NIRSPEC

UCLA Astrophysics Program	U.C. Berkeley	W.M.Keck Observatory

George Brims	October 12, 1995

NIRSPEC Cryomechanics Design Note 18.00

Instrument support structure

Introduction

The instrument has to be supported on the Nasmyth platform so that the window is centered on the optical axis 1.5 meters above the floor. In addition it has to move on tracks so that it can move out of the way of DEIMOS on one Nasmyth platform, and also roll onto a temporary position on the mezzanine level so that it can be transferred to the other platform to work with the AO system. The support structure will also have to incorporate the electronics racks, a pair of cabinets bolted together and placed under the instrument. We would like the electronic equipment to travel with the instrument when it moves around on the tracks, so that the minimum of dismantling is required when making such a move.

Design goals

The main requirements for this design are listed below.

The structure must support the weight of the instrument and other equipment and be stable and rigid when the telescope moves. This includes being secure during earthquakes.

There must be wheels which can run in the tracks provided at the telescope. These wheels could be detachable, so they could be moved out of the way once the instrument is in place.

There must be mounting facilities on the Nasmyth platforms that locate the instrument in exactly the same place each time it is moved and away and replaced.

The structure should have provision for adjustment of the instrument alignment, so that NIRSPEC can be brought back into precise alignment with the telescope optical axis after a move.

The electronics equipment racks should be accessible when mounted underneath the instrument. In addition there should be a simple way to remove the racks as a unit for access to the bottom surface of the dewar.

The story so far...

At this stage we don't have much of a design beyond what is shown in the picture below. This sketch was made to illustrate the idea of putting the electronics underneath the instrument instead of behind it. There is only one dimension on his sketch, because that is the only one that is really fixed. The size of the dewar is slightly exaggerated since the optical path has been shrunk a bit since this sketch was made. The height of the supports is also arbitrary, since the optical path was assumed to pass through the middle of the dewar front face. In reality, we will try to make all our optical elements be as close as possible to the optical plate. Since the optical axis remains fixed with respect to the floor, this will effectively raise the whole instrument.

The electronics racks are placed so that the front and rear doors face to the right and left of the instrument (where "front" means the face with the input window and "right" is the longer face showing in the picture). The structure should not have cross-members that stop us from opening the front and rear doors for access to the electronics. The support structure for the electronics racks is not shown.

Also missing from the sketch are the transport structure (wheels or bogies), and the adjustment mechanism for alignment of the instrument.

The gauge of the tracks already in use at Keck for other things that have to move is 1.5 meters (60"). This is a good bit wider than NIRSPEC, which will be about 36" wide. To match this gauge we can place the wheels outboard of the simple support structure shown above, but that will then mean having them stick out and create a safety hazard. We should probably make the transport equipment detachable, so it can be stowed out of the way when the instrument is in place (probably for several months at a stretch). The plan so far is that the instrument will rest on fixed static mount points once it is in place on the platform. The wheels will be used to get it close to the right place, then the wheels will be raised (or the instrument lowered, whichever way you want to look at it) so that it drops into its locating points.

The alignment concept is that the mount points on the floor of each Nasmyth platform will ensure that the instrument can always be put back in exactly the same place. However these positions, although reproducible, will be arbitrary, so we need to have a way to tweak NIRSPEC into exactly the right alignment once the structure has been located on the floor. Ideally the settings for each Nasmyth platform could be measured or indexed, so that NIRSPEC could be returned to the nominal position on each platform as soon as it has been moved. There might be a certain amount of slop in the fixed mounts so that a final adjustment was still necessary.

What next?

The first thing to be done is to get a better sketch put together including more realistic dimensions for the instrument, placement of optical path and size of the electronics racks. For example if the optical path is very near the bottom surface of the instrument, that face will be raised from the floor and we will be able to use taller racks for the electronics (there is a danger we might have to go to a third rack if we can't get enough height to fit in all the components). Ascertaining this dimension is vital to getting a good handle on what the whole structure will look like.

We also have to decide the form of the locating points on the floor, and the kinematic mount and adjustment for the instrument. We will have to work in cooperation with Dave Cowley from UCSC, who is taking responsibility for the layout of the tracks and the location of the load-bearing points on the Nasmyth platform.