Collimator

CUBIC will utilize an innovative collimator design to achieve moderate spatial resolution () below 1 keV, where galactic spatial structure is interesting, while maintaining a high A above 2 keV, where the sky is nearly isotropic and the incident photon flux drops roughly as . This is achieved by a two-part collimator. An square aperture defines a (Y Z) field of view (FOV) for each pixel, and acts as a coarse pinhole camera. It is surrounded by a concentric window composed of m Be foil, which is opaque below 1 keV and nearly transparent above 2 keV, where it provides a FOV.

The result is a four-fold increase in sensitivity to photon energies in excess of 2 keV, where the incident intensity is dropping sharply with increasing energy. The low and high energy fields of view for a single pixel are shown in the left half of Figure 6.

This pinhole camera design provides some coarse spatial resolution, since the fields of view of pixels on opposite sides of a given CCD differ somewhat. The plate scale of the instrument is 0.399 cm/degree, which corresponds to 148 pixels/degree or 0.406 arcminutes/pixel. The total 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 (0.0277 sr) at low energies and (0.0699 sr) at high energies, as shown in the right half of Figure 6. The displacement of the two CCDs provides an even larger FOV for the entire instrument: at low energies and at high energies. This coarse angular resolution 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 6 adjacent regions.

The response of this collimator to a point source is shown in Figure 7 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.

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