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Originally Posted by Hemi
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That's a great help- thank you. Although the second article had some clearly false data in it. In the first sentence under Introduction the author says exit pupil is independent of the telescope and the magnification. Yet all the maths for exit pupil are totally dependent on telescope focal length, aperture and F ratio. Unless he meant something else and expressed himself poorly. It sounds like English is not his first language, but apart from that it was helpful.
The first article was very clear and very well written.
In a round about way exit pupil is much the same as magnification on telescopes so I see the same concerns expressed in terms of ideal magnifications.
I did like the explanation for lower powered eyepieces showing eye induced optical imperfections. I noticed that with my APM 30mm UFF. After viewing through 12.5mm and 17.5mm eyepieces where the stars are beautiful and stable pinpoints the 30mm showed some "flare" which moved when I moved my eye. That was what Don pointed out to me on another forum.
Good to know.
It also explains why there seems to be a sweet spot just like say the sweet spot in a tennis racket. For my AP130GT F6 819mm focal length that sweet spot is around 17.5mm and 7-12.5mm for higher mag.
A 3.5mm Nagler is good when the seeing is great but a bit fuzzy otherwise
(not the eyepiece or scope's fault).
So in the end it boils down to you need 3 types of focal lengths - the high, medium and low power eyepieces.
Too short a focal length is not going to be very nice just as too long a focal length is not going to be very nice.
Greg.