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Telescope EyepiecesConsiderations when choosingMagnificationTelescope eyepieces determine your telescope's magnification. To calculate the magnification of telescope eyepiece, divide the focal length of the telescope by the focal length of the telescope eyepiece: Magnification = telescope focal length ÷ eyepiece focal length True Field of ViewTelescope eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will see (in degrees&41;, divide the telescope 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 viewThe field stop is the metal ring inside the telescope eyepiece barrel that limits the field size. It's projected by the telescope eyepiece so that it appears as a circle out in space when you look through the telescope eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each telescope eyepiece design. For example, Plössl telescope eyepieces have an AFOV of 50º, Radians have 60º, Panoptics have 68º, and Naglers have 82º. Low to medium Power-viewingFor low-power viewing of large objects, or to use your telescope as a
low-power finder, you will want a telescope 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" telescope eyepieces, it's 46mm). Then add
telescope eyepieces covering uniform increments in smaller field stops. For
example, if your widest field telescope 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 telescope eyepieces having field
stop diameters of approximately 40mm, 20mm and 10mm. To further fill in with
incremental steps, add telescope eyepieces with approximate field stop diameters
of 28mm and 14mm. Of course, avoid duplicating focal lengths. For example, if
you use a 31mm Nagler 7(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 telescope 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. |
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