Re: [ATM] Eye-pieces

[Lawrence Sayre <lsayre@neo.rr.com>]

Phill Powell wrote:

>Hi,
>
>I'm looking for some on-line information on the design of eye-pieces,
>particularly Plossls. 
>
>I'd be grateful I someone on the list could give me a push in the right
>direction.
>
>
>Thanks
>
>
>
>Phill
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>ATM mailing list http://www.atmlist.net/
>
>  
>
Phill,

I've made a few Plossl's.  Two 1.25" 'Woodshop Eyepieces' of 28mm and 
20mm, and also a 2" 40mm Plossl.  The ones I made are actually all 5 
element "Super Plossl's" (also know as 'Modified Plossl's', or 
essentially a form of 'Erfle'), but here are a few simple pointers off 
the top of my head for making true 4 element Symmetrical Plossls (all 
modern Plossl's are actually Symmetricals, though the original Plossl's 
were not Symmetricals):

Choose two "identical" coated (or better yet multi-coated) 'positive' 
cemented achromats of about F/2.  Try not to go below F/2, but it's OK 
to go above F/2 for the achromats.  Divide the achromats FL by its 
diameter to determine its F/ratio.  F/2 is the target ideal, but you are 
often limited here by achromat availability.  The abberations will get 
to be too severe if you go below F/2.  Likewise, abberations can 
actually be improved if you choose achromats above F/2, but the AFOV 
(apparent field of view) of the eyepiece will suffer.  F/2 is about the 
best overall compromise, and should result in eyepiece AFOV's of ~50 
degrees, as is typical of commercial Plossl's.  

The convex sides of the achromats face each other within the eyepiece.  
Try to position them so they are nearly touching, but not touching.  You 
want them as close together as you can practically and safely get them.  
Try to allign them as true as possible.

The focal length of the resulting eyepiece will be:

[(FL_ach1 x FL_ach2) - spacing] / (FL_ach1 + FL_ach2)

Where "spacing" is the measured distance between the "focal planes" of 
the two achromats as positioned within the eyepiece.  The "focal plane" 
position for each achromat can be assumed to be the position of the 
divide between the crown and the flint elements, or the "cement line" 
for each achromat.

You will need to install a field stop (FS) ring just below the 'field 
lens'.  Size it at (or preferably just a wee bit smaller than) the 
measured diameter of the "clear aperture" (unhindered lens diameter of 
the achromats as mounted in the eyepiece).  Place the focuser shoulder 
of the eyepiece as close to the inside location of the field stop as 
possible.

The AFOV of your eyepieces will be approximately:

AFOV = FS/FL * Pi/3

I believe that the original Plossl's (and probably most of them still 
today) all used achromats which were plano (flat) on one outside face 
and convex on the other, but the (once patented, but patent expired in 
~1990) TeleVue improvement of the Plossl design came from using 
achromats which are concave on one outside face and convex on the 
other.  If you can find achromats of this latter type, grab them.

Lawrence Sayre

-- 
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

 

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