The hyperbolic secondary constitutes 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 maximum movement of the secondary mirror is 20mm about the nominal focus position. For the f/15 system this produces a shift of 241mm 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.
As of 1998 the Harmer-Wynne f/8.06 focus has been decommissioned.
|Finder (doublet)||Sph. surfaces||200mm||3250mm|
|Focal length (mm)||15000||3250|
|Field diameter (arcmin)|
|no vignetting||34||10,30,90( 2)|
|Diameter of central obstruction (mm)||406|
|Focus/mirror shift( 1)||11.7|
|1) Movement of focal position for unit movement of secondary mirror|
|2) Depends on eyepiece used|