to be driven by a AAT staff member. Again exposures need to be long compared to the internal lamp.

Wavelength Calibration Lamps

A Westinghouse hollow cathode CuAr lamp is mounted inside the RGO spectrograph. Atlases at low dispersion are given in Appendix F. These are intended to aid identification of the wavelength region - for final calibrations see the medium dispersion arc atlas, and offline atlases at higher dispersion for the blue and red regions in the Arc folder in the control room (also available from your support astronomer on request).

A variety of lamps are available in the chimney (Figure 3.3). There is a small, portable switch board which controls the lamps. To use these lamps, ensure that the central dust cover is open (ask a technician), insert the diffuser disk and select your required lamp. When you remove the diffuser disk, an alarm sounds until it is fully out of the way.  These lamps are useful for regions of the spectrum poorly covered by Ar lines and for long slit spectra since the internal arc lamp does not illuminate the edges of the slit. Again, the chimney arc lamps are considerably weaker than the internal lamp. The current lamps in the chimney are CuAr, FeAr, CuHe and CuNe, but others can be mounted if requested well in advance. Some atlases are given in Appendix F. Offline atlases are available for these lamps in the Arc folder in the control room, or again, contact your support astronomer.

Figure 3.3. A view of the chimney over the (partly-opened) mirror cover. The lamps are mounted within the chimney.

3.4 Operation of the RGO Control Console

3.4 Subsections: Viewing Mode | Filters | Internal Lamps | Dekkers | Slit | Calcite | Below Slit Filter | Shutter | Collimator | Hartmann Shutter | Grating | Warning Light

The RGO spectrograph is usually operated using the control console in the control room. A schematic of this console is shown in Figure 3.4. The unit is divided into three areas. The top third of the panel is now defunct. The middle third contains most of the controls for the spectrograph, arranged from left to right in roughly the same order as the hardware occurs in the light-path (see Figure 1.2). The bottom third contains indicators which show the status of the entire system. The controls are described below, starting at the middle third of the panel, moving left to right.

Figure 3.4 Diagram of the remote RGO Control Console


Viewing Mode

The buttons on the left determine the mode of viewing. The top button (STAR) directs light to the spectrograph slit. The middle button (ARC) directs light to the internal calibration lamps. The bottom button (FIBRE) is no longer used. Note that the chimney lamps are accessed in STAR mode.


The filters described in Section 2.4 are controlled by 4 rotary switches. The top two switches move filters into the light path of the telescope, and the bottom two filter the internal calibration lamps. In each case the first position is clear. The other positions are labelled in the indicators in the bottom panel, which light up when the position is selected. The LEDs around the rotary switches light when the filter has actually reached the selected position.

Internal Lamps

The internal lamps can be powered on using the flick switches below the filter knobs, labelled ARC and TUNGSTEN. These switches can be left on for the duration of use, as the lamps are actually switched off automatically by selecting STAR viewing mode, and on again by selecting ARC viewing mode. Please avoid leaving the lamps on unnecessarily in ARC viewing mode, as the lamps have a limited lifetime.


The dekker is selected by using the thumbwheel control to enter the coded number, listed in Appendix D. The dekker wheel then is moved by holding the flick switch below to DRIVE until the correct number is reached. On releasing the switch, the dekker should clamp. If a red warning light in the bottom panel indicates that the dekker is not clamped, try driving the wheel a little further, or possibly selecting a number with one added or subtracted to it, since these values can sometimes drift. If you have further problems contact a technician - you should not continue with the dekker unclamped. If the dekker is at 50 the light-path is clear.


The slit width (in mm) is displayed in the next section. It is changed by using the flick switch under the right end of the display. Hold the switch to OPEN to increase the slit width, or CLOSE to decrease it. The next flick switch should be set to SLOW as even at this setting the slit changes very rapidly. The minimum slit with is generally about 10 µm, so don't attempt to drive below this figure, as the jaws sometimes jam. One arcsecond on the sky is equivalent to a slit width of 150 µm.

The switch on the left shows the width from the centre to JAW A (the left) and to JAW B, to enable you to check that the values are equal. This is usually only necessary if you are experiencing problems.


The calcite block used for spectropolarimetry is selected using the flick switch below the slit controls. Always check the indicator in the bottom panel to ensure that the calcite block is either in or out as required.

Below Slit Filter

The below-slit filters are selected using the rotary switch below the slit controls. The middle position is clear. The two filters are labelled on the indicator in the bottom panel.


The next panel is the shutter indicator and manual switch. If the toggle switch is at MANUAL then the two buttons will OPEN and CLOSE the shutter. Once the CCD has been installed, the toggle switch should be at CCD, and pressing the buttons will have no effect. The buttons should light, however, to indicate whether the shutter is open or closed. There is also an indicator in the bottom panel, which is more reliable.


The collimator is displayed in the next section. It is moved by holding both flick switches below the right of the display, until the required value to reached. Always use FAST - this is about snail's pace. TO SLIT increases the value, and FROM SLIT decreases the value. The flick switch on the left allows you to display the CERVIT ROD setting, but this is usually only of interest to the technicians.

Hartmann Shutter

The Hartmann shutter is controlled using the two flick switches below the collimator controls. Use the LEFT switch to OPEN or CLOSE the left half of the shutter, and the RIGHT to move the right half. These should only be closed during a Hartmann test to determine focus.


The last panel on this level displays the grating angle of the grating. Note that this should be at 40° for changing gratings or closing the 25 cm dark slide. To change the grating angle, flick the left switch to DRIVE (note that the middle position between CLAMP and DRIVE does nothing), flick the middle switch to FAST or SLOW, and hold the right button to + or - until the required grating angle is reached. In this case both speeds are useful, as SLOW is usually needed to accurately select a value. If the grating refuses to drive, it often recovers by selecting CLAMP, wait a little, and try again. At the end, make sure that the grating CLAMPS successfully.

Warning Light

In addition to the indicators in the bottom panel, a warning light below the grating controls lights if: the grating or dekker isn't clamped, a filter wheel is out of position, if a Hartmann shutter is closed, or if ARC viewing mode is selected, so it's worthwhile watching out for it during observing.

3.4 Subsections: Viewing Mode | Filters | Internal Lamps | Dekkers | Slit | Calcite | Below Slit Filter | Shutter | Collimator | Hartmann Shutter | Grating | Warning Light

3.5 Acquisition Light Paths

Before entering the spectrograph the light beam can be viewed on the Acquisition Camera by inserting a mirror using the TV DIRECT control on the telescope console. The TV image has a field of 2.4 × 3.2 arcmin and is ideally suited for star recognition. When the mirror is removed by depressing the TV SLIT button on the telescope console, the TV monitor views the slit jaws and dekker. Slit guiding is carried out in this mode. To view the spectrograph slit it is necessary to depress the STAR button on the left side of the spectrograph control panel. Note that ~ 2 magnitudes of sensitivity is lost in SLIT view, so offsets from nearby brighter stars can be useful.

Sections: The Spectrograph | Grating Changes | Calibration Lamps | RGO Console (subsections)| Acquisition Light Paths
Previous: Planning the Observing Run | Next: At the Telescope | CONTENTS

Ray Stathakis
Last updated 2/4/2002