Optics

Details of the individual optical elements are collected in Table 1. Table 2 gives the most important parameters of the available optical configuration, illustrated in Figure 3. The parabolic primary has a clear diameter of 1.000m and a focal length of 4.596m.

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.

 

Figure 3: Optical layout of the JKT, showing the f/15 Cassegrain configuration

 

Table 1: Summary of mirror characteristics for the JKT

Element	Shape	Asphericity	Working	Focal	Separation from	Material
			Diameter	length	primary
Primary	Paraboloid	-1	1000mm	4596mm		Cervit
f/15 Secondary	Hyperboloid	-3.545	307mm	1811mm	3342mm	Zerodur
Finder (doublet)	Sph. surfaces		200mm	3250mm

 

Table 2: Optical characteristics of JKT foci

Focal station	Cassegrain	Finder
Focal length (mm)	15000	3250
Focal ratio	f/15	f/16.25
Field diameter (arcmin)
no vignetting	34	10,30,90( 2)
50% vignetting	50
Scale (arcsec/mm)	13.8	63.5
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