Quote:
Originally Posted by La_Displuke
Can anyone elaborate further on this topic and help me understand how to match them up.
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OK...
1. maximum and minimum focal lengths.
The maximum useful ep focal length is one that gives an exit pupil around 5-6mm diameter in your scope. This is equivalent to 6mm x the focal ratio of the scope.
The minimum useful ep fl. is one that gives magnification around 1.5X per mm of aperture. This is equivalent to the focal ratio of the scope (numerically in mm) divided by 1.5.
2. field curvature and coma.
The common telescopes for visual use do not have flat focal planes. In particular Newtonians, Schmidt cassegrain, Maksutovs and most refractors all have curved focal planes and the radius of curvature can be surprisingly short. What's more the focal plane may be convex, or concave towards your eyepiece.
What this means is that supposing you have an eyepiece that has a flat focal plane, when it is focussed perfectly for the stars at the centre of the field, the stars off-axis will be defocussed increasingly badly towards the edge of the field.
If on the other hand your eyepiece happens to have a curved focal plane than is a fair match to that of the scope, images should be nicely focussed from centre to the edge.
Secondly off-axis coma, which fast Newtonians have very conspicuously when you look at a wide field with a simple low-power eyepiece.
3. focal ratio, and depth of field.
In a fast scope (f/6 or less) the depth of focus is very shallow which means that if there is a mismatch due to field curvature it will be quite visible. In a much slower scope (e.g. f/10 SCT's, or my f/15 Maksutov) the depth of focus is considerably greater and the defocus due to field curvature is not noticeable.
By way of example, in a fast Newtonian, say a 2-40cm f/4 - f/4.5 scope, the humble Plossl eyepiece performs remarkably well despite having just 4 elements because it happens to have a curved field that is a fair match for these newtonians. But putting a Plossl into a fast refractor, say f/6, the result will probably look pretty awful because the refractor has a field curvature opposite to that of the Newtonian.
Yet with a typical Meade or Celestron SCT working at f/10, most eyepieces will be fine for the simple reason that at f/10 the depth of field is enough to accommodate the curved field without you noticing it in most eyepieces.
It is also evident that several types of premium eyepieces are predominantly aimed at users of fast Newtonians on the basis that on any other scope it will be "good enough" though not perfect.
4. Eye relief, barrel size and weight
Eye relief is important - many observers - some like Kunama and myself prefer 20mm or so and loathe using high-power eyepieces where you really have to screw your eye onto the lens - e.g. short plossl, orthoscopic and monocentrics under 10mm focal length. These may be OK on a warm night but I have once frozen my cornea to an eyepiece in subzero conditions and its not something I wish to repeat (very painful).
Barrel size is worth considering as well. If you have a smaller scope, stick to 1.25" eyepieces. If you have a larger scope with a 2" back, consider whether to use a series where all eyepieces will fit a 2" back so you're not swapping 1.25/2" adapters all night.
Weight - the big low power eyepieces are often heavy - some exceed 500g. Swapping to a smaller high power one may cause you to rebalance your scope. I got sick of this long ago and one of the reasons I have a set of LVW's is that they're all close enough to the same weight that rebalancing the scope is not required.
