I figure that the reducer without coma correction is superior. However, I have read about a number of people on Cloudy Nights using 0.63X SCT reducer-correctors on Maksutov-Cassegrain telescopes. Even Sky-Watcher recommended using the Celestron 0.63X SCT reducer-corrector on their YouTube Channel. I would be curious to see a photo of what that might look like.
Would the coma corrector add coma if the telescope is already coma-free? And if so, would the severity of the aberration be proportional to the angular distance from the optical axis (like coma is)?
The point about aberrations being additive is this... The aberrations of the complete assembly are the sum of the contributions from each optical element. They can add positive, or negative amounts.
Most reducers are designed to match a specific type of scope - small APO's (which have field curvature but not coma), newtonians (which have coma and field curvature) or SCT's (variable combinations depending on classic SCT, ACF or Edge).
Since your gregory-mak already has:
Spherical - zero
Astigmatism - zero
Coma - very low essentially zero
Field curvature - slight, concave like a refractor or SCT
Distortion - very low, essentially zero
what you should look for is a plain reducer, no field flattening and no coma correction. While a little field-flattening might be desirable, bear in mind most of the flatteners are designed to suit small refractors with FAR WORSE field curvature than your scope has, so the net result may be worse, not better.
One for an SCT should work OK. And so will others. Yet others for Newtonians or small refractors will be terrible.
NB a reducer designed to suit a small newtonian will have a lot of coma correction AND a strong field curvature of the wrong sign for a mak, and the result won't be pretty. A Parracor is one example, there are others.
The downside with trying to find the right one to match a specific scope is that it does involve some trial-and-error, unfortunately; many resellers do not state what sort of scope the reducer is designed to suit. Some of the better ones do (Stellarvue, Astrophysics, Celestron) but many do not. You may be lucky and make a good choice, maybe you will have to try two or three before you find one that suits.
Edited by luxo II, 24 November 2021 - 03:19 AM.