Catadioptric Telescopes: SCT, Maksutov and RASA

Schmidt-Cassegrain, Maksutov and Hybrid Designs

All catadioptrics combine a mirror with a corrector lens to fold the light path. The differences between them come down to the corrector, the obstruction size and how quickly the tube cools.

• Schmidt-Cassegrains: A thin aspheric corrector plate with a moderate central obstruction, typically f/10. The most versatile long-focal-length design and the best supported by accessories.
• Maksutov-Cassegrains: A thick meniscus corrector giving very high contrast and sharp planetary images, usually f/12 to f/15. Slower to cool because of the corrector's thermal mass.
• Maksutov-Newtonians: A Maksutov corrector on a Newtonian layout, giving refractor-like contrast at a faster focal ratio with a small obstruction.
• Advanced coma-free and hybrid designs: Modified Cassegrains that eliminate off-axis coma, aimed at imaging with large sensors at long focal length.
• Reducer-correctors: Typically 0.63x for SCTs, turning f/10 into a much more practical f/6.3 for imaging and widening the field considerably.
• Mirror shift and focus: Moving-mirror focusing can shift the image slightly; an external focuser and mirror lock make long-exposure imaging far more repeatable.

Why Choose David Astro for Schmidt-Cassegrains and Maksutovs

Long focal length is unforgiving of mount error, seeing and thermal management. Buying one well means planning for all three.

• Mount sized for the focal length: At 2000 mm, guiding tolerances are several times tighter than at 500 mm. We size the mount to the focal length rather than to the tube weight alone.
• Cooling advice for Canadian conditions: A closed tube taking an hour to reach ambient will show soft planetary detail all evening. We discuss cooling fans and setup timing before you buy.
• Back-focus and reducer spacing: SCT reducers have specific spacing requirements, and we supply the adapter list to hit them exactly.
• Authorized dealer: Full manufacturer warranty and verified authenticity, with service handled domestically in Canada.
• Ships from Canada: No duties or brokerage fees for Canadian buyers on heavy optics.

Schmidt-Cassegrains and Maksutovs in Canada — Frequently Asked Questions

SCT or Maksutov for planets?

A Maksutov usually gives slightly higher contrast at the same aperture because of its smaller obstruction, but is slower to cool and available in smaller apertures. An SCT gives more aperture for the money and more accessory flexibility.

How long do these take to cool?

An 8 inch SCT typically needs 45 to 60 minutes from a warm house to a Canadian winter night; a Maksutov of the same size can take longer. A rear cooling fan reduces this substantially.

Can I image deep sky with an SCT?

Yes, with a 0.63x reducer and a capable mount. Native f/10 is demanding for deep sky; reduced to f/6.3 it becomes practical for galaxies and planetary nebulae, though guiding must still be excellent.

What is mirror shift and does it matter?

The primary mirror moves on a baffle tube to focus, and can tilt slightly, shifting the image as you focus. For imaging, use a mirror lock and an external focuser to eliminate it.

Planning a long focal length setup?

Tell us your targets and mount and we will match the optic and reducer. Ships from Canada with setup support.