The field of view of CUBIC is defined by a mechanical collimator.
We use a coarse ``pinhole camera'' collimator
consisting of a square aperture mounted
above our baseplate near the edge of the satellite.
The aperture is located 9.0 inches from the CCDs and has an opening
that is 
wide.
This aperture defines a field of view of 
(Y 
Z) for any pixel of the CCDs, giving a solid angle at low energy of
square degrees (9.58 millisteradians).
This design has the advantage of being very simple,
inexpensive, and easy to calibrate. It has the further advantage of
eliminating any possibility for low energy scattering off collimator walls,
which produces
a strongly energy dependent solid angle that is difficult to calibrate
accurately.
The geometry of this collimator design, which is shown in Figure 15, is such that the CCDs view the sky in slightly different directions,
displaced symmetrically about the satellite X axis. In fact, the
experiment is ``cross-eyed''. Furthermore, each CCD views a total
solid angle on the sky nearly four times as large as that seen by a
single pixel, as shown in the right-hand part of
Figure 15.
The field of view (FOV) of a single CCD, after taking into account the
portion of the CCD exposed to the sky through the mask, is 
(90.83 square degrees).
The displacement of the CCDs provides an even larger FOV for the entire
instrument: 
at low energies.
The coarse angular resolution implied by
this statement will permit us to choose between making a relatively
short measurement of the average spectrum within a 
region on the sky, or using a longer exposure to obtain
simultaneous spectra from up to 8 adjacent 
regions.
The plate scale of the instrument is 0.399 cm/degree,
which corresponds to 148 pixels/degree or 0.406 arcminutes/pixel.
Above 2 keV the sky is isotropic and the incident photon flux drops
roughly as 
. In order to increase our sensitivity to
this cosmic X-ray background component, and to ensure that the sky flux
dominates over the expected particle background over our entire energy
band, we have included a secondary collimator
(dimension 
) that defines a 
field of view for energies above 
keV (shown by the dotted lines in Figure 15.
This provides a solid angle of 127.7 square degrees, or 38.9 millisteradians.
This is
accomplished by means of a border of 
m Be foil surrounding the
open aperture of the collimator. The Be foil is opaque below 1 keV.
The result is an energy-dependent field of view:
for energies below 1 keV, and 
for energies above 3 keV, with a transition region at
intermediate energies.
(NOTE: for convenience, we will often refer to these as the 
and 
fields of view, but the actual open angles are given
above.)
Because the field of view is determined by
geometry and by the transmission of the Be foil, the energy dependence
can be easily calculated and calibrated. This design results in a
four-fold increase in the sensitivity of CUBIC \
to the cosmic X-ray background without sacrificing angular resolution for
studies of the galactic diffuse background.
The response of this collimator to a point source is shown in Figure 16 as a function of off-axis roll angle.
If we place a bright point source 
off the X-axis in the
roll direction, it will be observed at the peak of the response of
one CCD, but will be out of the field of view of the other CCD.
This
will permit us to make simultaneous on-source and off-source measurements,
which will be very useful for studying bright point sources, supernova
remnants, and
enhancements in the diffuse background, since it eliminates variations in
the non-cosmic background rate (charged particles, terrestrial and solar
X-rays, etc) between the source and background regions that
can make such measurements very difficult with a nonimaging instrument.
We note that the Be foil is quite fragile and is mounted in an exposed location on the payload. We will provide it with a protective cover to prevent accidental destruction during ground testing. However, during launch this aperture will be unprotected, and it is essential that no debris be present within the payload shroud which could damage the foil. Be is highly toxic if ingested, but should pose no danger unless the foil is broken.
The baffles prevent off-axis X-rays below 10 keV from reaching the CCD detectors.
