Gindex

SYSTEM DESIGN NOTE

SDN0014.02 - Instrument Access and Service

Prepared by	Date	Approved by	Date	Rev.	Rev Date
J Elias	6/2/99	N. Gaughan	6/8/99

1. Introduction

The purpose of this document is to discuss the requirements on instrument design as they relate to access for service, alignment, and other similar purposes. It is important to understand, in reading this, that the intention of these requirements is to save effort when aligning or servicing the instrument. Therefore, the cost of the solution must be weighed against the savings in effort it produces.

2. General Considerations

The basic principle governing access to the instrument should be that work on a given major assembly should not require subsequent adjustment of any other assembly. Ideally, it would not involve any interaction with any other assembly, but this must be treated as a goal and not as a requirement. As a specific example, it is not good to expose the science detector to atmosphere for extended periods, so one would remove the detector assembly as part of any extended servicing of any other part of the instrument. The design requirement is that it be possible to replace the detector assembly without having to realign it.

2.1 Modularity

The instrument is composed of several major sub-systems, each of which may contain several assemblies. The systems that are inside the instrument comprise:

Thermal system, which includes

Cryocoolers
Radiation shields
Thermal distribution system (“strapping”)
Pre-cool system (if present)
Heaters

Support structure, which includes

Cryogenic bench (not necessarily monolithic)
Support trusses

On-Instrument Wave-Front Sensor (OIWFS), which includes

Field lens
Collimator/camera lens groups (two optics groups in single mount)
Gimbal mirror
Filter wheel
Detector/optics assembly

Spectrograph, which includes

Fore-optics assembly (Offner relay plus pick-off & fold mirrors)
Filter wheels
Slit/Decker/IFU assembly
Collimator mirror
Acquisition mirror mechanism
Prism turret
Grating turret
Camera turret
Focus mechanism
Detector assembly

2.2 Alignment Requirements

The two main optical systems, the OIWFS and spectrograph, are aligned separately. A final relatively alignment of the two systems is, of course, required, but it is highly desirable to be able to work on the two systems separately. Thus, it would be desirable if the OIWFS bench (in addition to individual mechanisms) could be removed from the instrument and set up separately for alignment and test. If this arrangement compromises structural rigidity, it is worth noting that all the OIWFS assemblies except for the field lens are in close proximity. An alternative arrangement might then be one where a smaller plate is removed with all of the OIWFS except the field lens, and the field lens is removed separately if needed.

Alignment of the spectrograph should be possible without removing the optical bench from the rest of the instrument structure. Note that the definition of the “optical bench” is somewhat arbitrary; in particular, it probably makes more sense to view the fore-optics assembly as an assembled (and aligned) unit that attached to the optical bench rather than as a series of components that are in turn attached to the optical bench. In general, modularity within the spectrograph system should be driven by the combined requirements of structural rigidity and optical alignment (SDN003.20).

With the OIWFS removed, there should be ample access to the spectrograph through the top of the optical bench structure. (There may be some flexure of the instrument with the OIWFS removed, but alignment tolerances are generally much looser than flexure tolerances, so that this should not be a problem. If analysis shows otherwise, a dummy WFS plate could be made with access ports. A dummy WFS plate would also permit cryogenic tests of the spectrograph without the WFS.)

All of the above operations obvious require at least partial removal of the radiation shields. However, cold stationing of wires and the thermal distribution system should be routed to a central location so that removal of radiation shields for access does not involve thermal or electrical disconnections.
The alignment plan requires some use of flats and other alignment optics. Mounting holes for these should be provided in the optical bench. If for some reason the final alignment plan requires access other than through the top of the bench, access ports (with cover and light seal) should be provided.

2.3 Service

Service of the instrument falls roughly into two categories: reconfiguration and maintenance. The former category includes things like filter changes, while the latter includes everything from getter replacement to bearing replacement.

Only a few parts of the instrument lend themselves to easy reconfiguration. The components that can be readily replaced are basically only the filters, slits, and detector. The three turrets are designed to hold specific prisms, gratings, and cameras, so substitution of one of these elements would require at least some redesign of the turret and fabrication of parts.

The instrument design should permit either replacement of filters without removing the filter wheels (ideal), or removal of individual filter wheels without touching the rest of the pre-slit portions of the instrument. Similarly, changing slit plates should be possible removing only the slit slide (best case) or slit and decker slides together (acceptable). The detector assembly must be removable in any case for its own protection.

Most servicing requirements have to do with moving parts. Thus one would like the ability to remove an individual mechanism for inspection, adjustment, or repair. There are a total of ten mechanisms (3 OIWFS, 7 spectrograph). Wiring should include connectors so that mechanism can be removed without cutting wires. The mechanisms should be mounted (locating features) so that alignment is not lost when the mechanism is removed.

The main additional service needs have to do with the vacuum system. There will be getters (molecular sieve) mounted on the cold heads, and the getter material will need to be replaced periodically (in situ baking can postpone this, but not indefinitely). Access for this purpose should require a minimum of disassembly of the instrument.