Telescope View Finder--Which Way Do The Lenses Go?

[Wayne Watson]

I recently damaged a view finder, 8x50, on a Meade telescope by accidentally exposing it to the sun.
I was pretty certain it was damaged beyond repair. Upon taking it apart, I discovered the damage was
less than I thought; however, I failed to notice the orientation of the two lenses in the eyepiece.
They are plano-convex. After making a few repairs, I assembled it again and it looks alright, but I
thought I'd ask if it makes sense. Here's what I have:

Eye, plano-convex, plano-convex, plano-convex (50 mm objective)

It seems like something ought to be convex-plano. I'm not certain of the objective orientation,
since it's in a threaded tube about 1.5-2" thick. The lens is in the middle.
--
Wayne T. Watson (121.015 Deg. W, 39.262 Deg. N, 2,701 feet), Nevada City, CA

"The greatest thing you can do is surprise yourself." -Steve Martin

Web Page: <home.earthlink.net/~mtnviews>
Imaginarium Museum: <home.earthlink.net/~mtnviews/imaginarium.html>

[Wayne Watson]

It seems more likely that the objective is convex-convex. It's pretty hard to tell. I see no difference
in looking through it from either direction. with it held close, 3-4", from my eye.

[Sam Goldwasser]

Here's a link to a tutorial of sorts on eyepieces:

http://www.skygazer.net/files/Eyepiece%20Design.pdf

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[Bob May]

The objective is actually two lenses and there will be a convex-conved in
the front with the essentially plano-concave in the back. Thus the light
hits the objective convex surface first, going through the lens to the
essentailly plano back surface. It next hits the focal plane where the
crosshairs are and then on to the eyepiece of the telescope which normall is
the convex sides of two lenses together. This is the typical Plossl design
which does a good job of getting the light back to where it can be seen by
the eye.
Focusing process is to focus the eye lens on the crosshairs and then focus
the objective on the crosshairs. This will allow the eye to see both the
field of stars and the crosshairs at the same time in the view. The view is
in focus when you can move the eye about and the stuff focused doesn't move.
If it moves, you are not there yet.

--
Bob May
Why is there an Ozone Hole at the South Pole but Not at the North Pole?
Somebody's been lying to you!

[Wayne Watson]

Yes, I quickly realized the objective is a simple convex. However, what's up with the two
plano-convex lenses? Do I have them in the right orientation? Their curved convex surfaces are both
on the same side as the objective. eye->plano-convex->plano-convex->convex-convex.

Bob May wrote:

--

[Wayne Watson]

Thanks, but the pdf seems more applicable to eyepieces than viewfinders. My finder is about 10"
long. The portion of the view finder telescope that looks like an eyepiece could be either a Ramsden
or Huygens according to the pdf. It gives no way to tell. My guess is a Ramsden and both curved
surfaces should point to one another. The second 'eyepiece lens' (not the third one, which is the
objective) looks like maybe it could be a double convex glued into a convex, which makes it look
like a plano-convex.

Sam Goldwasser wrote:

--

[Sam Goldwasser]

Wayne Watson <mtnv...@earthlink.net> writes:

> > http://www.skygazer.net/files/Eyepiece%20Design.pdf

> Thanks, but the pdf seems more applicable to eyepieces than
> viewfinders. My finder is about 10" long. The portion of the view
> finder telescope that looks like an eyepiece could be either a
> Ramsden or Huygens according to the pdf. It gives no way to tell. My
> guess is a Ramsden and both curved surfaces should point to one
> another. The second 'eyepiece lens' (not the third one, which is the
> objective) looks like maybe it could be a double convex glued into a
> convex, which makes it look like a plano-convex.

Right, the link deals with eyepieces, but a finder scope is a telescope
so it should apply to its eyepiece.

The other way to figure it out is to try all combinations - one will be
correct and produce the least distortion. It's not like the Universe will
explode if you guess wrong. :)

[Repeating Decimal]

in article 3EEE03F0...@earthlink.net, Wayne Watson at
mtnv...@earthlink.net wrote on 6/16/03 10:52 AM:

As a general rule of thumb, you want the elements to be arranged so that a
ray bends the same amount at each air to glass surface. That means that the
bulging of the first element of a lens is toward the distant object being
used. The concave or least curved element will be toward the film.

Bill

[Wayne Watson]

Before I posted this message, I had some reluctance on trying combos for a couple of reasons: 1. All
but the back end of the view finder had been put in storage and was difficult to get to, 2. a plug
that holds the various 6 components (3 cylinders a crosshair section, and 2 lenses) was somewhat
broken and very hard to remove. It would take many minutes of prying to get it out. Things have
changed in recent hours. I got the finder back out of storage. I did find a combo that worked, but
will try another that also looks like a possibility.

Actually, the universe did almost end. :-) It's how I got into this fix. I was looking at the Sun
and didn't notice the view finder scope didn't have a cap on the end. Smoke!! I managed to repair
things but forgot to note the orientation of parts, since I though the whole thing was going to be
toast.

Sam Goldwasser wrote:

--

[Wayne Watson]

Yes, I believe that's the correct solution. I plan to try the scheme you just mentioned shortly. I
did find that with both curved elements pointing to the eye worked. I found that odd, because I had
finally discovered the concept you mentioned a short time ago from examining the pdf Sam mentioned.
See my last post for how I got into this fix. BTW, I had a solar filter over the main 8" objective.

I'll post my final solution later.

[Sam Goldwasser]

Wayne Watson <mtnv...@earthlink.net> writes:

> Before I posted this message, I had some reluctance on trying combos
> for a couple of reasons: 1. All but the back end of the view finder
> had been put in storage and was difficult to get to, 2. a plug that
> holds the various 6 components (3 cylinders a crosshair section, and
> 2 lenses) was somewhat broken and very hard to remove. It would take
> many minutes of prying to get it out. Things have changed in recent
> hours. I got the finder back out of storage. I did find a combo that
> worked, but will try another that also looks like a possibility.

> Actually, the universe did almost end. :-) It's how I got into this
> fix. I was looking at the Sun and didn't notice the view finder
> scope didn't have a cap on the end. Smoke!! I managed to repair
> things but forgot to note the orientation of parts, since I though
> the whole thing was going to be toast.

I hope you didn't mean you were looking the Sun through the main
scope. A burnt finderscope would be the least of your problems. :( :)

[Acme Optics]

Hi,

Call Meade and ask them. They will probably tell you. It isn't like
you're asking for a prescription. If that fails, call Scope City in
Simi Valley or Starizona in TUCSON.

Acme Optics

[Wayne Watson]

Yes, I called Meade and I was not really confident with their answer. Their tech support does not have a
lot of scope savy. I've pretty well determined the curved surfaces of the two lenses need to point to one
another.

Acme Optics wrote:

--

Wayne T. Watson (121.015 Deg. W, 39.262 Deg. N, 2,701 feet), Nevada City, CA

"It is better to know some of the questions than all of the answers."
-- James Thurber, writer, humorist

[MJones4106]

sounds like the eyepiece is a Ramsdem or Kellner type eyepiece. You may see a
"R" or "K" engraved on the eyepiece.

Ramsdens are made from 2 plano-convex lenses with the convex side facing each
other. The larger lens should be closer to the field stop. A Kellner has an
achromat eye lens and sometime the field lens is also achromatic, but the
convex surfaces are still facing each other.