Details of the individual optical elements are collected in Table . Table gives the most important parameters of the two optical configurations available, illustrated in Figure . The two optical systems share the same parabolic primary, with a clear diameter of 1.000 m and a focal length of 4.596 m. Two secondary mirrors are available on interchangeable top ends, as follows:
(a) a spherical secondary which, together with the primary mirror and an afocal doublet, constitutes the f/8.06 Harmer-Wynne system described in Monthly Notices of the Royal Astronomical Society, vol 177, p25p 1977. This configuration gives a highly corrected flat focal plane, 90 arcmin in diameter, located 515mm behind the primary mirror. The angular scale is 25.6 arcsec/mm. As can be seen from the spot diagrams reproduced in Figure , this system gives images smaller than 0.5 arcsec in diameter over the unvignetted field for incident light wavelengths in the range 3650 Å to 8521 Å. It is not envisaged that the f/8.06 focus would normally be used without the corrector; astronomers interested in using this system for non-standard applications (as for example with their own instruments) should discuss their requirements with RGO staff.
(b) a hyperbolic secondary constituting, with the primary, a conventional f/15 Cassegrain system. The curved focal surface is located 760 mm behind the pole of the primary mirror, giving a 34.4 arcmin diameter field with a scale of 13.8 arcsec/mm. As shown in Figure , the total aberrations on a flat focal surface passing through the nominal focal position on axis are calculated to be up to about 2 arcsec in diameter over the unvignetted field. Clearly, the off-axis images can be slightly improved by focussing the telescope at a compromise position for the field of view of interest.
The maximum movement of the secondary mirrors is 20 mm about the nominal focus position. For the f/15 system this produces a shift of 241 mm about the optimum focal position; the on-axis image size is calculated to grow approximately linearly with focus shift up to a maximum diameter of 0.3 arcsec. Use of the Harmer-Wynne f/8.06 system at other than the nominal focal position is not recommended without prior ray-tracing.
Figure 2.21: Optical layout of the JKT, showing the f/8.06 Harmer-Wynne and f/15 Cassegrain configurations [ TIFF ].
Table: Summary of mirror characteristics for the JKT
Table: Optical characteristics of JKT foci
Figure 2.22: (a) Spot diagrams for images at the f/8.06 focus of the JKT for various wavelengths and field radii. The diameter of the circle is 1.0 arcsec. [ TIFF ]. (b) Spot diagrams for a flat surface at the f/15 Cassegrain focus of the JKT [ TIFF ].