Any combination of apertures and filters was allowed. Polarizers were only used with the P2 filter and the aperture. At long wavelengths the Airy disc is significantly larger than the smallest aperture available. Section 2.9 gives more details about the calculation of , and . Intensity fraction of a point source entering the apertures of PHT-P. A theoretical monochromatic point spread function centred in the aperture has been adopted.
All apertures are circular except if noted.
The precision of the ISO pointing had implications on the achieved photometric accuracy. Ga with elements. Increased photon absorption was achieved by total reflection of a 30 wedged pixel surface and an integrating cavity. This corresponds to the projection of the array on the sky. PHT C200 consisted of 4 pixels arranged in a matrix.