This document describes the procedure for installing and aligning the spectrograph optics of the Gemini Near-infrared Integral-Field Spectrograph (NIFS). The procedure is based on the concentric integral-field unit (IFU) optical design. The procedure is applicable when the optics are being installed on the cold work surface plate in the NIFS cryostat, as well as on the setup work surface plate outside the NIFS cryostat.

2 Applicable Documents

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3 Introduction

The optical elements of the Gemini Near-infrared Integral-Field Spectrograph (NIFS) will be mounted off the cold work surface plate in the NIFS cryostat. It will be necessary to precisely align these elements after they are installed on the cold work surface plate to obtain optimal optical performance from the NIFS integral field unit (IFU) and spectrograph. The procedure to be used to align the NIFS spectrograph optics is described below. The procedure assumes that the vacuum jacket, cold work surface plate, On-Instrument Wavefront Sensor (OIWFS), and all necessary electrical wiring are installed, and that the assembly has been vacuum and cold tested before alignment begins.

The various components of the NIFS spectrograph are shown in Figure 1.

Figure 1: NIFS spectrograph optics mounted on the cold work surface plate.

3.1 Pick Off Probe Assembly

The NIFS cryostat is first placed spectrograph-end down on a service trolley in the RSAA clean room. The cryostat is then opened on the OIWFS side by removing the vacuum jacket end-plate, floating shields, and radiation shield. There is now ample space to install the pick off probe, focal plane mask wheel, pre-slicer optics, and the OIWFS baffle on the OIWFS side of the cold work surface plate. The electrical wiring to the focal plane mask wheel mechanism can then be connected and tested, and the OIWFS-side radiation shield, floating shield, and vacuum jacket end-plate are re-assembled. The cryostat is then turned over with the aid of the clean room overhead hoist. There are no NIFS alignments on the OIWFS side of the cold work surface plate, other than the OIWFS test projector. This alignment will be pre-set in the optical laboratory before installation in NIFS.

3.2 Pre-Slicer Optics to Cold Stop Alignment

The NIFS spectrograph is now assembled. The cryostat is already on the clean room trolley standing on the OIWFS end. The vacuum jacket end-plate and front section on the spectrograph side are removed first. The floating shields and radiation shields on the spectrograph side are then removed using the overhead hoist. This allows ample access to the spectrograph side of the cold work surface plate. The cold work surface plate is 70 mm below the edge of the vacuum jacket center section. An alignment telescope (e.g., the Taylor Hobson) attached perpendicular to the vacuum jacket center section now sights in through the cryostat window to the center of the pick off probe. While sighting from the pick off probe onto the cold stop, the focal ratio converter mirror is adjusted to center the cold stop in the alignment telescope. The focal ratio converter adjustment screws are accessed through the large hole in the cold work surface plate. The filter wheel, pre-slicer fold optics, IFU-1 mount, and the image slicer are now installed. The pre-slicer fold optics are then adjusted (one mirror only) to sight on the central slice of the image slicer using the same alignment telescope position.

3.3 Integral Field Unit Alignment

The alignment telescope is now removed from its position in front of the cryostat window and is replaced by a test projector. The test projector sends an f/16.2 beam in through the cryostat window to focus on the focal plane mask wheel. This provides an artificial star that is used to align the remainder of the optical system.

The optical centerline of the NIFS spectrograph is 90 mm above the cold work surface plate and 20 mm above the edge of the vacuum jacket center section. This allows the alignment telescope to be carried on a table and bracket clamped perpendicular to the side of the vacuum jacket. The alignment telescope then sights in horizontally at the height of the optical centerline. The IFU components deliberately have an odd number of mirrors so that alignment can be performed on the central mirror. The IFU-2 mount, the pupil and field mirror arrays, and the triple fold mirror are now installed on the cold work surface plate. All support structures carry accurately scribed lines 90 mm above the cold work surface plate to aid alignment using the alignment telescope.

The alignment telescope is clamped to the side of the cryostat such that it can view past the IFU-2 mount to focus on the image slicer. The telescope is set 90 mm above and parallel to the cold work surface plate using movable targets, and is then used to check the height of the projected spot on the central slice of the image slicer. The alignment telescope bracket and table are now moved to the other side of the cryostat and placed such that the telescope sights along a line through the Bouwers collimator, off the central mirror of the triple fold mirror, and onto all three central mirrors of the IFU. The image slicer, pupil mirror array, and field mirror array are pre-aligned in the optical laboratory in one assembly. The complete IFU assembly is then installed on the cold work surface plate. The height and centering of the three central IFU mirrors are checked using the spot from the test projector. Small corrections in alignment can be made after installation if necessary.

3.4 Bouwers Collimator Alignment

The Bouwers collimator mirror, Bouwers corrector shell, and their supports are now installed on the cold work surface plate. The alignment telescope is moved around the vacuum jacket center section to a position behind the grating and arranged to look into the last of the facets of the triple fold mirror and adjusted to the optical centerline. The Bouwers collimator mirror and corrector shell are first adjusted onto the optical centerline and then adjusted for collimation and focus onto the central element of the field mirror array. This is an iterative process. The alignment telescope is now focused to a point near the grating pupil, the test projector is moved up and down on the vacuum jacket surface, and the grating pupil is viewed to ensure that it is formed correctly near the expected position.

3.5 Baffle Installation

Most of the sheet metal baffling is now installed and checked for clearance using a laser. A flat tie-plate ties the filter wheel, image slicer, mirror arrays, triple fold mirror, and collimator component support structures together. The tie plate is placed in position and clamped to each optical support. The alignment telescope is still in position so the alignment can be checked as the tie plate to installed. The alignment telescope is then removed and the grating wheel and filter wheel mechanisms are installed, wired and tested. Lastly, the gratings and the baffles of the zero order trap are installed.

3.6 Spectrograph Camera and Detector Preparation

The NIFS spectrograph assembly now moves to the optical laboratory. A dummy cold work surface plate is placed on the laboratory optical table. The spectrograph camera is assembled, fitted to the dummy cold work surface plate via the tongue and groove alignment system, and is aligned using collimators and the HAWAII-2 multiplexer. The detector enclosure, detector circuit card, and the HAWAII-2 multiplexer are now assembled and fitted to the tongue and groove alignment system on the dummy cold work surface plate. The HAWAII-2 multiplexer is then sighted through the spectrograph camera using a collimated telescope (e.g., Hilger Watts Collimator). The detector assembly is translated in the tongue and groove system using a micrometer to bring the HAWAII-2 multiplexer into focus. The detector mount is tested for squareness to the optical axis by separately focusing on each corner of the multiplexer. Any tilt in the detector mount is corrected by machining the locating ceramic defining ring.

3.7 Spectrograph Camera and Detector Installation: Grating Alignment

Assembly now moves back to the clean room where the spectrograph camera is located on the tongue and groove system and attached to the cold work surface plate in the NIFS cryostat. The detector assembly is placed at the previously determined camera focus position, with allowance for the calculated shinkage. The detector ribbon cable is installed and the detector enclosure panels are closed. Now using white light and spectral lamps in the test projector, all five gratings and the direct-imaging mirror are aligned onto the multiplexer at visible wavelengths in second or third grating order. This will be a long process and will require suitable arc lamp maps.

The detector ribbon cable labyrinths are installed, the spectrograph cover is installed, and the radiation shields are assembled. Finally, the vacuum jacket is assembled and NIFS is ready for pumping.

Appendix A: List of Figures

Figure 1	spec on cws.bmp