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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Received on Wed Aug 27 15:06:10 2008
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