next up previous contents
Next: Telescope control Up: The 1.0-m Jacobus Previous: Summary of mechanical


Details of the individual optical elements are collected in Table gif. Table gif gives the most important parameters of the two optical configurations available, illustrated in Figure gif. 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 gif, 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 gif, 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
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
Figure 2.23
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 ].

next up previous contents
Next: Telescope control Up: The 1.0-m Jacobus Previous: Summary of mechanical

Tue Aug 15 16:42:46 BST 1995