Gemini Near-Infrared Integral-Field Spectrograph (NIFS)
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Duplicate NIRI Components
A large part of the NIRI instrument will be duplicated by IfA to form the superstructure, cryostat, and OIWFS for NIFS. The duplicated NIRI components will be unmodified to the greatest extent possible. However, certain changes are required to accommodate the NIFS spectrograph or to remedy problems that have arisen in commissioning NIRI. This section discusses the NIRI components that will be duplicated and identifies and justifies any proposed modifications.
An initial parts list of NIRI items to be duplicated is included as an Appendix (§15).
6.1 NIFS ISS Interface Plate
Figure 1 shows an exploded view of the NIFS-ISS interface plate and vacuum jacket. The interface plate is at the top of the figure. The NIFS vacuum jacket and carrier plate attach directly to the interface plate. Thus the interface plate transfers the entire weight and bending moment of NIFS to the ISS. The whole subsystem of the interface plate, its corner brackets, and the attached alignment system will be duplicated for NIFS.
Figure 1: NIFS-ISS interface plate (top) and an exploded view of the vacuum jacket.
6.2 NIFS Carrier Frame
The NIFS carrier frame is a large welded frame measuring 1300´1300´2100 mm (Figure 2). This frame is the largest NIFS component and supports the main counterweight, electronics cubicles, cables and lines, the balance ballast, and the interface to the Gemini instrument handling equipment (i.e., air pallet and instrument platform lift). It is unlikely that there will be any changes made to the current design for this part other than the addition of more ballast to the frame as NIFS is substantially lighter than NIRI. All of the frame, ballast, registration fittings to the instrument handling equipment, and support structure for the electronics cubicles will be duplicated for NIFS.
Figure 2: NIRI cryostat with carrier frame and two thermal electronics enclosures.
6.3 NIFS Cryostat
6.3.1 NIFS Vacuum Jacket
All of the vacuum jacket components shown in Figure 1 will be duplicated for NIFS including the entrance window and the window cover which are not shown in the figure. IfA has agreed to move the large O-rings from the vacuum jacket front and back sections to the center section to ease assembly of the vacuum jacket. A tapped hole to permit the use of a lifting eye will be added to the center of each end plate, if it is not already included.
6.3.1.1 Cable Feed-through Issues
The lower face of the center section in Figure 1 has three through holes and a number of tapped holes for the Wildfire array controller used by NIRI. These should be deleted from the NIFS specific vacuum jacket. The near face of the vacuum jacket center section has a through hole in the lower left below the closed-cycle helium cooler port. This hole should be deleted as it lies under the NIFS SDSU controller for the science detector. A new hermetic connector port on the upper left of this vacuum jacket face to the left of the closed-cycle helium cooler port will be specified by RSAA to IfA within a reasonable time frame so that detail drawings can be modified. The decision to move the science detector hermetic connector is justified in §8.5.
6.3.1.2 External Attachment of Racks and Cables
The SDSU controller for the NIFS science detector attaches vertically to the near face of the vacuum jacket center section in Figure 1, to the left of the cooler port and below the new hermetic port. Tapped holes to attach the new controller will be specified by RSAA to IfA within a reasonable time frame so that detail drawings can be modified.
6.3.1.3 Lifting and Handling Points
IfA has agreed to add lifting points to at least the center section of the vacuum jacket. Lifting points may be added to the front and back sections if suitable attachment points do not already exist.
6.3.2 NIFS Cold Work Surface Plate
It is envisaged that two CWS plates will be manufactured by IfA. A minimal plate will be used at RSAA to speed integration and testing of the NIFS spectrograph. Meanwhile, the complete CWS plate will be installed in the duplicate cryostat to allow integration and testing of the vacuum jacket and OIWFS.
6.3.2.1 OIWFS Side
Figure 3 shows the large cryostat parts for NIFS. The CWS plate is at the center of the figure with the titanium tines and pads attached. To the left is the science side radiation shield. To the right is the OIWFS optical table and gimbal carrier. All of these components will be duplicated for NIFS. There will be relatively few changes made to the OIWFS side of the CWS plate. At the present time, only the addition of a ring of tapped holes around the beam transfer hole to mount the pick-off probe and some tapped holes for anchoring cables are envisaged.
Figure 3: NIFS cryostat components from left are the science side radiation shield, the cold work surface plate, and the OIWFS optical table.
6.3.2.2 Spectrograph Side
Extensive changes will be made to the science side of the CWS plate. The attachment holes for the NIRI optical table will be deleted and the cable trenches and focuser fittings will be removed. The temperature sensor and heater unit will be moved slightly and the cover light trap groove will be moved inwards by ~ 6 mm. The NIFS spectrograph side of the CWS plate will have tapped holes for the spectrograph cover, optics carrier housings, lens carrier tube, detector carrier, filter wheel, grating wheel, sheet metal baffles, and cable tie down points. This will amount to a total of perhaps 100 holes in all.
6.3.3 NIFS Radiation Shields
6.3.3.1 OIWFS Side
The whole of the NIRI OIWFS side radiation shield will be duplicated for NIFS. The only modification envisaged is the addition of a large hole and ring of tapped holes to the upper panel to carry the OIWFS baffle tube.
6.3.3.2 Spectrograph Side
The whole of the NIRI imager side radiation shield will be duplicated for NIFS. The only modification envisaged is the addition of a second cable saddle and baffle in the inboard frame to give ribbon cable access to the new SDSU hermetic connector. A slot may need to be provided for the science detector ribbon cable in the floating shields if these shields extend over this area.
6.4 Closed-Cycle Helium Coolers
NIRI uses two Leybold Coolpower 130 Cryocoolers. These coolers are the source of significant vibration in NIRI. This model has been superseded by a more recent version with lower vibration, but also lower cooling capacity. It is expected that NIFS would reach a temperature of ~ 75 K using this cooler. NIFS does not need to reach as cold a temperature as NIRI (because NIFS uses a 2.5 mm cut-off detector) so the lower cooling capacity is not expected to cause problems. However, a colder temperature would be required if NIFS used a 5 mm cut-off HgCdTe/CdZnTe MBE detector. The newer model cooler may be used for NIFS. If it is, the external cooler mounts and internal cooling attachments will all require modification.
6.5 NIFS On-Instrument Wavefront Sensor
The NIFS OIWFS provides closely coupled x,y image motion sensing for instrument autoguiding and focus sensing via two prisms in a collimated beam for active control of the telescope focus. This OIWFS has two great advantages over more conventional guiding systems. Firstly, the wavefront sensor is mounted from the same cold work surface that carries the NIFS spectrograph and is cooled to the same temperature. This mechanical arrangement ensures minimum differential flexure between the OIWFS and the NIFS image slicer. Secondly, the wavefront sensor uses a near-infrared detector and so senses x,y drift and focus change at the same wavelength as the spectrograph. This ensures the best possible guiding and focus control for NIFS.
Figure 4 shows the NIRI OIWFS assembly. The NIFS OIWFS will consist of most of these parts. The entire optical train of lenses, mirrors, prisms, filters will be duplicated. All supports and housings, brackets and plates will be duplicated. Both of the gimbal mirror and filter wheel mechanisms will be duplicated, but not the focus mechanism. This focus mechanism will be replaced by a simpler focus unit that is adjustable when the cryostat is warm. The beam onto the OIWFS detector is slow and we believe that the simpler focus mechanism will be adequate. The OIWFS optical table and photon shield will be duplicated and IfA may consider deletion of the NIRI focal plane wheel slot and fittings in the optical table. This will reduce scattered light inside the OIWFS optical table.
Figure 4: NIFS OIWFS assembly. The large field lens (upper right) passes light via fold mirrors (lower middle) to the X-Y gimbal mirror (upper left). The beam then passes through an optical train to the detector (middle right).
The two fold mirrors in the lower part to Figure 4 rotate the telescope field by 45° with respect to the ISS cardinal axes. This means that the rows and columns of the OIWFS detector will be at 45° to the NIFS image slicer. For control systems simplicity and best guiding performance, we may find it advantageous to rotate the science field using fold mirrors in the NIFS spectrograph.
6.5.1 Cryostat Entrance and Pick-off Probe Baffling
NIFS does not use the large and heavy NIRI beamsplitter wheel. This omission leaves a large region of the OIWFS cavity unbaffled. NIFS will be fitted with a ribbed cylindrical baffle attached to the OIWFS radiation shield to prevent radiation entering this cavity. This closely baffles the OIWFS field-of-view and extends down to, but does not touch, the OIWFS optical table. The NIFS pick off probe enters the baffle through a slot and does not touch the baffle.
6.5.2 OIWFS Calibration
6.5.2.1 Star Projector
NIFS will be fitted with a star projector to allow engineering tests of the OIWFS during the day or when the instrument is not on the telescope. This projector is partly enclosed inside the pick-off probe and consists of a lamp or LED, pinhole, and lens system. The projector forms an f/16.22 star image at the entrance focal plane of the OIWFS directly under the pick-off probe. This simulates a fairly faint star. The artificial star is not movable but provides an internal reference for testing the repeatability of the OIWFS gimbal mirror. No remote control will be provided for the projector; the lamp will be switched from a nearby manual control panel.
An internal system in ALTAIR generates an artificial star for testing the AO system. Under some conditions this star can be made to continue on to NIFS and be directed down beside the pick-off probe to the OIWFS. This will provide two stars simultaneously and allow daytime testing of differential flexure between the NIFS OIWFS and ALTAIR on a moving telescope.
6.5.2.2 Flat Fielding
The Gemini Calibration Unit (GCAL) mounted on the ISS can provide a flat field source for the NIFS OIWFS. Illumination on the OIWFS detector will be limited to a small section of the detector by the field stop at the OIWFS internal focus. However, this flat field can be sampled at any point in the large OIWFS field-of-view by tilting the X-Y gimbal mirror.
6.6 NIRI Duplication Risks
6.6.1 Vibration Coupling From Closed-Cycle Helium Coolers
NIRI has experienced substantial problems with vibration coupling from the closed-cycle helium coolers to the CWS plate, sufficient to smear out test images at the detector. The large Leybold coolers used on NIRI appear to have little vibration due to the microstepper motor drive but large transient vibration due to helium valving at ~ 2-4 Hz . Vibration applied to a cryogenic instrument also causes fretting corrosion to bearings and lock systems over relatively short periods of time and may introduce microphonic noise in the detector system.
Modifications were made at IfA to place the helium cooler units on rubber mounts. This helps the vibration transfer problem. However, it is not yet known what overall system performance will be achieved under actual observing conditions.
6.6.2 OIWFS Repeatability
At the time of writing, operation of the NIRI OIWFS had not yet been demonstrated. The NIFS OIWFS is required to reliably set with an accuracy of < 0.005˛ defined to be one tenth of the half slit width step that will be required to “sub-sample” spatially perpendicular to the slit as described in the NIFS Operational Concept Definition Document. The performance of the NIRI OIWFS will be closely monitored as NIRI is commissioned. Modifications to the current design may be required for NIFS.