I think I need to correct a misinterpretation of my post ...
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Originally Posted by Don Pensack
My original advice was based on the idea that a particular scope should start out with a 1X/2X/3X magnification scheme, where X= a particular number based on the aperture. For instance, on an 8", X=50, on a 12.5", X=70, and on an 18", X=80.
That's because the larger scope can use higher magnifications and for the same f/ratio, larger scopes have higher magnifications at the same exit pupil.
Is that all the eyepieces you would need? Probably not as seeing conditions are rarely the same everywhere (people under very steady skies tend to use higher magnifications) and you might find yourself wanting a magnification in between two that are derived this way.
Such a recommendation flies in the face of exit pupil considerations.
Take the 18" f/4.5 scope with a Paracorr. The effective f/ratio is f/5.18,
so a 2mm exit pupil would be a 10.4mm eyepiece = 228X. Most places, that would be too high a magnification for a mid-power eyepiece, even though it is a mid-sized exit pupil.
so though the exit pupil idea seems to work well for small scopes, it doesn't seem to work very well on large scopes, which is the reason I came up with a 1X/2X/3X idea for a basic set of eyepieces to build a collection around.
In my scope, that was 70x, 140x, 210x, or 26mm, 13mm, 9mm and I have built around that idea by splitting the 9mm into an 8 and 10mm and going beyond in the high end to 6 and 4.7mm.
But the idea of a basic set of 3 around which you can build a collection is not a bad one.
And if you choose to use exit pupils to do so, that's fine, but just remember it doesn't work very well in big scopes. I have a friend with a 28" f/4.2 scope. With a Paracorr, his focal ratio is f/4.8. A 9.6mm eyepiece would yield 358X. Where he observes, he can use that magnification fairly often, but not much higher. But that's a 2mm exit pupil, which is mid-power by exit pupil standards.
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Don, if you're referring back to my post, with its advice to use f/stop intervals based on exit pupil, as you seem to be, you're overlooking a very important step.
The first step I recommend (as do many people) is:
Quote:
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Originally Posted by Astro_Bot
... picking your lowest and highest practical magnifications (based on a number of factors, including how often you will get to use it/them), selecting corresponding eyepieces at those extremes ...
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In this scheme, you would use the f/stop interval (exit pupil) to fill-in
between the magnification extremes, hence whether the smallest exit pupil is considered mid-sized or not is irrelevant.
The absolute size of the exit pupil has to be considered anyway - that's largely how an owner would arrive at the lowest practical magnification along with consideration of effect of a central obstruction (if any). In your method, you address this with a "look up table" of aperture vs. lowest magnification, and whilst that's certainly one way to go, IMHO, it's an unnecessary approximation that doesn't suit all users, although as an eyepiece vendor, if you're there to apply it to each purchaser, I'm sure it would work well. For everybody else, better to calculate exit pupil and use it directly. (In Stephen's case, he already had a 27mm Pan with 6mm exit pupil, so that was not an issue here).
The bottom line is that, whatever method is used, it remains only a rough guide, and neither of us is Stephen.
