Choosing your Eyepieces by Al Nagler

CHOOSING YOUR EYEPIECES

‑ by Al Nagler

MAGNIFICATION

Eyepieces determine your telescope's magnification. To calculate the magnification of an eyepiece in your telescope, divide the focal length of the telescope by the focal length of the eyepiece: Magnification = telescope focal length ‑‑‑ eyepiece focal length

TRUE FIELD OF VIEW

Eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will

see (in degrees), divide the eyepiece field stop diameter by the telescope's focal length and multiply the

result by 57.3:

True field of view = eyepiece field stop diameter/telescope focal length x 57.3

THE FIELD STOP AND APPARENT FIELD OF VIEW

The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plossl eyepieces have an AFOV of 500, Radians have 600, Panoptics have 680, and Naglers have 820.

LOW‑TO‑MEDIUM POWER VIEWING

For low‑power viewing of large objects, or to use your telescope as a low‑power finder, you will want an eyepiece that delivers close to the maximum possible true field of view (note that for 1.25" eyepieces, the maximum field stop diameter is 27mm; for 2 eyepieces, it's 46mm). Then add eyepieces covering uniform increments in smaller field stops. For example, if your widest field eyepiece has a 40mm diameter field stop and you choose a decreasing increment diameter factor of 2 (which results in a 4x decrease in area size), you'll end up with eyepieces having field stop diameters of approximately 40mm, 20mm and 1Omm. To further fill in with incremental steps, add eyepieces with approximate field stop diameters of 28mm and 14mm. Of course, avoid duplicating focal lengths. For example, if you use a 3lmm Nagler (with a 42mm field stop diameter), you would not need a 32mrn Plossl (with a 27mm field stop diameter).

In general, for each field stop size, choosing eyepieces with shorter focal lengths and larger apparent fields of view will allow you to see more detail and fainter stars. In addition, you'll have a smaller exit pupil to better match your eyesight.

EYEGLASSES AND EYE RELIEF

If you do not need eyeglasses to correct astigmatism, don't use them when observing. If you wear glasses to correct astigmatism, make sure they're multi‑coated, and try to choose eyepieces that have at least 15mrn to 20mm of eye relief, to minimize any field reduction (vignetting). However, you will find that with small exit pupils such as l mm or less, you probably will not need eyeglasses, and can therefore use eyepieces with less eye relief.

EXIT PUPIL

The exit pupil is the image of the objective that is formed by the eyepiece. It's where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope's focal ratio:

Exit pupil = eyepiece focal length ‑ telescope f/#

For reflector telescopes, it's best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.

IMAGE AMPLIFIERS (BARLOWS AND POWERMATES)

You can also choose a long focal length eyepiece with comfortable eye relief and use Barlow lenses to increase power. Tele Vue makes Barlows and Powermates (an improvement to the Barlow‑type design) in magnifications factors of 2x, 2.5x, 3x, 4x and 5x.

HIGH‑POWER VIEWING

Once you've selected an eyepiece set based on field stop sizes, calculate the magnifications produced with your scope. For planetary or double star observing, you'll want an eyepiece in at least the 150x range. For determining maximum power, a good rule of thumb is to use no more than 50x per inch of aperture for scopes with apertures up to 6". You can also consider magnifications of 200x to 250x if the "50x per inch of aperture" rule is maintained. Realistically, the atmosphere will usually limit your planetary observing to a maximum magnification of about 300x, no matter how large your telescope aperture.

Basically, you'll be choosing low and medium power eyepieces by field stop increments to "frame" the subject, and high power eyepieces by magnification increments (based on your scope's aperture), to reach the optimum contrast and resolution for viewing planets and double stars.

TELE VUE EYEPIECE SPECIFICATIONS

Tele Vue recommends choosing low and medium power eyepieces in ratios of field stop diameters. For exam factors of 1 .4 or 2.0. When choosing higher power eyepieces, use ratios of magnification.