Quote:
Originally Posted by Wavytone
Tony is correct.
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Unfortunately eyepiece manufacturers do not state what the field curvature is, which is unfortunate as a lot of observers waste money on eyepieces that aren't a particularly good match for their scope...
The issue is that the curvature of the focal plane can be convex or concave towards the objective (and sky). For refractors, SCTs, maks and cassegrains it is concave towards the sky. For Newtonians however the focal plane is CONVEX.
The reason this matters is that the curvature inherent in the Masuyama (and for that matter a nice 38mm ProStar eyepiece I have) is that the curvature in the focal plane of the eyepiece is a fair match for that in small refractors and cassegrains.
Much like stacking two soup bowls on top of each other, a close fit.
But in a Newtonian the curvature is opposite to that of the eyepiece and the result is like stacking soup bowls back-to-back - they touch at one circle (in focus) but for the most part are far apart (out of focus). In a scope, you can't get the whole field to focus and have to rack the eyepiece in and out only to find there is no sweet spot where most of all of it is in focus.
While at f10 or even f13 it might not seem much of an issue, it is noticeable in sharp optics at f7. At f5 it is irritating to the extent you'll be looking for something that is a better match. And while field flatteners may help they don't really fix this especially for fast Newtonians.
Conversely the humble Plossl has field curvature that is a great match for Newtonians, but not refractors.
Televevues main market is the US where big dobsonians rule. So it is no surprise their eyepieces are a good match for these. But in Japan and Europe smaller refractors are the biggest seller and hence their eyepieces are intended primarily for these.
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Well, I am humbled. I have not heard this concept before, which is remarkable since I have been in the hobby for over 50 years. It shows that one continues to learn something every day!
I assume from the above quote that orthos are a better fit for refractors than reflectors? (It certainly seems valid from my experiences).