Quote:
Originally Posted by silv
... Can we tell from the build type ... which EP is designed for correcting concave (Newts) and which convex (refr., CS, Mak)?
For example, if a lens design ends with a concave one nearest the eye-end, it'll correct best in a newt? Or vice versa, a lens design ending with a convex will best correct in a newt?
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No, you cannot.
FIRST, four general observations:
A. The problem depends on focal ratio - squared. In scopes with fast f-ratios (f/7 and under) matching the eyepiece to the scope is a significant issue, and at f/4 it is a huge problem. OTOH at f/10 (Celestron/Meade SCTs)it is not an issue and almost any eyepiece made in the past 100 years will work fine.
B. In the US the telescope market is dominated by large fast dobsonians and Celestron SCTs. The two main US eyepiece brands - Televue and Explore Scientific - both have tailored their eyepiece designs specifically to suit fast newtonians, not refractors. What that means will be evident below.
C. In Europe and Japan the telescope market is dominated by small refractors, which have field curvature opposite to that of newtonians, and no coma.
So it should be no surprise that the premium Japanese eyepiece manufacturers - Vixen, Pentax, Takahashi, Nikon, and Masuyama - all make eyepieces optimised for refractors, not Newtonians.
D. The cheap Chinese "budget" eyepieces such as GSO, Prostar and numerous clones... aren't optimised for anything and you're taking pot luck there. But one thing is for sure - they'll all work reasonably well in SCT's.
DETAILS: The real problems are:
1. The aberrations of a complete system (ie telescope + eyepiece+ eyeball) are the sum of the aberrations contributed by each element, ie telescope and eyepiece and eyeball.
A good choice of eyepiece can negate the aberrations of the scope (and the result is very good). For example, Newtonian scope (coma and positive field curvature) plus Plossl eyepiece (negative field curvature) = very good.
A poor choice of eyepiece makes matters worse. For example, Newtonian scope (coma and positive field curvature) plus Erfle eyepiece (positive field curvature) = quite poor.
2. Field curvature of the eyepiece is a parameter that manufacturers do not disclose - in order to keep people guessing and shelling out $$$ to find out the hard way.
3. Coma, or the lack of it, from the scope and whether the eyepiece cancels that. From trying eyepieces in the field with various scopes over 40 years it is quite evident all the Televue eyepieces MUST have some negative coma to cancel the coma produced by fast Newtonians, but those from the Japanese makers do not.
Alternatively if you really want to use an f/3 Newtonian visually, you had better add a coma corrector (eg. Televue Parracor, there are others).
3. Astigmatism, in the observers eye. Televue make a gadget to correct for this - the Dioptrix.
FWIW I use Vixen SLV and SSW eyepieces with my scope, which is a 9" f/13 Rumak. It has a fully corrected flat field - no need to pay the Televue Tax - and doesn't do low power... it starts at 100X and runs to 660X.