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If you had just one eyepiece, what should it be?

Posted by Michael Covington   12/28/2005 00:00:AM

If you had just one eyepiece, what should it be?
Here are the criteria for the selection of that eyepiece:

(1) The magnification should be high enough to show most, if not all, of the fine detail that the telescope picks up, so you can view planets and double stars.

(2) The magnification should be low enough to show faint nebulae and galaxies well.

(3) The field of view should be wide enough that a computerized ?go-to? telescope will generally get objects within it.

(4) The eyepiece should bring out the best in the human eye, whatever that means.

Start with the last criterion first. The human eye is sharpest with a pupil size of about 2 mm. When the pupil is larger, there are more likely to be problems with astigmatism and with imperfections at the edge of the lens of the eye. When it?s smaller, opaque particles inside the eye begin to be visible as spots.

So let?s consider eyepieces with a 2-mm exit pupil. That means the focal length of the eyepiece is 2 mm times the f-ratio of your telescope: 8 mm for an f/4 telescope, 20 mm for an f/10 telescope, and so on. Most amateur telescopes are between f/5 and f/10.

Interestingly, when you choose a 2-mm exit pupil size, the other requirements fall neatly into place. Consider magnification. The resolving power of the human eye is about 60 arc-seconds. That of a telescope is 4.55 arc-seconds divided by the aperture in inches.

We want to magnify the telescope?s true resolving power so that it looks like 60 arc-seconds in the magnified image. That means that, for a telescope of diameter D inches, we need magnification M, where M = 60 / (4.55 / D) = 60 D / 4.55 = 13.2 D.

That is, we want a magnification of about ?13x per inch.? That means we want an exit pupil of about 1/13 inch ? which is ? guess what? ? almost exactly 2 mm.

But is a 2-mm exit pupil suitable for deep-sky observing? Beginners say no; a 5-mm or 7-mm exit pupil obviously gives a brighter image. But experienced deep-sky observers find themselves using higher powers and smaller exit pupils nonetheless. The reason is contrast. With a 2-mm exit pupil, the image is darker, but so is the sky background, and the extra magnification makes faint galaxies more visible. So, again, a 2-mm exit pupil wins.

What about field of view? Under average conditions, a ?go-to? telescope is accurate to about 1/4 degree. Amateur telescopes are usually about 4 to 10 inches in aperture. At the large end, a 10-inch telescope with a 2-mm exit pupil will have a magnification of 125x. With a wide-field eyepiece, the apparent field will be 60 degrees or more, giving a diameter of half a degree, or a radius of 1/4 degree, just what we wanted. Smaller telescopes are even better off because they have wider fields.

The conclusion? The ideal eyepiece is about 20 mm for an f/10 telescope, or 30 mm for an f/15, or 8 mm for an f/4.

The Pentax XL20 XW and Tele Vue Radian 18mm jump out as ideal choices for a main eyepiece for an f/8 or f/10 telescope. I know that when I put my older Pentax XL21 on my 8-inch f/10 SCT, I rarely feel the need to take it off, even though I also have 40-, 32-, 14-, and 8-mm eyepieces handy.