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Telescope EyepiecesConsiderations when choosingEyeglasses and Eye ReliefIf 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 telescope eyepieces that have at least 15mm to 20mm of eye relief, to minimize any field reduction (vignetting). However, you will find that with small exit pupils such as 1mm or less, you probably will not need eyeglasses, and can therefore use telescope eyepieces with less eye relief. Exit PupilThe exit pupil is the image of the objective that is formed by the telescope 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 telescope eyepiece by your scope's focal ratio: Exit pupil = telescope eyepiece focal length รท 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 telescope 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 ViewingOnce you've selected a telescope eyepiece set based on field stop sizes, calculate the magnifications produced with your scope. For planetary or double star observing, you'll want a telescope 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". You can also consider magnifications of 200x to 250x if the "60x 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 telescope eyepieces by field stop increments to "frame" the subject, and high power telescope eyepieces by magnification increments (based on your scope's aperture), to reach the optimum contrast and resolution for viewing planets and double stars. |
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