Selecting an Eyepiece
   Eyepiece Specifications
   Panoptic      Nagler

SELECTING AN EYEPIECE SET

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

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" and is a fixed property for each eyepiece design: Plössl eyepieces have a 50 degree apparent field of view, Radian eyepieces have 60 degrees, Panoptics have 68 degrees, and Nagler eyepieces have an apparent field of 82 degrees.

TRUE FIELD OF VIEW

Eyepieces also determine the true field you see in the sky. To calculate the true field of view (in degrees) that you will see, divide the eyepiece field stop diameter by the telescope's focal length and multiply the result by 57.3:

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

The following key spec summary for all Tele Vue eyepieces will help you calculate the true field size you will achieve with your telescope in combination with Tele Vue eyepieces. Please note that eyepieces are listed according to their field stop diameter, from largest to smallest. Eye relief is given for reference only.

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 10mm. To further fill in with incremental steps, add eyepieces with approximate field stop diameters of 28mm and 14mm.

Of course, avoid duplicating focal lengths, when filling in. As an example, there's no need for a 32mm Plössl (27mm f.s.) if you use a 31mm Nagler (42mm f.s.).

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.

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 the telescope'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.

EYE RELIEF

If you wear glasses to correct astigmatism, try to choose eyepieces that have at least 15mm to 20mm of eye relief.

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 telescope. 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 limit your planetary observing to a maximum magnification of about 300x, no matter how large your telescope's aperture is.

Basically, you'll be choosing low and medium power eyepieces by field stop increments, and high power eyepieces by magnification increments based on your scope's aperture.

Be assured that by choosing Tele Vue eyepieces, you'll have the sharpest and highest contrast images, not only at the center of the field, but at the edge as well.

For more in-depth technical background on the optical factors that affect choosing your magnifications, see the article    "Choosing Your Telescope's Magnification" by Al Nagler, President of Tele Vue Optics, Inc.

If you have any questions, do not hesitate    contact us!

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