How do you identify a Plossl eyepiece?

How do you tell if you have a Plossl eyepiece?

Thanks

On 6 Sep 2003 13:42:07 -0700, (Doc) wrote:

How do you tell if you have a Plossl eyepiece?

Thanks

Not easy unless you take it apart to look - not really recommended but
if you feel comfortable about it... (just remember to put things back
exactly how they come out!).

A plossl is normally a pair of matching plano-convex doublets with the
two convex surfaces facing each other - close together but not quite
touching. You can normally see if each lens is a doublet by looking at
their edge, you'll see a line where the two components are cemented
together.

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

"Doc" wrote in message
om...
How do you tell if you have a Plossl eyepiece?



If it says it is...otherwise, what brand is it? If it says absolutely
nothing then it probably isn't.
FOV is the give away..plossls have wider FOV's than say an ortho (OR).

"ChrisH" wrote in message
...
A plossl is normally a pair of matching plano-convex doublets with the
two convex surfaces facing each other - close together but not quite
touching.

That would imply that the first lens surface at each end should be flat. Is
that right?

I suspect that this comes into the 'true but not useful' category.

..

On Sun, 7 Sep 2003 00:29:11 +0100, "OG"
wrote:


"ChrisH" wrote in message
.. .
A plossl is normally a pair of matching plano-convex doublets with the
two convex surfaces facing each other - close together but not quite
touching.

That would imply that the first lens surface at each end should be flat. Is
that right?

I suspect that this comes into the 'true but not useful' category.


Actually, it's in the 'interesting but wrong' catagory...

I went and had a closer look on the web and it appears the eye-lens is
not plano-convex but the outer (eye) lens has a concave surface.
Having said that, my Vixen 22mm Plossl (as written on the barrel)
definitely has a flat surface on this face of the eye lens. It seems
there are a lot of minor variants of the basic Plossl design which are
then (wrongly) lumped together and called 'Plossls'. The design I
descibed above is correctly known as a 'symmetrical' or 'dialsight'
eyepiece.

Here's an interesting monologue describing the development of eyepiece
designs over centuries:
http://www.brayebrookobservatory.org...fEYEPIECES.pdf
(lots of other interesting information on the root page-
http://www.brayebrookobservatory.org...LICATIONS.html
but someone really ought to do something about that background image!

Other interesting sites I found we
http://members.shaw.ca/quadibloc/science/opt04.htm
Which has pretty coloured scross-sections of eyepiece designs - but as
I said, I would not treat these as definitions.

http://www.bpccs.com/lcas/Articles/scope4.htm#eyepieces
Interesting basics on how eyepieces and filters work

http://www.observers.org/beginner/ey...s.freeman.html
Jay Reynolds Freeman's page on his eyepiece selection for different
telescopes.


ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

A plossl is normally a pair of matching plano-convex doublets with the
two convex surfaces facing each other - close together but not quite
touching.

That would imply that the first lens surface at each end should be flat. Is
that right?

I suspect that this comes into the 'true but not useful' category.

Hmm, well, from what I remember being told, a plossl needn't be plano;
just that the _most_ convex surfaces are facing each other. In reality,
if I remember correctly, that means the crown (convex) halves of
the achromats are facing each other. But I'd be interested to hear if
this is wrong.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

"Doc" wrote in message
om...
How do you tell if you have a Plossl eyepiece?

Thanks

It's easier to tell if you don't.

There is no sharp line between Plossl and Symmetrical and some other
four-element designs.

A Plossl has two components, each of which is a doublet (a two-element
cemented achromat) and they are roughly comparable in thickness. The eye
relief is very roughly 75% of the focal length.

If the element away from the eye is a single (thin) lens, you have a Kellner
or variation thereof (Modified Achromat is a common name).

If the elements are a singlet and a triplet, then you have an Orthoscopic.

And if it's more complicated, you have something else



--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope