20" f12 Classical Cassegrain 20" f12 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 0.47746 degrees.                   Another way to view the chart is through this total field spot diagram. Center is on axis, white circle is the 16.0 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 astemtry work.                     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  6096/240" f3.0 & 1524/60" 431.8/17.0"  50.8/2.0"  139.75/5.50" 4.00             M1 to M2 spacing** M2 baffle diameter Aspheric Constant M1 Aspheric Constant M2 Aspheric Constant M1 Aspheric Constant M2  1132.84/44.6" 186.16/7.33" -1 -2.7778 -1.0432 -3.1764  Classical Cassegrain Classical Cassegrain Ritchey-Chretien Ritchey-Chretien     * 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.