NIRSPEC

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

George Brims	May 7, 1996

NIRSPEC Cryomechanics Design Note 18.01

Instrument support structure

1 Introduction

NIRSPEC has to be mounted on a stable support structure at the telescope, on either of the two Nasmyth platforms of Keck II. The instrument will not be permanently installed in one place, but will sometimes have to move between the two Nasmyth platforms, so the support structure will run on tracks. To make the transfer, NIRSPEC must roll onto a temporary position on the Nasmyth deck (the "balcony" around the dome where the elevator comes up). The telescope will then be rotated so that NIRSPEC can be rolled onto the opposite platform. On the right hand Nasmyth platform it will also have to move out of its observing position and into a standby position in the corner of the platform, to allow DEIMOS to get to the focal plane. On this platform there will be a railroad style turntable to enable both NIRSPEC and DEIMOS to rotate and then move off into the corners of the platform.

After each move the instrument will have to be re-aligned to the optical axis, preferably with the minimum amount of work. To deal with this we need to ensure two things: the support structure must re-locate exactly each time it is moved back into one of its observing positions, and we need a reliable method of accurately and reproducibly re-aligning the instrument on the structure.

The support structure will also house the electronics racks, which will consist of two or more cabinets bolted together and placed under the instrument. The optical axis is 1.5 meters above the floor of the Nasmyth platforms, so there is quite a bit of space under the dewar. There is also a floor space problem if we place the electronics cabinet beside or behind the instrument, especially on the left platform with the AO bench. The electronic equipment should travel with the instrument when it moves around on the tracks, so that the minimum of dismantling is required when making a move. We need to complete the layout and design of the electronics housing in conjunction with this design, since some of the space issues will be affected by things like the size of the racks after they have had insulation added.

We also need to be able to move the instrument around independent of the tracks installed at the telescope, such as when it is in the lab, and any time it has to be moved to an instrument prep room at the observatory, so it will need a second set of wheels. These will be hard rubber castors rather than the steel rollers used on the tracks. The track rollers won't allow the instrument to turn corners, and are only meant to roll on a smooth surface.

2 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.

The instrument will move on wheels which run on the tracks provided at the telescope.

The support structure must locate on kinematic mount points on the Nasmyth platforms, so that the instrument returns to exactly the same place each time it is moved and replaced.

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

The electronics equipment racks will be mounted underneath the instrument, but the equipment must still be easily accessible.

The electronics racks should be easy to remove once disconnected from the dewar.

Access to the underside of the dewar, which has all the vacuum and electrical feedthroughs, should be unrestricted.

There must be provision to attach additional wheels or castors for moving the instrument independent of the tracks, on any reasonably flat surface.

3 Summary of design

There are five main elements to the design: (1) the basic support structure or framework, (2) the transport mechanisms, (3) the kinematic mount to the Nasmyth platforms, (4) the kinematic adjustment of the instrument itself on the support structure, and (5) the mounting and placement of the electronics racks.

3.1 Support framework

The support framework should really be very simple. Figure 1 below illustrates the basic idea, with the dewar on its support structure rolling on the rails at the telescope. We will probably build the frame from 4" × 4" steel tube, which should be adequate to support the load. There are horizontal frames around the top and bottom. The telescope will rarely accelerate very fast except when tracking an object which passes near the zenith, but we need to be sure that the structure is rigid enough that it won't flex significantly and put the instrument out of alignment with the optical axis, so I have put in diagonal members across the front and rear.

The electronics racks are placed so that their 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 longitudinal members joining the front and rear wheels at the bottom might stop us from opening the doors for access to the electronics, so if necessary they will be removable.

3.2 Transport mechanisms

Any large heavy component which has to move around at Keck mounts on its own shuttle assembly. These shuttles run on a series of tracks installed on the Nasmyth deck and Nasmyth platforms. The track system consists of three tracks, as shown in Figure 3. The two outer rails bear the load, while the middle one is used for driving via a rack and pinion mechanism, and also keeps the moving shuttle in line on the outer tracks. The standard gauge of the outer tracks is 1.5 meters (60"). This is a good bit wider than the NIRSPEC dewar, which will be around 40" wide. To match the gauge of the tracks we will have to place the wheels outboard of the simple support structure, as shown above. The wheels will stick out to the sides and occupy a bit more floor space than the footprint of the dewar alone, but this shouldn't pose a problem.

The outer tracks rest in a 6" channel set into the floor, and have the profile shown above. The wheels are mounted in pairs so that one of them can take the load whenever the other crosses a break in the tracks, such as between the edge of the Nasmyth platform and the Nasmyth deck. These wheels have no flanges to keep them on the raised profile of the track, so something has to keep them in line. This is accomplished by the middle track. The middle track has a rack which is used to drive the equipment along the tracks, and angle steel welded on to make a guide rail. Wheels mounted at either end of the moving trolley run inside the angle to give the trolley lateral guiding, and also a vertical restraint. The trolley is driven by a pair of gears which mesh with the rack mounted along the middle track.

When it is in its observing positions the support structure has to rest on fixed mount points, but it will have to lift out of those a few inches before moving. There will be a set of screw jacks at the corners of the support structure, which will push down on the trolley and so lift the instrument and support frame up. An electrical interlock will ensure that the motor can not drive the instrument along the tracks until it is lifted clear of the mounting points.

The overall trolley used on the rails will be detachable, so that the instrument on its support frame can be lifted off, and the trolley will remain with the tracks. For rolling on normal floors the instrument will be placed on a similar trolley equipped with castors, as shown below. As with the rail system, we need a way to set the instrument down in a stable position once it has been moved. To achieve this we can put adjustable feet on the ends of the legs, and lower the instrument with the screw jacks until they rest on the floor. For alignment in the lab we could even re-create the mount points on the Nasmyth platform so that the instrument can always be re-located in the same place relative to our optical bench.

3.3 Kinematic mount to platform

The instrument has to rest on fixed static mount points once it is in place on the platform (and have safety bolts dropped into place to restrain it in the event of an earthquake). It will roll along the tracks until it is 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. We need to have three points for our mount so that the support is kinematic. The location of the mount points will have to be discussed further with Dave Cowley and his group at Lick, who are designing the mounting for DEIMOS and for the AO system. There is a problem in that there are no load-bearing points close up to the elevation bearing, where the front of both NIRSPEC and DEIMOS would have to rest in their observing positions. Kyle Kinoshita at CARA will be looking into adding something to the Nasmyth platform structure to take care of this. The load-bearing points are on a grid of approximately 1.25 meters, so there should be no problem having the two front points on or near the two new load-bearing points, and the rear one on a "duck foot" structure like those used for HIRES.

The mounting points need to constrain the instrument to remain in the same place but will need some provision for thermal expansion. We need the classical type of mount where one point constrains in both X and Y, and the other two constrain in only one direction.

3.4 Upper kinematic mount

The plan 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 when it has been moved. However these positions, although reproducible, will be different on each platform, 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 is still necessary even though the "right" settings for the kinematic mount are dialed into the mechanism.

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3.5 Electronics racks

The electronics racks will be two standard 19" rackmount cabinets placed underneath the dewar. They will be at least 2 feet square in the horizontal plane, and about 4 feet high. It looks like there is adequate room underneath the instrument to accommodate these racks, but there is the issue of placement. The underside of the dewar is the "working surface", where all the feedthroughs, vacuum ports etc. are situated. The scheme for bringing wiring out of the inner cold shield/optical plate enclosure dictates that the wiring will all come though near the edges, so the main constraints on placing the racks are the CCR head and the liquid nitrogen fill tube.

There is also the problem of insulating the racks so we don't dissipate too much heat into the atmosphere in the dome and affect the seeing. I recently calculated the amount of insulation required to meet the heat dissipation requirements of less than 50 Watts. In order to have the electronics inside the racks running at a reasonable temperature requires about two inches of PVC foam covering the racks. This does add a fair bit of bulk to the racks but it looks like it is still possible to fit the racks underneath. The main problem is in the left- right direction, since the racks must be off to one side to be clear of the CCR head. That pushes them right over to the right hand side of the transport structure, but they are just able to fit in, as shown in Figure 4.

A more serious problem is accommodating the liquid nitrogen fill tube. The fill tube is approximately in the middle of the bottom surface, so it falls right between the two racks. I had hoped to make the two racks a single mechanical unit, but it is obvious we have to split them six inches or so to allow easy access to the fill tube. This doesn't pose a space problem since there is enough room to move them towards the ends of the instrument, but we will have to provide some kind of cable duct between the two. It also increases the overall surface area of the whole enclosure, but the space in between could be completely filled with foam insulation below the level of the fill tube access, so that shouldn't be too much of a problem. Since the two racks are now separate we need to make sure they are adequately supported to maintain their relative position. The addition of a platform across the bottom of the support frame should take care of it.