Below is a comparison of a 20" f8.0 Ritchey-Chretien Cassegrain that is with (right column) and without (left column) a field flattener.

20" f8.0 Ritchey-Chretien 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.

with field flattener

RMS spot size at edge of field: 26.9 microns.

RMS spot size at edge of field: 6.2 microns.

Total field.

Total field. Same field as on the left but this time with the field flattner.



Point Spread Function (visual appearance) off axis, at the edge of the 2.0" diameter full field.

Point Spread Function (visual appearance) off axis, at the edge of the 2.0" diameter full field but this time a field flattener is utilized.

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







* 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.


From the graphic illustrations above it is readily apparent how advantageous the use of a field flattener can be for the wider field. Planetary work, if centered on the optical axis (using optics that are accurately collimated), does not benefit from such a flattener. But if wide field imaging with 1.5"+ CCD chips is the goal, then a field flattener can greatly benefit the optical system.

The RMS spot size at the edge of a 2.0" diameter field is 26.9 microns without a flattener and only 6.2 microns with the flattener used. This is greater than a four fold reduction in RMS spot size. Moreover the first chart, RMS spot size, shows that only at the very edge of the field does the RMS spot size cross above the airy disk size. A telescope with such performance would always be seeing limited, unless adaptive optics were employed.

Defocus of the uncorrected system allows for a maximum reduction in spot size to about half of the 26.9 micron inherent RMS spot size, at the cost of increasing the on-axis spot size past the airy diameter. Use of defocus on a field flattened system only degrades the optical performance.

 A field flattener offers a much greater reduction of the RMS spot size over the entire field, while keeping the on-axis RMS spot sizes as small as they would be if focused for on-axis (without a field flattener). A field flattener benefits the optical system far greater than using defocus on an uncorrected system. Although the latter is an excellent technique to maximize performance of an uncorrected system.

There are numerous mentions on the Web regarding Ritchey-Chretien telescopes having a flat field. This is simply not the case for every Ritchey-Chretien. In other words, the term Ritchey-Chretien does not inherently imply "flat field." A Ritchey-Chretien Cassegrain can be designed with its secondary and primary mirrors having the same radius of curvature, thus yielding a true flat field, but this is NOT how most commercially produced telescopes are designed and made. To do so increases the central obstruction (of the secondary mirror itself, not the baffles) typically 50% or higher. The SLOAN Digital Sky Survey is one Ritchey-Chretien (2.5m) that uses such an optical design. The Henrietta Swope Telescope is another example (1.0m).

Ironically it is much more feasible to make a Ritchey-Chretien set that does NOT have a primary and secondary of the same radius of curvature. There are numerous reasons for this. One is that both systems benefit greatly from the use of correctors, for moderate to wider field use. The corrector for a flat field Ritchey is much more complex and therefore more expensive than a simple field flattener that is required on a more conventional (curved focal plane) Ritchey-Chretien systems. 

 A conventional Ritchey-Chretien can have a secondary that is 37.4% central obstruction for a f8 system. The flat field Ritchey-Chretien (utilizing mirrors of the same radius of curvature) needs a 46.3% central obstruction (secondary mirror) plus the secondary itself is extremely hard to fabricate, which again drives costs for such a system skyward... The zero Petzval f8 Ritchey-Chretien (utilizing mirrors of the same radius of curvature) is also about 13" longer than the conventional Ritchey-Chretien that has an inherent curved focal plane.

A 20" f8 zero Petzval Ritchey-Chretien has a 15.8 micron RMS spot size at the same 2.0" diameter field edge. The conventional (curved focal plane) Ritchey-Chretien has a RMS spot size of 26.9 micron at the same 2.0" diameter field edge. So there is definitely some gain in spot size quality with the zero Petzval choice but it still doesn't compare to 6.2 micron RMS spot sizes of the field flattened conventional Ritchey-Chretien. The latter, even with the cost of an expensive field flattener (say $2000-$4000), would still cost far less than the zero Petzval optical set. The zero Petzval Ritchey-Chretien can benefit from a corrector but that corrector is more complex and therefore more expensive than the field flattener required for the conventional Ritchey-Chretien. So the zero Petzval optical set with a dedicated corrector becomes an extremely expensive endeavor. Plus central obstruction (of the secondary itself prior to baffling) is usually ~50%. A conventional f8 Ritchey-Chretien typically has a central obstruction (glass only) of ~38%.