The dewar should be adjusted so that the telescope exit pupil is matched to the internal cold stop. The positioning of the dewar is expected to be accurately reproducible in normal operation, so visual confirmation that the pupil image is unvignetted should be all that usually is required.
Acquire a bright star and drive the telescope out of focus so that you can clearly see the image of the telescope primary mirror. This is best done by zooming in on the image of the star in the Idle Display (see §5.3). The dewar should be tipped so that the image of the primary mirror is unvignetted. This can be done using either the normal direct imaging cold stop in position 2 of the Utility Wheel, or the alignment mask in position 1 of the Utility Wheel (see Table 18). Using the former, you see the full primary mirror aperture and must gauge whether the image is circular. Using the latter, you see the image through four small holes located at the periphery of the pupil mask and must gauge whether the four images have equal brightness. If the image ofthe primary mirror is vignetted, you will have to align the dewar. Switch MOPRA's console video to the monitor on the Cassegrain Access Platform by connecting the appropriate BNC cable to the BNC `T' on one of the RGB inputs at the rear of MOPRA's monitor. Disable the console terminal screen saver from the Session Manager menu. Stop the instrument rotator and position CASPIR towards the Cassegrain Access Platform by typing:
ROTATOR/REFERENCE=STATIONARY
ROTATOR 0
Then go to the Cassegrain focus and tip the dewar using the three adjustments on the mount to the IMB until the out-of-focus image is circular. To do this, loosen the Allen bolt slightly, loosen the inner locking nut, and adjust the outer nut. When completed, retighten the Allen bolt, and the inner lock nut.
Return the Cassegrain instrument rotator to position angle mode by typing, e.g.:
ROTATOR/REFERENCE=POSITION_ANGLE
ROTATOR 179.5