Friday, December 02, 2005

the unknown eyepiece

Part of reason I obtained All About Telescopes (AAT) book was so I could continue to learn about eyepieces. In particular, I'm trying to definitively, once-and-for-all, know what Mom's unindentified eyepiece (EP) is.

  • included with the Edmund 6" reflector (from Big Al)
  • 1-1/4" shaft diameter
  • front or field lens (near the 'scope) is concave
  • rear lens (near the eye) is convex
  • rear lens diameter is 0.90"
  • no markings or printing
I want to know it's focal length for then I can determine it's power. Of course, if I know the power, I can calculate the focal length. Then I know how this EP rates with the other 3 that Mom and I have. And if it's similar to my (more modern) Plössl then there's not much point using it...

I want to know it's apparent field of view so I can then determine the true field of view.

I'd also like to know the exit pupil size and the relief figures.

Finally, I'd like to know the construction type (Kellner, RKE, Plössl) as that will suggest it's strengths and weaknesses.


My personal perception initially was that it was similar in power to my Celestron 26mm Plössl. My timing of the true field of view supports this: the mystery EP is slightly less powerful.

It was in the '74 and '75 Edmund product catalog that I learned that the included low power eyepiece was rated at 48x. If Mom's EP is the "standard" one included with the Edmund 6" reflector, then it's focal length is 25.5mm or 1".

Somewhere on the web (can't remember where now), I saw something about the field lens (the one toward the 'scope or furthest from the eye) of an EP and if it was concave then the EP was an RKE. But the AAT book shows that a "type 2" Kellner has a concave field element.

AAT mentions a trick to determine the image or focal plane of an EP. Having done this little experiment, I'm confident Mom's EP is not a Huygens.

Power, focal length, etc., I'll determine later.

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