"BenignVanilla" wrote in message
...
Does anyone have a FAQ or care to engage in a short Eyepieces 101 thread?
I
have an 8'' Harden Optical Dob. I am very fond of the scope. I bought the
biggest bucket I could afford, now I would like to learn more about
eyepieces, in the hopes of adding to my default set. I don't know where to
start.

Short thread on eyepieces? Surely you jest.. :-)

Where to start?

First some (generalized) definitions:
Aperture - the diameter of the telescope's objective lens/mirror (unless
intentionally "stopped" down by a mask of some type, where the aperture is
then the diameter of the stop).

Focal Plane - the point at which all rays coming from the objectve
lens/mirror create a representation of the target image. (A "crossing of the
beams", so to speak.)

Focal Length - the distance light rays must travel to reach the focal plane

Focal Ratio - the ratio of a telescope's aperture, to its objective's focal
length.

Magnification - an ordinal value that indicates the number of times the
image at the focal plane is made larger, measured in diameters. Example: 50x
means that the image "in" the eyepiece is 50 diameters larger than the image
at the focal plane.

Apparent field of view (rating for eyepiece) - the field of view you would
get on the sky, when (if you could see) looking through an eyepiece without
a telescope

True field of view - the field of view you get on the sky when the eyepiece
is in the telescope. (And, not just any telescope, but a telescope of
specific focal length, and all other scopes of same focal length).

Eye lens (eyepiece) - the eyepiece lens closest to the eye.

Field lens (eyepiece) - the eyepiece lens closest to the telescope's
objective lens/mirror.

Field stop (eyepiece) - the diameter to which the field lens is restricted
by the eyepiece manufacturter

Eye relief (eyepiece) - the distance from the eye to the eye lens, where the
entire field of view is visible in the eyepiece

Exit pupil - the diameter of the light cone at the point where the entire
field of view is visible in the eyepiece (the eye relief point), when in the
telescope (And, not just any telescope, but a telescope of specific focal
ratio, and all other scopes of same focal ratio).

Eye pupil - the diameter of your eye's pupil, given the ambient light
conditions. Below age 40, consider 7mm to be the maximum, above age 40
consider 5mm to be the maximum, for fully dark adapted eyes. YMMV.

The formulae:
(1) Magnification = focal length of telescope / focal length of eyepiece

(2) Exit pupil = focal length of eyepiece / focal ratio of telescope

(3) True field of view = (diameter of eyepiece field stop / focal length of
telescope) * 57.3
(3a) True field of view (approx.) = apparent field of view of eyepiece /
resultant magnification of eyepiece in a telescope of specific focal length

Of primary importance when selecting eyepieces is using the best exit pupil
for the job. I think it was David Knisely who posted a short document on
useful magnifications, where he lists magnification per inch of aperture,
which in turn dictates exit pupil. This list, as well as others, indicate
what exit pupils are reasonable for the different objects one views in a
telescope.

The five steps to selecting an eyepiece:
1) Determine which exit pupils to work with (find the aforementioned
documents)
2) Based on your telescopes specifications, determine what focal length
eyepieces will give you those exit pupils.
3) Find which currently available eyepieces are closest to those focal
lengths.
4) Beg and borrow eyepieces in those focal lengths, but of different
designs, and try them
5) Select the ones that you can afford, keeping in mind that a premium
widefield eyepiece is worth every penny if you have the money to spare, but
that the primary goal is the correct focal length.

Final comment: wide field eyepieces are more important in undriven scopes
than in driven scopes. Consider the cost of an inexpensive EQ Platform for
your Dob when considering eyepieces. I have a set of very expensive wide
field eyepieces that I use in my Dob, but there are times when I'd rather
have tracking. In particular, the wide field eyepieces are important at low
powers, whether the scope is driven or not, as they aid in finding, and can
frame some of the larger objects. However, at higher powers, even the wide
field eyepieces, leave you with the requirement to nudge your scope along
quite frequently. Since high power views are generally aimed at planets and
small objects, where a large field of view isn't required, before spending
$900 on three higher power wide field eyepieces, consider spending $600 on
an EQ Platform, and $300 on three narrow, high quality eyepieces of a
simpler design.

If those numbers just blew you completely out of the water, in the interim,
Plossls are good eyepieces with a 50 degree AFOV. Other than the increased
nudging frequency, I still like the actual images in my 13mm, and 6.4mm
Plossls, which compare pretty favorably to the Naglers.

--
-Stephen Paul