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Old 20-07-2017, 01:45 PM
electric
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electric is offline
 
Join Date: Jul 2017
Location: Sydney
Posts: 6
Of course the focal ratio counts for something. As you say, it affects the radius of curvature of the focal plane along with the focal length of the telescope. This is most significant with photographic applications. One example being the requirement of scope specific field flattner and focal reducers. If your scope is an ED80 f/7.5 refractor, you need flattner specific for that scope - you cannot use one made for a 100mm f/9 refractor. Astrophotographers are most aware of this.

As for catadioptric cassegrains, these produce a convex focal plane due to the, shall we say "hyperbolic" or negative secondary mirror. It is the shape of the secondary mirror that increases the native focal length of the primary to what the telescope is specified as being, and changes the shape of the focal plane. Despite the concave primary mirror.

This can be seen with the way the convex specific eyepieces also perform well in catadioptric cassegrains. Also those cheap and nasty 0.5X focal reducers, these work ok as general purpose focal reducers in refractors and catadioptric scopes, but not Newtonians. Even in slow Newtonians these reducers do not perform as well as with Cats.

Field curvature also be seen in convex specific eyepieces when used in fast refactors. These eyepieces are also cheaper. Even the older Masuyama eyepieces, SOME individual focal lengths can exhibit a little field curvature in very fast refractor. Remember, the original Masuyama eyepieces were designed when refractors were typcially slower than what is available today. Yet the degree of field curvature is nowhere as significant as in a Newtonian. Again, this is not a flaw in the eyepiece, but a factor of the design parameters of the time thirty years ago. Whether these new Masuyama eyepieces are designed for slower refractors as its predecessors or the more contemporary faster refractors, we have to see. I would suspect as yes to faster refractors due to the very large apparent field of view that they offer. But to dump all telescope designs as being the same with regards to shape of focal plane and only making the distinction as "faster" and "slower", that's disingenuous.

It comes to understanding the pedigree of the eyepieces you are using and coupling them to the most appropriate instrument. Unfortunately this is the hard part, and many reviews are written by people who do not understand this enough or have a vested interest.

Anyone have an f/5 APO to try out one of these? Oh, and most fast APOs come with built in field flattners, making things even more complicated, like the Skywatcher Esprit line.

Collin, did your refractor have a field flattner in it when you used the Masuyama?
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