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Eyepiece Diameter Vs Image Size

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Posted by Steve Bodenheimer on June 15, 2001 16:56:03 UTC


Your question is a good one. Here's my answer; others may have additional views.

The controlling factor on image size at the eyepiece is magnification at the eyepiece (calculated by dividing focal length of the scope by eyepiece value, i.e., a 2000 mm focal length scope used with a 20 mm eyepiece gives a magnification of 100). Thus the image size of, say Mars, will be twice as large at a magnification of 200 than at a magnification of 100, regardless of the eyepiece diameter. But the larger diameter eyepiece will typically have a larger field of view (width of the portion of the sky that can be seen) than the smaller diameter eyepiece so that the larger eyepiece may allow seeing more of the magnified image than the smaller one.

As you must have heard by now, a larger aperture (diameter of primary lens or mirror) scope will allow for a greater magnification than a smaller aperture scope. Thus, with a small scope, the larger, 200 magnification image might be dark and fuzzy, while the same size 200 magnification image might be relatively bright and detailed in a substantially larger scope. Quality of the scope and eyepiece also have a substantial impact on quality of the magnified image.

You should try to locate a local astronomy club and visit during a monthly star party to see these differences for yourself.

Hope this helps.

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