Case study: a fast Maksutov-Ritchey-Chretien of F/1.3

Published 12.12.2014

Design of a very fast Maksutov-RC type catadioptric telescope, with an internal image plane.

Continuing my line of reflective telescopes case-studies, I designed a Maksutov-Ritchey-Chretien mix catadioptric telescope. It has an aperture of 200mm and a focal length of 260 mm, making the relative aperture an extremely fast F/1.3. The layout and the optical tube assembly along with characteristics would make this design a good addition to an automated Near Earth Object detection system.

I based this design on a similar Cassegrain design featured in Lens Design by Milton Laikin (4th ed, page 192). Interesting in this design was the multiple refractive element part that produced an excellent color correction despite being made from only one glass type. I retained this feature with the modification of using glass that has a good transmission even in infrared. The following analyses were made in visual range, though. Also the last meniscus lens from the Laikin design was removed, and central obscuration reduced to tolerable levels.

The basic layout looks like a Maksutov. First a three-element all spherical (a positive lens followed by a negative and a meniscus) lens unit is placed in front of the RC type reflective system (both mirrors aspherical to 8th order). The flat 40 mm diameter image circle (corresponding to a FFOV of 9°) is formed inside the system, between the mirrors. That (and the image side NA of 0.36) and relatively short back working space of 50 mm pretty much excludes the use of commercial cameras. A specialized sensor or an array can still be supported through the central hole of the primary. On the outside, the layout is a tube twice the length of the diameter. Optical components would be heavy, but they are nicely distributed through the short OTA, making balancing easier.

Even with the central obscuration (of 40%), the MTF curves over the field is excellent even with 100 cc/mm resolution. The lower wavelengths escapes somewhat at the edges, but polychromatic spot diameter remains below 4 microns throughout the full field.

The lenses can be made from any material, even Schott N-BK7, but should be adjusted to transmission range, and also to the refractive index that would result in best curvatures for a wavefront correction. With almost a Full Frame image size, an array of various sensors of different wavebands could be used for imaging of respective parts of the field of view. Rotation of the OTA or the sensor array would enable quick imaging of the whole FOV in short time and multiple bandwidths.

The design has very little distortion, only 0.34%. Distortion doesn't affect image quality, but does move image from where it should be. For a hardcore, mosaic-building stargazer, 0.34% is quite unacceptable, but as an automated NEO detector, where the computer is only interested whether any stars have moved in relation to picture taken a day before, this level of distortion is not a problem as it can be easily compensated in post-processing. The layout is very compact and nicely balanced for such a robotic system, and easy to protect from the elements.