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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 12929
Loc: Amargosa Valley, NV, USA
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Re: Strange Results in Eyepiece FOV Comparisons
05/18/08 04:39 PM
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I found a field stop listed as 10.7mm for the Hyperion 8mm on a pdf linked from the Alpine Astronomical page. This is close to what I calculated using a derivation of Lawrence's forumla RFOV(nonexistent as it may be) * fl / 57.3 = Field Stop.
Using his formula for distortion, there is a 12.7% positive distortion for the Hyperion, and a 4.9% negative distortion for the 2.5mm Nagler. Knowing this, the drift test results I got aren't "strange" at all. I do, however see the utility of the "RFOV" concept. it allows you find an equivalent field stop and predict the true field of view of an eyepiece in any Telescope, even if you don't have access to the written specifications. And you don't have to disassemble the eyepiece to do it, you only have to use it.
If you generalize this, you realize that a 68d Hyperion and a 82d Nagler of the same focal length should share almost the same total field of view!
Looking at it this way, I gain extra respect for both the Hyperion AND the Nagler: The Hyperion shows sharp detail and excellent transmission over a very wide field, as does a Nagler, but does it at 1/3 the price.
So what makes the Nagler superior? Two things:
1. There is less overall distortion in the field of view of the Nagler. Less than half as much. Views of extended objects are likely to be more acurate.
2. The nature of the distortion is such that in the Hyperion, objects would appear to drift more slowly near the edge of the field, and speed up as they pass through the center. It would be just the opposite in the Nagler. Objects would drift more quickly into and out of the field at the edges, but would spend more time near the central area of the field where they are most easily seen.
This thread has been quite the learning experience!
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"Since the process of science generates more mysteries than it solves, I predict that we'll never learn everything: and we'll continue to generate new ignorance at the speed of knowledge."
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