Newbie Eyepieces 101

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.

--
BenignVanilla
Pond Site: www.darofamily.com/jeff/links/mypond

see:
http://www.brayebrookobservatory.org...OLUTIONofEYEPI
ECES.pdf

You will need to put the entire URL together, and will need a PDF viewer and
some time to download this....

Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

"Bill Foley" wrote in message
...
see:
http://www.brayebrookobservatory.org...OLUTIONofEYEPI
ECES.pdf

You will need to put the entire URL together, and will need a PDF viewer
and
some time to download this....

Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

Downloading now...thanks for the link.

BV.

P.S. Your sig reminded me...My neighbor's son is a major gearhead. Last
night I had my head stuck to my scope, when I heard the roar of his Mustang.
I clinched my eyes shut waiting for his headlights to blind me, as he parks
near my curb. The sound of his engine began to roll away, but no light? I
opened my eyes to see that he had gotten in his car, and backed down the
street with just his parking lights on, in order to not blind me with his
headlights, nor backup lights. Thanks for the wish in your sig...it worked.
I had both last night. I think I got my first glimpse of M51.

"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

Awesome thanks!

"Stephen Paul" wrote in message
...
"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

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.

There are lots of possibilities and options out there. Do some reading,
www.cloudynights.com has lots of comparisons and reviews, all of course to be
taken with a grain of salt.

For someone on something of a budget, I suggest considering the Orion Expanse
eyepieces or the similar (identical) eyepieces offered by other vendors.

These are 6, 9, 15 and 20 mm 66 degree FOV eyepieces. I have the first three
and have been very pleased with them, as sharp in the center as plossls, very
acceptable edge performance, I rarely use anything else these days. They do
have some ghosting issues but this does not seem to bother me.

Prior to acquiring these, I used a variety of decent plossls (Ultima, Parks,
and some standard Celestron) and some UO ortho's.

The other day (as I have pointed out previously), I was using the 6mm with an
Ultima barlow, paracorr with my 10inch brother of your scope and was able to
get nice sharp airy disks and diffraction rings, this amounts to about 500X so
that eyepiece can't be all bad.

I think these make a nice set of eyepieces for use in your 8 inch F6 DOB. Add
an Orion Deluxe shorty (on sale) and you have lots of nice possibilities.
Certainly you can spend more money and may will want to in the long run, but
these seem to do a decent job.

jon

BV,

My starting array of EP's would consist of a 25mm plossl, a 12.5mm plossl,
and a 6mm orthoscopic. The orthoscopic, while offering a smaller FOV,
serves up the contrast desired at higher powers.

Don't break the bank on the plossls you acquire at this point. . .something
in the 50 to 75 dollar price range should offer sufficient performance.


Martin

"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.

I'm also a newbie and owner of the 8" Hardin dob. Trying to figure which
ep's to buy is as hard as figuring out which first scope to buy. It's enough
to drive you batty.

I have purchased 2 ep's and the Orion Shorty + 2x Barlow. I picked up the
BW Optiks 2" 80 degree ep. I also got the UO Konig 16mm 65degree ep. My
thinking is I'll start with the wider FOV ep's to make it easier to locate
those hard to find objects.

My next plan of attack is to make a list of other ep's I'm interested in
and take the list to my next starparty. The Orion expanse, OU Orthos and
meade 4000's to name a few. I'm going to try out as many of these I can, and
see what works for me.

Mike
--
BenignVanilla
Pond Site: www.darofamily.com/jeff/links/mypond