|TeleVue.com: Advice Article List > Choosing Eyepieces|
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:
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:
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, Plössl eyepieces have an AFOV of 50°, Radians have 60°, Panoptics have 68°, Delos have 72°, Naglers have 82° and Ethos eyepieces have 100° or 110°.
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. For example, if you use a 31mm Nagler (with a 42mm field stop diameter), you would not need a 32mm 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.
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 15mm to 20mm of eye relief, to minimize any field reduction
(vignetting).* You may find that with small exit pupils such as 1mm or
less, you probably will not need eyeglasses, and can therefore use eyepieces
with less eye relief.
*Many Tele Vue eyepieces accept DIOPTRX, the unique addition that replaces prescription eyeglasses to correct eyesight astigmatism.
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:
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.
You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers 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.
If you have a Newtonian or Dobsonian reflector that's f/5 or faster, you should seriously consider using the Paracorr to eliminate coma, so your full field eyepiece sharpness is not compromised. Paracorr also acts like a 1.15x Barlow, so for example, a 1000mm f/4.5 scope becomes 1150mm f/5.2.
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 60x per inch of aperture for scopes with apertures up to 6". Higher magnifications may still be pleasing but will not likely reveal any additional detail. 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.
Thank you for considering Tele Vue eyepieces. Every Tele Vue eyepiece is Q.C. tested at f/4.0, and can be serviced or repaired by Tele Vue. For more individual advice on specific applications, you can always call Tele Vue during east coast business hours.