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Changing the gratings or dichroic in AAOmega

Changing the gratings in AAOmega is simple, but the gratings and dichroic are heavy and fragile and are in a confined space close to precision optics. Grating changes should only be undertaken by experienced AAO staff. If at any time you are unsure of any aspect of the procedure, contact the afternoon technician or the instrument scientist. The necessary procedure is below:

  1. Figure 1 shows a top-down view of the AAOmega spectrograph. The door to the Coude room (on the 4th floor) is at the bottom right of this image. As noted in the caption, the spectrograph is shown with the Red camera in high dispersion mode, the Blue in low dispersion mode. Note the restricted access to the blue grating.

  2. AAOmega

  3. First, look at the spectrograph and decide if the camera must be moved to allow you to access to the gratings. If the cameras are to be moved use the control task in the control room (ensuring that the camera tracks are free from any obstructions.)

  4. Close the dark slides to the two cameras. This protects the dewar aspheric windows.

    The gratings and dichroics each have aluminium rectangular box covers (stored in the Coude room cupboard). The correct covers for each element are a close fit. Each cover has a slot cut in the top so that the handle of the element fits through. Ideally these covers should be fitted before attempting to replace either the gratings or the dichroic but in certain instrument configurations fitting these covers may be more risk to the optics than carefully removing the grating first.

  5. You will most likely need to stand on the AAOmega bench to move the elements.

  6. The grating mounts are located in kinematic seats on the two grating turntables. Each grating is locked into position by three CAPTIVE bolts which should be FINGER TIGHT. Do not use an allen key to undo or tighten these bolts. Undo each bolt by hand until it turns freely, gently lift the bolt, it will not come free of the mount completely.

  7. With all three bolts free, position yourself above the grating and take hold of the handle on top of the grating mount. The grating mount is heavy and must come straight up-and-out of the spectrograph without being allowed to touch any other parts.

  8. Lift the grating, smoothly, straight up from its mounting position, and clear of the surrounding optics. The mount should come away from the turntable easily, once the weight is taken up, and so if there is any resistance stop and confirm that all of the CAPTIVE bolts are correctly freed.

  9. Cover the grating with its protective aluminum cover and store it in the Coude room cupboard. Locate the new grating, remove its cover and repeat the grating extraction process in reverse.

    Note that the correct alignment of the new grating with the turntable is indicated by a white mark on the grating mount and the turntable. The three balls in the turntable based will align with the three grooves in the grating mount.

  10. Carefully tighten the three CAPTIVE bolts BY HAND, Do not use an allen key.

  11. Open the dewar dark slides.

  12. In the control room check that the new grating/dichroic has been detected and set the central wavelengths for the next observations. The current grating/dichroic are listed in the AAOmega Full Status Dialog window. To change the central wavelengths click the Configuration button in the Spectrograph Control Dialog window and select Set Blue/Red Arm Wavelength. Enter the new Centre and Blaze Wavelengths and click OK. If it finds it difficult to find your new wavelength, click the Home First option and try again.

Sarah Brough (sb@aao.gov.au)