The PFADC was planned with a CCD imager as the default instrument. The design assumed that a 4mm BK7 filter would normally be placed within the back focal space (distance 11 in Table 1) in front of a 6mm window of fused silica. With the corrector as built, theory predicts the best imaging with the focal plane of the detector 91.9mm behind the rear face of the nominal corrector. Under these conditions, the focal plane is very nearly flat and the system is achromatic. So long as the surfaces of the window and filter are flat, their precise locations within the back focal distance have almost no effect on the optical behavior of the system.
Argus places its fiber tips directly in the image plane. As a result, there is 10mm less refractive material in the system than the design calls for, which causes an image shift and a small amount of chromatism. To maintain the same optical distance between corrector and detector, the Argus fibers must be placed closer (88.6mm at 4400A) to the rear surface of the nominal corrector. Over the entire field, more than 90% of the transmitted light at all wavelengths from 3500A to 10000A falls into a .7 arcsecond circle when the system is focussed through a B filter. This image quality is sufficient to do efficient broadband spectroscopy through Argus' 1.86'' diameter fibers.
The nominal thickness of the filters used in the Prime Focus Camera is 2mm. For best imaging with 2mm filters, the film surface should be 89.3mm from the nominal corrector. The filters normally used for photography have different thicknesses and compositions. The image quality is much better than was achievable with the old triplet correctors.
In all three cases, the instruments have been mounted with back focal distances which are nearly optimum for the respective configurations in the nominal design. The actual "as built'' and measured distances (+/-.1mm) between the rear of the corrector and detector plane are 91.6mm, 88.5mm and 89.2mm respectively for the PFCCD, Argus and the PF Camera.