20" f10 Classical Cassegrain

20" f10 Ritchey-Chretien Cassegrain

Above chart shows on axis lower left on chart and off axis upper right in the chart. Off axis is at the edge of a 2.0" diameter field. Total field at 2.0" diameter is 0.57294 degrees.

Another way to view the chart is through this total field spot diagram. Center is on axis, white circle is the 13.4 micron airy disk diameter, outer spots outer edges of field (corners being the farthest out). Vertical white bar just left of center is 76 microns tall.



Yet another way to view the star pattern. This however is the star's Point Spread Function (visual appearance) off axis, at the edge of the 2.0" diameter full field.

Both of the above optical systems have the following parameters:

system focal length

M1 focal ratio & focal length

back focus*

 100% illumination diameter

M2 size

M2 magnification


f3.0 & 1524/60"





M1 to M2 spacing**

M2 baffle diameter

Aspheric Constant M1

Aspheric Constant M2

Aspheric Constant M1

Aspheric Constant M2







 Classical Cassegrain

Classical Cassegrain



* Back focus is defined by the distance from the front optical surface of the primary to the focal plane.

** This parameter is measured from the optical surface of the secondary to the optical surface of the primary. Since these two optical surfaces face each other, this measurement does not include the thickness of each mirror.