The simple way ....Get your eyes dark adapted and have someone else come up
with a mm scale ruler and hold it near your eye to estimate pupil size. Red
light OK; I've also heard of people using a mirror to do it themselves.
On Wed, Aug 27, 2008 at 3:56 PM, Rich Girard <rrgirard@No-Spam> wrote:
> 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)
> >
> >
> >
> > ---
> >
> > Aug 25, 2008: TAC Web Page Updated
> > http://observers.org/TAC.cgi/Announcements/
> > CalStar - Sept 25-27, BYO Party! http://www.sjaa.net/calstar/
> > TAC mailing list - join or leave here:
> > http://seds.org/mailman/listinfo/sf-bay-tac
> >
>
>
>
> --
>
>
>
>
>
> http://goldenstatestarparty.blogspot.com/
>
> ---
>
> Aug 25, 2008: TAC Web Page Updated
> http://observers.org/TAC.cgi/Announcements/
> CalStar - Sept 25-27, BYO Party! http://www.sjaa.net/calstar/
> TAC mailing list - join or leave here:
> http://seds.org/mailman/listinfo/sf-bay-tac
>
---
Aug 25, 2008: TAC Web Page Updated http://observers.org/TAC.cgi/Announcements/
CalStar - Sept 25-27, BYO Party! http://www.sjaa.net/calstar/
TAC mailing list - join or leave here: http://seds.org/mailman/listinfo/sf-bay-tac
Received on Wed Aug 27 16:03:24 2008
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