Sections: The Spectrograph
| Grating Changes | Calibration
Lamps | RGO Console
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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°.
Figure 3.1b. View of the RGO spectrograph showing the reverse side
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.
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.
3.3 Calibration 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.
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.
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
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 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 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.
3.4 Subsections: Viewing Mode | Filters | Internal Lamps | Dekkers | Slit | Calcite | Below Slit Filter | Shutter | Collimator | Hartmann Shutter | Grating | Warning Light