The following web pages highlight the differences in f8, f10 and f12 systems in Ritchey-Chretien, Classical Cassegrain and Dall-Kirkham (at f12 only). You can also view a comparison of a 20" f8.0 Newtonian to the f8.0 Ritchey-Chretien and Classical Cassegrain.

 For a brief description of the optical design differences between the designs listed above, click here.

August 2008 article outlining inherent optical performance of Classical Cassegrain vs. Ritchey-Chretien Cassegrain can be read here.


20" f8.0 Classical Cassegrain

20" f8.0 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 0.7162 degrees.


Another way to view the chart is through this total field spot diagram. Center is on axis, white circle is the 10.8 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.

Notice how the RC design creates star patterns that are much more circular in nature. Excellent for astrometry work and for "pretty pictures" it creates round stars.



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.

To see this system with a field flattener, click here.

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.