With its coated surfaces and 58.5mm thickness of UBK7 on axis, the triplet has a spectral transmission approximately as shown in Fig. The triplet appreciably enlarges the image scale, giving an effective focal length near the axis of about 13.54m, corresponding to an image scale of 15.2 arc sec per mm. The distortion can be very closely fitted by a simple polynomial. Note that the spot diagrams indicate some radial shift in images with changing colour. Figure 1.4 Spot diagrams for the triplet corrector.
Axes are in millimeters.
Figure 1.5 . Optical properties of the anti-reflection coatings on the triplet corrector. The optical transmittance of the triplet corrector on axis, with and without the anti-reflection coatings. The guide probes and filters in the prime focus cage are operated by hand, so an observer needs to ride in the cage for all but the simplest operations. The prime focus camera has an offset guiding microscope adjustable over the ranges shown in Figure 1.5 .