The figure below shows a top down view of AAOmega. The door to the Coude room 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. While access to the red may appear easy with this setup, there will be some tight tolerances on clearances with the red camera in this position. The first task is to inspect the working area and decide if the camera must be moved to allow access to the gratings. If the cameras are to be moved, either using the control task on the 6th floor, or the engineering interfaces in the 4th floor prep. room, first ensure that the camera tracks are free from any obstructions.
- Close the dark slides to the two cameras. This gives some small measure of protection to the dewar aspheric windows. Ideally the aluminum grating, aspheric and dichroic covers should be fitted before attempting to replace either of the gratings. In certain configurations of the instrument, fitting these covers may actually represent more risk to the optics than carefully removing the grating cell first. the simple rectangular box covers come in a variety of sizes to suite each optic (two sizes for the gratings). The correct covers are a close fit. Each cover has an off center slot cut in the top so that the handle of the cell protrudes.
- The grating mounts are heavy and rather large. Change only one grating at a time, and remove the original grating, storing it in the 4th floor prep. room cupboard with its aluminum cover in place, before retrieving the new grating from said cupboard and removing its protective covering. Do not balance grating mount precariously on benches that are too small and do not place gratings in transit on the floor.
- The grating mounts are located in kinematic seats atop the two grating turntables. Each grating is locked into position by three CAPTIVE bolts which should be FINGER TIGHT. Do not use an allan 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, and when in its upper position turn the bolt to hold it in this upper position.
- With all three bolts free and in their upper state, 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 swing to the side, which might endanger the optics. This requires some thought as to how best one will position ones self within the spectrograph, and will depend to some extent on the spectrographs current configuration. Standing on the optical bench is permissible (since you will have ensured your shoes and clothing are free from detritus before entering the Coude room) and is probably required for most people, but care must be taken to avoid contact with delicate electronics and mechanisms.
- 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.
- Transport the grating to the prep. room cupboard (currently located in front of the main doors to the dome). Cover the grating with its protective aluminum cover and store it in the 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 groves in the grating mount.
- A design feature of the system includes a small vertical post in the turntable which can make alignment rather tricky. The grating must come down onto the turntable perfectly straight for the last 30mm of travel, or this post will prevent the correct seating of the grating. An in-experienced operator will require some trial and error to position the grating, but when correct, the seating is positive and will leave no doubt in the mind of the user.
- Carefully tighten the three CAPTIVE bolts BY HAND, Do not use an allan key. Following the usual best practice, half tighten each bolt in turn before firmly locking them into position.
- Open the dewar dark slides.
- Due to the confined working space, it is quite common, even for an experienced user, to trip one or more of the many limit switches in the region when changing one of the gratings. This will prevent AAOmega operations until the electronics rack is reset, but does not place the system in any danger. Once both gratings are in place and the Coude room resealed, reset the "instrument enable" toggle switch on the AAOmega electronics rack in the prep. room. The system should now have correctly identified the gratings that are in place, and the spectrograph can be set up.
Note, at this time (August 2006) we do not have a detailed database of camera focus settings. The only option available is as follows :
- Use the AAT database to locate the most recent AAOmega run which mounts your chosen gratings and wavelengths.
- Locate a likely frame in the database and then find this frame on disc at : /net/aatlxy/data/aatobs/OptDet_data/YYMMDD/CCD_#/
- Examine the headers (ensure you have the correct CCD) and find the previously used camera focus values.
- These value should be used as the starting point for a full AAOmega focus sequence.
Rob Sharp (rgs@aao.gov.au)