CASPIR is a cryogenic reimaging camera with a
50 mm long, 10.4 mm diameter collimated beam section. The optical
layout is shown in Figure 21, and the mechanical
structure is shown in Figure 22. The camera body is
cooled to 
60 K by the first stage of a closed cycle helium
refrigerator , and the detector array is cooled to
32 K by the second stage of the cooler. The dewar incorporates
a novel design which uses five 16-position annular wheels mounted
coaxially around the cooler to produce a compact vacuum system. The
wheels are driven by motors located on the dewar base plate.
Figure 21: Layout of the CASPIR optics for the 0.5''/pixel scale (left)
and the 0.25''/pixel scale (right). In both cases light
enters at top left and passes through the field lens/dewar window,
is reflected down to focus at the aperture wheel (marked APERTURE),
passes through the collimator to the pupil position (marked PUPIL),
and is brought to focus on the detector array by either the fast
(left) or slow (right) camera.
Figure 22: Assembly drawing of the CASPIR dewar. The annular wheels are located
around the closed cycle helium refrigerator which is mounted on the
dewar top plate. Motor drives for the wheels are located on the
dewar base plate. The f/18 telescope beam enters from the left and
passes through the dewar window/ field lens, the Aperture Wheel,
collimator lens, Upper Filter Wheel, Utility Wheel, Lower Filter
Wheel, and Lens Wheel to reach the detector at lower left. The
dewar is shown configured with a cross-dispersed grism and the fast
camera lenses in the optical beam. The slow camera lens is shown
out of the beam to the right in the Utility Wheel.
The CASPIR dewar mounts on port A of the IMB (Fig. 23).
The rotatable dichroic mirror in the IMB directs the f/18 telescope
beam to the dewar. The dewar window is a Sapphire/CaF 
doublet
which acts as a field lens to image the telescope
exit pupil (the secondary mirror) onto an internal cold
stop . A cold gold-coated mirror then directs the
beam down, parallel to the dewar axis. The telescope focus is located
immediately below this mirror at the Aperture Wheel . The Aperture Wheel contains baffles for the 0.5''/pixel
and 0.25''/pixel focal plane scales, a range of slits for the
grisms, coronograph masks, and the field mask used for imaging
polarimetry (Appendix E, Table
16). The diverging beam then passes to a fixed
MgO/CaF doublet collimator lens which produces
the collimated beam section. Immediately below this, in the
collimated beam, is the Upper Filter Wheel
which contains the filters listed in Table
17 of Appendix E. Next is the
Utility Wheel which is located at the
pupil plane. This wheel contains the direct
imaging cold stop, the MgO/CaF doublet slow camera lens for the 0.25''/pixel focal plane scale, the six
grisms, and the Wollaston prism polarimeter
analysers (Appendix E, Table 18).
Below the Utility Wheel is the Lower Filter
Wheel which contains the filters listed in Table
19 of Appendix E. Note that some
of the broadband filters require blocking
filters located in the Lower Filter Wheel. Both filter wheels contain
clear positions. The detector array should not be exposed to optical
light while cold, so the software
prevents both clear position being selected at the same time. Note
also that the Lower Filter Wheel is located in an f/10.4 converging
beam when the slow camera is used, so some refocusing between filters
may be required. The final wheel is the Lens Wheel
which contains the MgO/BaF fast camera
lenses for the 0.5''/pixel focal plane scale. These are rotated out
of the beam when the slow camera is used. The detector array is
located at the lower end of the dewar at the camera focus.
Figure 23: Schematic of CASPIR mounted on the IMB showing the CASPIR
dewar, the dichroic mirror, the calibration lamp module, the IMB X-Y
stage, and the Tip-Tilt dewar.