The Optics

(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, 177, 25. 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. 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. 

(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. 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.

Below is a summary of the optical characteristics of the JKT optics (more information can be found at JKT optics (ING Observers' Guide):