Quote:
Originally Posted by Wavytone
Well of course they suit an SCT - the field curvature is a fair match for that of the SCT which is concave towards the sky, like a refractor. And Alex does have an SCT as well as several dobs.
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There is rumour going round the amateur astronomy community that the focal surface curvature relative to the eyepiece of Newtonians is opposite to that of other telescope types. Because of this certain eyepieces are reputed to work best with refractors or SCT's and others are more suited to Newtonians. It's an interesting theory but unfortunately only partially true in general and quite incorrect in one fact - the focal surface curvature direction
relative to the eyepiece is the same for nearly all telescope types.
In anticipation of the storm of protest from the resident experts I could quote a number of supporting references but will use only Rutten and Van Venrooij, Telescope Optics here. Anyone who wishes to gain some understanding of the interaction between telescope objectives and eyepieces should read the whole book but particularly Chapter 16, Eyepieces for Telescopes.
Under the sub-heading The Performance of Objective-Eyepiece Combinations, on page 181 they quote "Between the tangential and sagittal focal surfaces lies the average curved field, a somewhat imaginary construct.
For most objectives -- refractors, Newtonians, and Cassegrain-like systems this surface is inward curving", by which they mean convex towards the eyepiece (or "concave towards the sky" as Wavytone puts it).
The origin of the myth lies in an incomplete understanding of raytracing and particularly the surface curvature conventions. In optical specifications and ray-tracing results, focal surfaces for SCT's and other Cassegrains, and refractors all have negative sign, those for Newtonians are nearly always positive*, but it is wrong to conclude from this that curvatures relative to the eyepieces are opposite.
To quote Rutten et al again:
Chapter 20 Optical Calculations sub-heading 20.4 Sign Conventions on page 240:
"Light entering the optical system travels from left to right.
Distances from left to right are signed positive; those from right to left, negative.
Curvatures with the convex side to the left are signed positive; otherwise they are negative."
For refractors and Cassegrain-like types the rays approach the focal surface from the left and the focal surface has a negative sign ie convex to the right and
towards the eyepiece.
For Newtonians the rays approach the focal surface from the right (the diagonal mirror is ignored - it has no optical power and turning the rays at right angles is not regarded as changing their direction). The focal surface has a positive sign ie convex to the left and
towards the eyepiece.
The interactions between eyepiece and objective are quite complex and involve a combination of astigmatism and field curvature with the former being usually dominant. Certain eyepieces will work better or worse with different telescopes but it does not depend only on the type of optical system. Again - read Rutten and Van Venrooij if you want to go some way towards understanding how it all works.
* There are of course some exceptions - Maksutov Newtonians have a focal surface that is convex away from the eyepiece ie. a negative sign of the focal surface radius.