Thanks for your very informative posts as usual Alex. I'm interested in learning more, could you recommend where to find out more about optical matches? Or does this only come with experience?
Cheers, Iain
Quote:
Originally Posted by mental4astro
... Telescopes we think of as focusing light at just one single point. Not so. In fact, a scope's focal point is actually a 'plane'. AND a curved plane a that! Depending on the design of the scope, this plane is either convex (refractors, SCTs, Maks) or concave (Newtonians). And depending on the focal ratio of the scope, the shape of this curve is either very deep (fast scopes) or shallow (slow scopes).
Eyepieces then in turn need to be designed to deal with one shape or the other. The way the EP is designed WILL influence the way it deals with the manner the focal plane of the scope falls onto it. This is why some eyepieces are brilliant in one scope, say a fast refractor, and poor in a fast Newtonian. Nothing wrong with the eyepiece. The two are just not an Optical Match. And unfortunately, many people write off this or that eyepiece because they see a whole bunch of aberrations, and those aberrations would not be there if they had put the EP into the right scope design!!!
Some eyepieces can work well with both convex and concave focal planes. Oddly enough, these also tend to be more expensive EPs too...  . BUT, they do not throw up the exact same image quality!!! There will be some compromise somewhere in the optical light train. Either a little field curvature, or a little softer in focus, or a little astigmatism right at the edge, or a shift in eye relief, or a little more eye strain, or eye placement becomes more critical... Something will change, however small. < WARNING: another cliche> - No Free Lunch here... But the nett result is still a very damn good image. ... |