Flexure testing NIFS will be a major undertaking requiring considerable time and staff involvement. The test envisaged here involves rotating just the cryostat containing the NIFS spectrograph and the OIWFS, and looking at differential movement between spot images at both NIFS and OIWFS detectors. This is a valid test as feedback from the OIWFS will control the telescope tracking during exposures and hence correct for flexure between the Cold Work Surface plate and the vacuum jacket, and between the cryostat and the ISS.

4 Approach

Figure 1 shows the illumination system for the flexure tests. The lamp house contains a tungsten lamp or arc lamp, and a diffuser, and passes a broad diffuse beam through the hollow center of the trunnion, then through the cryostat window and on into the cryostat. This beam floods the pick-off probe mirror so the central part of the diffuse beam is folded down to illuminate the NIFS f00ocal plane inside the focal plane unit. The focal plane unit carries a 0.1" diameter aperture in the Focal Plane Mask Wheel and this aperture is ultimately imaged onto the NIFS spectrograph detector and re-formed into a spot by collapsing the NIFS spectra. Most of the beam from the lamp house passes by the pick-off probe and floods a 150mm diameter aperture plate carried from the OIWFS Optable. This aperture plate is drilled with ~ 50 0.32" (200 μm) diameter holes and a central 3 mm hole. A diffuser carried over the aperture plate floods the holes in the plate such that the OIWFS pupil is uniformly filled for even the most radially distant hole in the plate.

Figure 1: Flexure Test Illumination System.

The central 3 mm diameter hole in the aperture plate allows a projected spot from the OIWFS test projector to pass directly to the OIWFS. If the 20" diameter aperture is used in the OIWFS filter wheel then the inner ring of 12 apertures in the plate and the single central test projector spot will be simultaneously imaged on to a 2 mm circle on the OIWFS detector. This test image will comfortably fit on the OIWFS detector. This single image allows for both an (x,y) and rotation flexure test and for a flexure check between the Optable and the spot projector mounted in the pick-off probe. The outer ring of holes in the aperture plate are ~ 75mm from the field center and allow testing of the reach and repeatability of the OIWFS gimbal mirror at any cryostat position.

The image scale at the telescope focus, for both NIFS and the OIWFS, is 0.6205mm/arc sec. The scale at the OIWFS detector focus is 0.1078mm/arc sec and the scale at the NIFS detector focus is 0.4242mm/arc sec.

5 Cryostat Window Axis

Figure 2 shows the assembled cryostat at the beginning of a flexure test. Not shown here are the two thermal enclosures that will be mounted on trolleys to allow the enclosures to follow the rotating cryostat.

Figure 2: Lifting trunnions on the cryostat window axis.

In the figure, the cryostat is fitted with a pivot support over the window cell and two trunnions are then fitted such as to allow rotation of the cryostat around an axis normal to the cryostat window. Two nylon slings are fitted to the trunnions and then to two separate overhead cranes. Once the cryostat is lifted clear of the trolley, a jack or third crane is used to rotate the cryostat. An inclinometer attached to the cryostat will be used to measure the cryostat rotation angles. During flexure tests, the cryostat will be initially rotated in 15° increments and images from both detectors recorded. If points of serious flexure or flop are discovered then the rotation increments can be reduced to only a few degrees. The cryostat can be rotated through a full 360° but will need to be put down in the inverted position to swap the nylon slings over the helium lines and cables.

6 Cryostat Long Axis

Figure 3 shows the cryostat rotated such that the long axis is horizontal in preparation for swapping the cryostat rotation axis. Once in the horizontal position the cryostat is lowered into V-blocks on the handling trolley. The rightmost of the V-blocks in the figure is shortened to clear the lower helium cryocooler.

Figure 3: Preparing to swap rotation axes.

Figure 4 shows the cryostat ready for lifting and rotation about the long axis. The two trunnions have been moved to the vacuum jacket end plates and the lamp house is now mounted directly onto the pivot support. The cryostat can be rotated through a full 360° about the long axis but will need to be put down in the V-blocks after 180° of rotation to swap the nylon slings over the helium lines and cables to the enclosures.

Figure 4: Preparing to rotate the cryostat about the long axis.

7 Third Axis

Figure 5 shows the cryostat rotated and under test about the third axis. In this view, one of the cranes has lifted one end of the cryostat such that the long axis makes an angle of 15° to the horizontal. Note that the nylon sling folds over the left corner of the cryostat and this probably limits the range of motion to ±15°. This arrangement gives an extra degree of rotation freedom when closely investigating any flop.

Figure 5: Rotating and tilting the cryostat about the third axis.

It is not easily possible to fit the trunnions to the third (cryocooler) axis of the cryostat. However, the cryostat can be rotated further about this axis by placing it between two trolleys and lifting one end. The trunnions are drilled to carry eyebolts for this purpose (Figure 6).

Figure 6: Rotating the cryostat about the third axis.

Appendix A: List of Figures

Figure 1	Flex Test Aperture.bmp
Figure 2	Flex Test 1.bmp
Figure 3	Flex Test 3.bmp
Figure 4	Flex Test 4.bmp
Figure 5	Flex Test 5.bmp
Figure 6	Flex Test 6.bmp