Gemini Near-Infrared Integral-Field Spectrograph (NIFS)
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Spectrograph Mechanical Design
5.1 Overview
The NIFS spectrograph will be mounted in a duplicate of the NIRI cryostat and use a duplicate of the NIRI OIWFS. The NIRI cryostat consists of a hexagonal vacuum jacket in which is supported a central vertical cold work surface (CWS) which acts as an optical bench for both the science instrument and the OIWFS (Figure 1). The NIFS spectrograph replaces the NIRI camera in the duplicate cryostat. A pick-off mirror protrudes on a probe over the OIWFS to deflect the science beam into the spectrograph. The discussion in this section concentrates on the mechanical design of the NIFS spectrograph. The NIRI cryostat and OIWFS have been thoroughly analyzed by the NIRI design team. The recent performance of NIRI is summarized in §6.6. Necessary modifications to the duplicate NIRI components are discussed in §6.
Figure 1: Detail of the duplicate NIRI cryostat with its vertical cold work surface near center, the NIFS spectrograph mounted on the right, and the duplicate OIWFS mounted on the left
Detail of the NIFS spectrograph mounted in the duplicate NIRI cryostat is seen more easily in Figure 2. In this view, the front and end sections of the hexagonal vacuum jacket have been moved aside to show the spectrograph optical components and detector near the center of the figure. The hexagonal vacuum jacket is ~ 1 m across. A large frame is mounted around the cryostat to carry two thermal electronics enclosures and ballast for balancing the instrument. This frame is not shown in Figure 2, but is discussed in §6.2.
Figure 2: The NIFS spectrograph mounted in the duplicate NIRI cryostat.
All of the components shown in Figure 2 are extremely stiff in order to minimize deflection as the entire two tonne instrument swings at the Cassegrain focus of the Gemini telescope. At the top of the figure is the Instrument Support Structure (ISS) interface plate that attaches the instrument to the Gemini telescope. The f/16.2 beam from the telescope enters the vacuum jacket via a window protruding through the center of the interface plate. The hexagonal vacuum jacket is initially pumped to a high vacuum and this vacuum is then maintained by cryopumping. Two large closed-cycle helium coolers (not shown Figure 2) cool the internal components to ~ 60 K and cool the cryopump getters. Cold and floating radiation shields (not shown in Figure 2) prevent radiation directly reaching the spectrograph. The CWS plate is located directly behind the spectrograph in Figure 2. An 8 mm thick, light tight, aluminum cover attaches to the CWS plate and encloses the spectrograph optics. On the far side of the CWS plate is the OIWFS which acts as a closely-coupled near-infrared autoguider for the spectrograph. The spectrograph proper begins on the far side of the CWS plate at the pick-off probe. The input optics then pass the beam through a hole near the top of the CWS plate. The small science field then passes to the IFU which slices the field into strips and stacks the strips end-to-end to form a long staircase slit. This long slit is fed to a medium resolution spectrograph that delivers spectra to the detector at the lower right of the CWS plate.
5.2 Cold Work Surface Plate
All of
the NIFS optical components are carried on the 40 mm thick cold work surface
plate. This plate is a modified version of the NIRI CWS plate, heavily modified
on the science instrument side to carry the NIFS spectrograph but largely
unchanged on the OIWFS side. Three V-shaped titanium tines support the CWS plate
at the center of the vacuum jacket. These tines are very stiff and limit
relative movement between the plate and the telescope. The titanium tines
restrict heat inflow to the instrument and flex inwards by more than 3 mm as
the CWS plate shrinks in cooling to 60 K.
Most of the NIFS spectrograph components are carried in a plane 90 mm above the CWS plate. They are cantilevered off the plate in optical table style. Figure 3 shows a view of the NIFS spectrograph looking onto the CWS from the science side of the plate. The figure is labeled with the main components of the spectrograph. The Focal Plane Mask Wheel is on the far side of the CWS plate and the optical beam passes inwards through the CWS plate to the spectrograph on this side. The beam then zigzags down through the IFU and spectrograph components to the detector. The extensive black sheet metal baffling that will be required between the folded optical system is not shown in Figure 3. This is needed to absorb scattered light. Where possible flat sheet sections with cut outs that graze the beam will be used to approximate an infinite cavity baffle. Particular care will be taken to baffle the grating and image slicer as these are expected to be the largest sources of scatter in the instrument. A sheet metal “zero order” trap is included to suppress direct reflection from the high dispersion gratings.
Figure 3: NIFS spectrograph optics mounted on the cold work surface plate.
5.2.1 Stability (Flexure)
NIFS is far lighter than NIRI and will place far smaller forces on the CWS plate and titanium support tines. The 8 mm thick aluminum cover over the NIFS spectrograph will add significantly to the stiffness of the plate. Once all the optical mounts are in place on the CWS plate and aligned, a 3 mm thick plate may be attached across the top of all of them. This would tie the entire opto-mechanical system together to form a very stiff structure. RSAA will continue flexure analysis during the course of the instrument design.
5.2.2 Thermal Conductivity Issues
Although the NIFS instrument is far lighter than NIRI and will cool faster, it is likely that the temperature difference across the CWS plate will remain as for NIRI. This is largely due to the unchanged heat load on the edge of the plate coming from the radiation shields. The current cold test of NIRI (Feb 2000) suggests that this temperature difference is ~ 5 K from center to edge of the plate. This should not cause major problems for NIFS. RSAA will continue thermal analysis during the course of the instrument design.
5.3 Spectrograph Pick-Off Probe
Figure 4 shows a cross section through the CWS plate in the vicinity of the input beam. The cantilevered pick-off probe shown in the figure protrudes through a cylindrical baffle above the OIWFS and folds out the NIFS science field. This small part of the telescope field comes to focus at the Focal Plane Mask Wheel. The cylindrical baffle around the probe passes the full 3˘ telescope field to the OIWFS and prevents scattered light from entering the cavity surrounding the OIWFS. NIFS does not use the NIRI beamsplitter wheel so without the baffle there would be large scatter into this cavity. The baffle is carried by the OIWFS radiation shield and it does not contact the pick-off probe. The pick-off probe is cantilevered closely from the CWS plate in order that it be as stable as possible with respect to the spectrograph and to ensure that it becomes as cold as possible.
Figure 4: NIFS OIWFS baffle and pick-off probe area.
5.3.1 Field-of-View and Baffling
The NIFS field is ~ 2 mm square and the pick-off probe is ~ 94 mm above the telescope focus. Rays passing by the probe continue to the OIWFS. The probe is 10 mm thick and carries two small diamond turned aluminum mirrors at the center of the telescope field, the upper of these mirrors folds out the NIFS science field. The closest usable unvignetted guide star to the center of the NIFS field is at a little over 8 mm or 12.7˛ from the field center (Figure 5). Guide stars will pass through to the OIWFS at a little over 5 mm from the field center, but will be partially vignetted and will not properly illuminate the two-part focus prism in the OIWFS. The OIWFS will not then deliver accurate focus corrections to the telescope. There may be usable position angles where the prisms are equally illuminated but this would require that the object be rotated with respect to the NIFS slit and this may not be scientifically desirable.
Figure 5: NIFS pick-off probe showing OIWFS guide star availability.
The cantilevered pick-off probe is drilled with a stepped hole to tightly baffle the NIFS science field through to the Focal Plane Mask Wheel. Bright stars near the science field will scatter light into the drilled hole inside the pick-off probe. Scatter inside the pick-off probe is of considerable concern. Preliminary analysis with the OptiCAD non-sequential ray tracing package is shown in Figure 6. This figure shows rays reflecting off the probe mirror and both passing through the probe to the top and scattering off the OIWFS baffle. The models used in this analysis are in stereo-lithographic format (.stl) composed of interlocking triangles. Tracing such a complex object with these files has proven to be difficult. We will soon be able to transfer IGES solid body objects to OptiCAD and will continue scattered light analysis of this area of NIFS.
Figure 6: OptiCAD scattering analysis of the NIFS pick-off probe region.
5.3.2 Wavefront Sensor Test Projector
A second diamond turned mirror, facing downward from the end of the probe, folds a beam from a test projector built inside the probe to the OIWFS. This projector creates a star image at the focal plane of the telescope directly under the probe that will allow the OIWFS to be tested during daytime or when NIFS is not on the telescope. This feature is discussed in more detail in §6.5.2.1.
5.3.3 Stability (Flexure)
It is essential that deflection of the pick-off probe not cause significant star movement in the NIFS focal plane. Deflection of the probe shown in Figure 5 has been analysed. Although tapered, during analysis the probe was assumed to be parallel and of the smallest cross section (54´10 mm). Analysis using standard case formulae for the 175 g probe showed a deflection of ~ 1mm at the probe tip. While parallel deflection of the pick-off probe will not cause focal plane shift, rotation of the mirror will. Analysis showed a rotation of 6.3 mrad that would result in an image shift of about 1 mm in the telescope focal plane or ~ 1 mas. The probe is very stiff in the shear direction; the calculated shear deflections are just a few nanometers in any direction. Table 1 shows the working formulae for these results.
Table 1: NIFS Pick-Off Probe Deflection Summary
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Probe Material 6061 Aluminium Alloy: |
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Volume |
64680 |
mm3 |
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Mass |
174.636 |
g |
SG (6061 Al) = 2.7 |
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Force,
W |
0.174´9.8 |
N |
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Breadth,
b |
54 |
mm |
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Depth,
d |
10 |
mm |
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Length,
l |
84 |
mm |
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Section
properties across 10 mm. |
4500 |
mm4 |
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Ixx = (b´d3)/12 |
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Deflection
in bending under self load. |
-4.06876E-04 |
mm |
E (6061 Al) = 69 000 MPa |
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y = (W´l3)/(8E´Ixx) |
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Slope
in bending under self load. |
6.30987E-06 |
radians |
Reference:
Roark |
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f = (W´l2)/(6E´Ixx) |
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Position
shift at focus. |
6.37928E-04 |
mm |
L = 101.01 |
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z = f´L |
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Shear
deflection. |
4.84658E-06 |
mm |
Reference:
Shanley |
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d = (w´l2)/(2A´G) |
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G (6061 Al) = 26691 MPa |
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w = W/l = 0.0198 N/mm |
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A = 540 mm2 |
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5.4 Spectrograph
Focal Plane Mask Wheel
A twelve position Focal Plane Mask Wheel is located at the folded telescope focus. The wheel is completely enclosed in a cold aluminum housing carried on the OIWFS side of the CWS plate and directly attached to it. The wheel rotates on lubricant-free stainless steel ball bearings and is driven and locked using a miniaturized version of the NIRI drive system. Magnetic encoding of the type used in NIRI will encode the wheel (§7.1). Drive and position locking of this small wheel are critical especially when occulting disk inserts are in use. The locking mechanism will be the subject of substantial design evaluation.
5.4.1 Mask Inserts
The focal plane
wheel carries a clear aperture for general integral-field spectroscopy plus an
array of masks for special uses and a blank to form an instrument shutter.
Apart from the clear mask which will be an open frame, all other masks will be
metalized sapphire or glass disks. Each disk will be 6 mm in diameter and 0.5
mm thick. Mask patterns will include occulting disks of various sizes, holes to
simulate stars, and a Ronchi screen for calibrating the spectrograph. The wheel
will initially be placed under an alignment microscope in order to align the
occulting disks and Ronchi screen. This should be a once only operation.
5.5 Spectrograph Input Optics
The NIFS input optics include the focal ratio converter mirror, the cold stop mirror, and two fold mirrors. These two fold mirrors fold the long f/256 beam to the image slicer. If tests show suitably low scattering values can be achieved then all four mirrors will be machined in 6061 aluminum alloy with integral mount and adjustment points and have diamond turned and coated optical surfaces. As most NIFS parts are 6061 alloy, the mirrors and mounts should shrink by the same amount with cooling and should retain alignment.
The 4 mm diameter cold stop mirror will be directly mounted to a cold housing without adjustment. This small mirror is diamond turned without edge chamfer to provide a sharp edge to cleanly cut off the telescope pupil. A black painted 1 mm wide cavity is formed in the housing around the cold stop to provide a beam dump for rays outside the telescope pupil. Both the focal ratio converter mirror and the cold stop mirror are deeply buried in an aluminum housing. Small stepped holes provide tight baffling. This baffling prevents rays from passing under the focal ratio converter mirror, by-passing the cold stop, and reaching the image slicer directly.
5.5.1 System Adjustment
Two of the four input mirrors will be adjustable (Figure 7). The focal ratio converter mirror has a three point mount adjustment to steer and focus the re-imaged telescope pupil onto the fixed pupil mirror. One of the two fold mirrors has a three point mount to steer the re-imaged f/256 focus onto the center of the image slicer.
Figure 7: NIFS input optics showing the Focal Plane Mask Wheel, focal ratio converter mirror, cold stop mirror, and Filter Wheel.
5.6 Spectrograph Order Sorting Filter Wheel
An eight
position filter wheel carries the spectrograph order sorting filters and is
supported in a housing mounted on the science side of the CWS plate. This
housing tightly baffles the wheel and prevents out-of-band light from entering
the spectrograph space. The filters are carried as near as possible to the cold
stop to maximize the beam footprint through the filters and prevent point
defects in the filters from skewing the bandpass to any one point in the
spectrograph field. This housing also carries both of the input fold mirrors.
The filter wheel drive and encoding system will be similar to that used in NIRI
with Phytron stepper motor drive and magnetic encoding (§7.1). Filters
are accessible from, and may be changed by, removing the vacuum jacket end
plate, floating shield, radiation shield, and spectrograph cover. An extensive
strip down of the vacuum jacket is not required.
5.7 Spectrograph Integral-Field Unit
The IFU
reformats the 3.0˛´3.0˛ NIFS field into 29 separate 0.1˛ wide slitlets and joins the slitlets
end-to-end to form a long staircase slit for input to the NIFS spectrograph.
The IFU parts are carried above the CWS plate on separate brackets; IFU-1
carries the image slicer, and IFU-2 carries the pupil and field mirror arrays.
At assembly, the tops of these brackets are tied together with a cross-plate to
stiffen the IFU structure.
5.7.1 Image Slicer
The NIFS IFU image slicer is constructed from a stack of 1.0 mm thick aluminum sheets clamped together in a frame. This stack is then diamond machined across the edge of the stack to form a flat surface and then diamond machined to a shallow concave sphere in the center of the stack. The stack is then released, each slice is rotated by a small angle then the stack is re-clamped. We envisage using a pinned or machined staircase system for controlling the slice rotation. Figure 8 shows the image slicer mirror stack after diamond machining of the spherical mirror shape and after the mirror stack has been rotated. The slicer assembly is carried from the CWS plate on the IFU-1 support. The slicer assembly is adjustable in rotation about two axes to steer the 29 separate telescope pupils onto the 29 mirrors of the pupil mirror array.
Figure 8: IFU image slicer stack after machining of the spherical optical surface. The stack is shown aligned for machining (top) and after each slice has been rotated (bottom).
A
half-size pinned prototype image slicer has been constructed to demonstrate the
feasibility of this approach. The alignment of the prototype was tested on an
optical bench and shown to be acceptable. However, it will be necessary to
construct a prototype to the specification of the concentric IFU design to
convincingly establish the viability of this approach.
A machined staircase is an alternative assembly system. This can align all the slicer mirrors from one master surface to the required 30 mrad angular tolerance slice-to-slice. Figure 9 shows part of the staircase assembly system. The surrounding housing has been omitted for clarity. The part shown here forms the front of the slicer housing and is machined in one piece from 6061 aluminum alloy to match the expansion of the mirrors. The master surface and the two opposing staircases are machined in one operation to maintain accuracy. Opposite pairs of defining edges on the staircases are 54 mm apart and the step-to-step height is ~ 57 mm and varies a little from the central slice towards the top and bottom slices. To meet the 30 mrad tolerance, the required step accuracy is ~ 1.6 mm per span or 0.8 mm per step. This should be feasible on a good quality CAM milling machine.
At assembly, each of the 29 slices in the stack is placed gently down onto two defining edges, one on each staircase. Each slice rotates around the common focal line that passes down the center of the stack. The mirrors face the staircase and each mirror touches the staircase defining edges on the flat diamond turned areas adjacent to the mirror, and spans the beam aperture. This beam aperture defines the length of each slitlet and hence the field of the IFU. It must be accurately machined. Stops in the surrounding housing align the slices endways to the necessary 30 mm. Anvils on the top and bottom of the stack and rods through the holes in the stack are then clamped to form a permanent assembly.
Figure 9: Staircase image slicer alignment system.
5.7.2 Pupil Mirror Array
Both the pupil and field mirror arrays are diamond turned into similar 6061 aluminum alloy blanks. Figure 10 shows detail of these blanks. The blanks are roughed to shape on a CAM milling machine, then transferred to a CAM diamond turning machine with x,y,z axis control. A spinning fly-cutter then generates 29 slightly toroidal, concave pupil mirrors. Figure 11 shows the mirror cutting procedure. The spinning fly-cutter is represented by the flat disk. The curved edge of the disk represents the industry standard diamond tip radius of 0.76 mm which produces a smooth finish. The center of the spinning cutter follows the looping CAM path shown in Figure 11 and cuts the 29 mirrors on a 448 mm radius. There are no steps between the mirrors.
Figure 10: Detail of pupil and field mirror array blanks.
Figure 11: NIFS pupil mirror generation procedure. The top figures show the pupil mirror blank and the path of the fly-cutter. The lower-left figure shows the blank rotated by 4°. The lower-right figure shows the looping CAM path. The vertical paths follow the curve of the mirror.
This fly-cutting procedure does not generate a true spherical surface (§4.21). To minimize departures from a sphere, the array blank is rotated in a jig by 4° to place the two equators of the generated spheres at the center of the mirrors. This generation process requires that all three axes of the diamond turning machine to be active for all but the center mirror. This process requires a freeform generator such as the Moore 500FG DT machine and there are relatively few of these in the world.
A
simplified version of the required tool path has been programmed and executed
on a prototype pupil mirror array using a CAM milling machine at RSAA with
acceptable results. A much more elaborate tool path would be required to
manufacture the actual pupil mirror array.
5.7.3 Field Mirror Array
The field mirror array is generated in a similar manner to the pupil mirror array. All the mirrors are spherical with a radius of 60.834 mm. This array is convex about the image slicer and compliments the concave pupil mirror array. Figure 12 shows the crowded region around the pupil and field mirror arrays mounted on the IFU-2 support structure.
Figure 12: NIFS IFU-2 assembly carrying the pupil and field mirror arrays. The triple fold mirror is seen at right.
A
prototype linear field mirror array was produced with a CAM milling machine at
RSAA. While demonstrating the feasibility of the approach, a much more
elaborate tool path is needed to manufacture the actual field mirror array.
5.7.4 Triple Fold Mirror
The three principal arms of the NIFS optical system are longer than the CWS plate is wide. The triple fold mirror (Figure 13) folds all three arms of the optical system into the vacuum jacket from one multi-faceted mirror. This mirror is constructed from a 90 mm square piece of 6061 aluminum alloy with mounting points directly machined into the blank. All three flat mirrors are diamond turned into the blank in one accurate operation in a diamond turning machine of the type used to cut polygon scan mirrors. The triple fold mirror is mounted to the IFU-2 support structure without adjustment.
Figure 13: NIFS triple fold mirror schematic.
5.8 Spectrograph Collimator
NIFS uses a de-centered Bouwers optical system as a spectrograph collimator. The collimator consists of a spherical primary mirror and a concentric meniscus corrector lens. The meniscus is manufactured from a calcium fluoride disk ~ 100 mm in diameter, two opposite sides of the blank are then trimmed to leave the 55 mm wide meniscus. The meniscus is mounted with respect to the de-centered camera axis and appears tilted in the optical layout. A compliant mount that allows for temperature induced strain carries the meniscus and allows for some coarse x,y adjustment on the cold work surface. The lens has little power so we do not envisage that any z adjustment will be required.
With a reflective surface measuring 160´70 mm, the collimator primary mirror is the largest optical component in NIFS. We envisage diamond turning the mirror from 6061 aluminum alloy for two reasons; Firstly, no mirror cell is required and the three point mount is machined directly into the aluminum blank before diamond turning. This provides a more stable mount for this large component than a glass mirror in a cell. The three point adjustment system is necessary to both focus and properly center the grating pupil on the grating. Secondly the aluminum mirror will shrink in step with the aluminum cold work surface. This will allow us to focus the collimator onto the pupil mirror array using an optical table at ambient temperature and have it retain focus at 60 K. The mirror has a focal length of 434.3 mm and the temperature induced change of focal length is 1.76 mm at 60 K. A temperature difference of 5 K between the aluminum mirror and the cold work surface will defocus the collimator by 13 mm. This should have little effect on total image quality.
5.9 Spectrograph Grating Wheel
NIFS carries seven gratings and one mirror in a large grating wheel. This is the largest and heaviest of the three wheels in the spectrograph. The wheel uses a replica of the NIRI wheel drive and locking system, the gear is driven from a Phytron stepper motor, and a NIRI-style magnetic encoding system is used.
All seven gratings and the one mirror are replicated on 6061 aluminum alloy blanks (Figure 14). A three point adjustment is machined directly into each blank to allow each grating in turn to correctly place spectra onto the detector. Most of the gratings fill the detector completely so the adjustment points are as far apart on the blanks as is possible in order to make for a sensitive adjustment system. The reflective surface of the mirror is parallel to the back of the blank. Each grating blank in turn is machined with the required grating angle with respect to the back of the blank. Each blank is individually fitted with a sheet metal mask to limit the diffracted pupil size as seen from the collimator. Figure 15 shows the complete grating wheel. The gratings are skewed on the wheel face in order to place the wheel as near to the CWS plate as possible for stability reasons.
Figure 14: NIFS grating and mirror blanks.
Figure 15: NIFS spectrograph grating wheel.
Each of
the gratings and mirror has a mass of about 300 g giving a total mass of 2.4
kg. These eight gratings plus the 2.5 kg wheel rotate on preloaded deep groove
ball bearings that are lubricated with vacuum spluttered moly-disulphide
lubricant. These bearings are ~ 80 mm in diameter, have a ~ 80 mm track
spacing, and are preloaded in X configuration to about three times the 48 N
force applied by the mass of the components. A total pre-load of 145 N is small
for bearings of 80 mm diameter even when dry lubricated.
The NIFS spectrograph camera has a focal length of 288 mm and a detector with 18 mm pixels. To meet the Gemini requirement that the image be stable to 0.1 pixel per 15° change in attitude, angular grating movement must not exceed 3 mrad with respect to the camera. This is a demanding but achievable condition. It requires that the bearing and latching system be preloaded to eliminate backlash, and that the wheel and mounting structure be very stiff to avoid excessive gravitational sag. The gratings also need to be stable to at least this tolerance on their three point adjustable mounts. Indeed, similar levels of stiffness are required in many areas of the instrument.
We may be able to apply extra bracing to the edge of the wheel in at least one direction, most likely the spectral direction with a modification to the NIRI style wheel mechanism. The NIRI mechanism uses a narrow lock arm rotating on a flexure pivot. We may be able to make the NIFS grating lock arm triangular with flex pivots in two corners of the triangle. With strong spring pressure on the lock arm we can stabilize the edge of the wheel in one direction to the required tolerances of about 3 mrad. RSAA will continue to investigate this area as the NIFS design progresses.
5.10 Spectrograph Camera
All five elements of the refractive spectrograph camera are elastically mounted in a single aluminum tube. This tube is carried in turn by brackets from the CWS plate. A tongue and groove system allows the camera to be easily removed and replaced on the cold work surface and maintains alignment to the detector. This alignment groove in the cold work surface extends under the detector package to maintain alignment between the camera and detector housings. The input end of the camera abuts the sheet metal zero order trap and the output end carries part of a labyrinth seal to seal the camera to the detector housing but does not touch the detector housing. The interface between the end of the lens housing tube and the camera will receive considerable design attention as it is important that it not leak radiation directly to the detector.
5.11 Spectrograph Detector Package
Figure 16 shows plan and elevation views of the proposed detector mounting arrangement. At assembly, the ceramic package for the Rockwell HAWAII-2 detector is inserted in a 19´19 Pin Grid Array socket carried on the multi-layer detector circuit card. This card is then placed in the detector housing, the ceramic package is pressed against a Macor ceramic defining-ring, and the card is spring loaded to maintain contact between the package and the ring with cooling. The detector housing is a 6061 aluminum alloy box with removable top and side panels. A protruding tongue on the bottom of the box mates with the alignment slot in the CWS plate to maintain camera to detector alignment. The defining ring has no tip-tilt adjustment with respect to the detector housing but is pre-aligned on an optical table using an empty ceramic package. The detector housing is positioned when warm and then repeatedly cold tested for proper focus. Two strip clamps along the top and bottom edges of the housing clamp the detector housing to the CWS plate. Push-pull screws and a small micrometer fitted to the CWS plate will aid the focus setting.
Figure 16: NIFS science detector mounting arrangement.
The Rockwell HAWAII-2 detector is cooled by conduction through the central 15x15 pins of the Pin Grid Array socket. Heat passes along solid copper wires soldered to each of the central pins, these wires loop to adjacent copper blocks. Heat now passes from the blocks to copper braids, through a labyrinth light seal in the lower housing wall, to the second stage closed-cycle helium cooler connection located 80 mm from the detector housing.
Figure 17 is a plan view of the detector housing, braid, and ribbon cable systems. Low capacitance electrical ribbon cables begin at plugs on the lower corner of the circuit card and pass to a hermetic connector on the nearest point of the vacuum jacket center section. These ribbon cables consist of very narrow copper tracks printed on and between black woven Teflon fabric. The ribbon cables pass down from the circuit card, through a labyrinth seal in the rear wall of the detector housing, through a second labyrinth on the NIFS cover, through a third labyrinth under the radiation shield, through a slot in the floating shield, then to the hermetic connector. Each of these labyrinth seals provides successive light sealing and thermally shunts heat inflowing along the ribbon cables to the CWS plate. The SDSU detector controller is mounted vertically on the vacuum jacket center section. It is necessary to place it near the science side closed-cycle helium cooler. Fortunately the cooler microstepper drive motor, with possible stray magnetic fields and RF interference, is on the far side of the cooler and should not affect the SDSU controller. Special precautions will be taken to shield the input wiring to the controller.
Figure 17: NIFS detector to controller cabling.
5.12 Spectrograph Cover and Vent
A cast and fully machined 6061 alloy cover 8 mm thick will cover the NIFS spectrograph and make a light-tight seal to the CWS plate. Two cable labyrinth fittings located in the edge of the cover will allow cable access for the detector and the two mechanisms inside the cover. Lifting points on the cover will make for easy handling during instrument assembly.
During
vacuum pumping or unintended air admission, the NIFS cavity will not be able to
breathe with sufficient flow rate through the pick-off-probe and past the focal
plane wheel. This could apply destructive forces to the spectrograph cover. A
high capacity baffled vent on the cover will allow direct air admission through
the cover and prevent any damage to the spectrograph.
5.13 Spectrograph Mechanical Design Risks
5.13.1 Spectrograph Flexure
The
spectrograph optics must be held accurately to meet the Gemini requirement of
< 0.1 pixel flexure per 15° change in attitude. The large grating wheel has
been identified as a particular concern. However, similar levels of stiffness are required in many areas of
the instrument. This should be achievable with careful design followed by
empirical adjustment.
5.13.2 Spectrograph Alignment
The telescope pupil must be accurately aligned to the 4 mm diameter cold stop to reduce thermal emission from the telescope in the K band. There is no active adjustment of the cryostat orientation at the telescope. This adjustment will have to be made by mounting an alignment telescope on the ISS interface plate and visually setting the adjustment of the focal ratio converter mirror when the instrument is warm. The adjustment will be checked when the cryostat is cold by sighting the cold stop.
Alignment
of the IFU image slicer elements and of the pupil and field mirror arrays will
be particularly crucial. Two alternative mounting arrangements for the image
slicer elements have been proposed; a pinned system, and a machined ramp
system. These will be tested further. Alignment risks for the mirror arrays
will be mitigated by manufacturing the arrays as monoliths.
Procedures
for aligning the cold stop, IFU, and spectrograph optics will be developed
using a setup copy of the CWS plate at room temperature. The aligned optical components will later be
transferred to the cryostat and re-aligned.
5.13.3 Spectrograph Baffling
Elimination of stray light is crucial for achieving ultimate performance at J and H. Light entering the science instrument must be efficiently baffled at the cold stop, and light scattered within the spectrograph must be eliminated by light baffles. The scattering properties of proposed designs will be modeled using the OptiCAD non-sequential ray trace program.