You all have me wondering what my dark adapted exit pupil size is. Is there
a simple way to figure this out or would I need a optometrist to tell me? My
Longest focal lenght eyepiece is Panoptic 22, in combination with 12.5 f5
Portaball. I'm thinking about getting Nagler 31, but wouldn't want to do
that if it's not optimal for me. BTW I'm 51 and require low power readers.
Rich Girard
On Wed, Aug 27, 2008 at 3:05 PM, Jay Reynolds Freeman <
jay_reynolds_freeman@No-Spam> wrote:
> I have been following the discussion of very long focal-length
> eyepieces ...
>
> Recall that the actual field of view of an eyepiece may
> be limited by the mechanical diameter of the barrel and of
> the focuser, as well as by the optical design of the lenses.
> The Plossl optical design has a usable apparent field of
> view of about 50 degrees, and a little geometry shows that
> when using two-inch eyepiece barrels, the barrel diameter
> begins to encroach upon the field of view as eyepiece
> focal lengths approach 50 mm. (Precise details depend on
> the thickness of the metal of which the barrel and the
> lens retaining rings are made.) In that case, the
> lenses of the eyepiece cannot be as large as optical design
> permits, or they won't fit in the barrel!
>
> Eyepieces whose optical designs permit a wider apparent
> field of view will run into the two-inch-barrel limitation
> at shorter eyepiece focal lengths. Thus if field of view
> is all that matters, and if you have a two-inch focuser,
> there is no particular advantage to, say, a 58 mm Plossl
> over perhaps a 40 mm Koenig or Erfle, or an even shorter
> focal length eyepiece of a more modern, extreme-wide-field
> design.
>
> The longer focal lengths will give lower magnifications, of
> course, with two possible adverse consequences: (1) The
> exit pupil of the eyepiece may be too large to fit into the
> pupil of the observer's eye, in which case light will be
> wasted, and (2) the sky background may appear bright, which
> may make detecting faint diffuse objects more difficult than
> at higher magnification, at which the sky background is
> darker.
>
> The exit pupil diameter of a telescope/eyepiece combination
> is equal to the clear aperture divided by the magnification,
> and also equal to the eyepiece focal length divided by the
> telescope f-number. (If you use a Barlow lens, don't forget
> to include it in figuring out either the effective eyepiece
> focal length or the effective telescope f-number, your choice.)
>
> Your dark-adapted exit pupil diameter is whatever it turns
> out to be for you, but a reasonable rule of thumb is that
> for young adults it is about 7 mm, and declines with age
> after about 40. That is the basis for the rule not to use
> an eyepiece longer than focal-ratio times 7 -- a fine rule
> of thumb for young adults, but make that 5 or even 4 for
> older folks.
>
> I have a University Optics 55 mm Plossl in a two-inch barrel,
> and don't use it much, except perhaps as a wide-field
> eyepiece on my Astro-Physics 10-inch Maksutov-Cassegrain,
> whose f/14.6 focal ratio means that even this extremely
> long focal-length eyepiece gives only a 3.8 mm exit pupil:
> I find that an exit pupil of around 4 mm is very useful
> for observing many deep-sky objects, though not necessarily
> small, faint galaxies. The 55 gives about the greatest
> actual field of view a two-inch barrel can provide, so it
> makes a good finding eyepiece as well. It does have a rather
> long eye relief, which means there is a knack to getting your
> eye in the right place to use it; fortunately, that knack is
> easy to learn. And it has two other virtues compared to more
> modern designs: First, it is light in weight! If you drop
> it on your foot you may still be able to walk back to the car.
> (Of course, it won't be as useful if you throw it at a mountain
> lion, either.) Second, it is quite inexpensive compared to
> more high-tech designs.
>
> An occasional use for a very long focal-length eyepiece, one
> that produces an exit pupil larger than the pupil of the
> observer's eye, is to make sure that the brightness per
> unit area on the observer's retina, of a large, faint object,
> is as high as it can possibly get. In an extremely dark sky,
> that may be the optimum condition for visibility. My first
> observation of the Sculptor Dwarf Galaxy was made in such
> conditions.
>
> Clear sky,
>
> -- Jay Reynolds Freeman, Deep-Sky Weasel
> ---------------------
> Jay_Reynolds_Freeman@No-Spam
> http://web.mac.com/jay_reynolds_freeman (personal web site)
>
>
>
> ---
>
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--
http://goldenstatestarparty.blogspot.com/
---
Aug 25, 2008: TAC Web Page Updated http://observers.org/TAC.cgi/Announcements/
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Received on Wed Aug 27 15:57:07 2008
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