Why do I need a Ritchey-Chretien?

A true Ritchey-Chretien is an optical design of Cassegrain geometry using two mirrors with hyperbolic reflecting surfaces. Observatory telescopes have utilised and refined this design for five decades, but now the technology is filtering through to the amateur market. Producing such surfaces is expensive and complicated.

Like other Cassegrain-based designs, the first mirror is concave with a central perforation and in this case of short focal ratio, namely F3. The secondary mirror is convex, diverts the light cone back through the opening in the primary mirror and the resultant focal ratio is F8.

Unlike ‘lesser' designs, these two mirror surfaces are hyperbolic in shape, which is very difficult and expensive to manufacture. The sets the Ritchey-Chretien apart from other reflector designs, and is free from coma and chromatic aberration, and has the largest flattest field of all reflecting telescopes. This makes them superb choice for astroimaging.

Originally designed by George Ritchey (US) and Henri Chretien (France) mainly for photographic purposes, in the 21^st Century these telescopes are a perfect match to the increasingly large-format CCD cameras used by astroimagers at all levels. These instruments represent the very best in optical design and construction that is available today.

Astrosib (Russia) has a long history of making quality optics and they have a worldwide reputation. We are proud to exclusivly offer them in the UK and add them to our own range of Elite instruments.

Meade Instruments Corporation (USA) has developed their own design, originally called RCX or Advanced Ritchey Chretien, now changed to 'Advanced Coma-Free' or ACF for legal reasons. This avoids the necessity for two hyperbolic surfaces but uses a front corrector plate. This design is not a true Ritchey Chretien but is very favourably reviewed worldwide.

Summary of advantages of the Ritchey Chretien design:

• Coma Free image
• Zero chromatic aberration
• Flatter Field - due to lower amplification factor of the secondary mirror. The Astrosib RC has a 2.67x secondary, most SCTs have a 5x secondary.
• Only two surfaces for reduced light loss - Most catadioptric types have Four surfaces and triplet APOs have 6 surfaces. Each surface degrades the image, and the effect is cumulative.
• No refraction of light - especially in Infrared where CCD cameras are most sensitive.
• Smaller Spot Size on and off axis
• Light weight
• Open tube for rapid thermal equalisation
• On-axis mounting for CCD cameras and other accessories making for easier balance & flexure control