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Posted by Daniel Johnson on November 17, 2002 04:06:26 UTC

First, I trust that the inexpensive eyepieces you refer to are Super Plossl or better, not the "modified achromat" type (modified achromat is also called Kellner, MA, or SMA). If you're referring to the $99 special they're running for a full set of Super Plossls with the purchase of a scope, GRAB IT! BUY IT. Although Plossls are now outperformed by some very expensive eyepieces, they are nonetheless a match for anything that was on the market when I entered astronomy 25 years ago, and they beat anything Charles Messier or William Herschel ever had.
For low magnification with nebulae, getting the f/6.3 focal reducer allows you to get reasonably low powers while sticking with 1.25" eyepieces. The LOWEST power that you can use with any scope is about 4 times its aperture in inches--beyond that, your eye can't dilate to accept all the light, and it can seldom use that low power well. I find that 6 or 8 power per inch of aperture works better, for several reasons:
1)It puts most of the light in the more accurate, central part of your cornea/lens system, instead of at the periphery, which focuses poorly.
2)If you are under any but the darkest skies, or if you are near or past retirement age, your pupil won't dilate fully anyway (with age, you lose a lot of your eye's aperture because dilation stops working well, and city lights also block dilation).
3) If the exit pupil (the cylinder of light leaving the eyepiece from each star) is maxed out to your eye's full dilation, you can't move your eye a single millimeter without losing light. Higher powers give smaller exit pupils.
I own a 10-inch SCT and don't use powers below 50--usually not below 60--for dim objects. A 32 mm Plossl and the f/6.3 focal reducer are a good match for low power for f/10 Schmidt-Cassegrains.
For planets on nights of average seeing, aim for an eyepiece that will give you 25-30 power for inch of aperture, which means the 12.4 mm eyepiece from Meade, with the 9.7 mm being useful many nights. Usually, higher magnification will just give more fuzz (250x is a nice number). On rare nights you may reach 500x, but don't hold your breath. You'll go months between such nights.
Also, can you get by without glasses? If so, you'll be able to use shorter-focal-length eyepieces from most manufacturers, though with short focal lengths (say, 10mm or less) you'll feel your eyelashes brush the glass with every blink. You'll find the short eye relief of the 6.4 mm to be annoying even without glasses, and you'll rarely be able to use it (at 470x). Televue's Radians give wonderful eye relief usable by most eyeglass wearers even at 6 or 8 mm focal lengths, but you can buy 4 or more Plossls at REGULAR price for the price of one Radian.
So if you're buying individual eyepieces, which ones do you need? If I could own only three, I'd probably get the 32mm, plus either the 15mm and 9.7mm, or the 20mm plus 12.4.
The 40mm adds very little. It gives lower power but NOT a wider field, since both the 32 and the 40mm take in all the light that a 1.25 barrel will admit. Plus, with the f/6.3 focal reducer, the 40mm is too low-power to use, for reasons described above.

Dan Johnson, 10-inch LX200 GPS user.

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