The coronograph mode in CASPIR is used to image faint emission near brighter objects with sizes comparable to the seeing disk. The coronograph does two things; first, it occults the direct light of the bright object over a region of the sky comparable to the seeing disk, and secondly, it blocks light from the bright object which is diffracted around the telescope secondary support structure.
This is achieved in CASPIR by selecting either the 2'' or 5'' diameter occulting disk in the Aperture Wheel (see Table 16), and selecting the pupil plane mask in the Utility Wheel (see Table 18). This means that the CASPIR coronograph can only be used with the fast camera (0.5''/pixel scale), as the slow camera is also located in the Utility Wheel. The size of the occulting disk is based on the expected extent of the faint emission being measured and the seeing, and is selected by typing, e.g.:
CASPIR/APERTURE=DISK2
Only one pupil plane mask is available. This is selected by typing:
CASPIR/UTILITY=MASK
Use of the coronograph masks in CASPIR is complicated by the alt-az
nature of the 2.3 m telescope. Because the 2.3 m telescope has an
alt-az mount, CASPIR is continuously rotated to maintain a fixed
orientation with respect to the sky. The image of the secondary
support structure then rotates in the pupil plane. The pupil
plane mask is a Maltese Cross shaped baffle with each section of the
cross having a half-angle of 15 
. The mask has a fixed
orientation with respect to the dewar. In using the coronograph pupil
plane mask, it is desirable to set the instrument rotator position
angle so that the secondary support structure remains vignetted for
the longest time. If the parallactic angle is increasing set
the instrument rotator position angle to the parallactic angle
plus 15 . If the parallactic angle is decreasing
set the instrument rotator position angle to the parallactic angle
minus 15 . The secondary support structure will then
be behind the pupil plane mask, but will move as the telescope tracks.
The rate of motion depends on position on the sky. Consult Figure 9.9
in the 2.3 m Telescope Observer's Manual to estimate this speed.
Field rotation at the Cassegrain focus is not monotonic and changes
most rapidly near the zenith. Consequently, coronograph observations
are best done away from the zenith.