Michael meant to write 'long' FL eyepieces, of course. With a decent aperture, it's not quite as severe an issue as it is with binos and smaller scopes. You won't get all the light the aperture has to offer, but you can still benefit from the resolution. And what light does make it to the retina can still be a goodly amount. There's none to spare with only 2 or 3 inches of aperture, but with 12", or more (16", 25", 33"...) even getting only 25%-50% is nothing to sneeze at. Another consideration is when you want as large a true field as possible, and can spare some aperture to get it. Medium-sized comets, for example. I have an old UO 55mm Plossl, that with my 10" scope produces a whopping 9.7mm exit pupil- but the true field of view is immense. Larger than my 32mm Widescan, even though it has an 82-degree apparent field, and the Plossl a 55-degree field. Taken to extremes, the limit with Newtonians anyway, is when the shadow of the secondary mirror gets large enough to completely fill your pupil- then you see nothing at all, on-axis! But Michael's point is a good one to keep in mind when searching for an object on the threshold of visibility with a given aperture. Adjust the exit pupil as needed to maximize throughput. It's a balancing act with exit pupil on one side, and FOV (magnification) on the other. One can easily see how contrast is thus affected. Something we should be doing with narrowband LPR filters also, BTW, for best contrast with those little wonders. --- Michael Carnes <michaelcarnes@earthlink.net> wrote:

You can sure save a bunch on eyepieces too. No point in getting a short F/L 2" eyepiece since most of the light won't make it through the pupil.

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