Re: ATM High Performance Refractor Design

["B. Pepin" <atmjedit@conceptsnet.com>]

Go for it Steve. Keep us posted, too.


----- Original Message ----- 
From: Steve Fejes <jsfejes@swva.net>
To: <ATM@shore.net>
Sent: Thursday, October 12, 2000 5:05 PM
Subject: ATM High Performance Refractor Design


> 
>   I have designed an achromatic refractor objective which I think should
> give good images.  My design goals are to make an oiled triplet
> objective out of common glass types.  I wanted to avoid very expensive
> and/or less stable glass types.  I also wanted to avoid steep curves
> which would be harder for amateurs to make well.  And if it is possible
> to obtain good results, I wanted to make all four of the inner surfaces
> have the same radius of curvature, which should simplify fabrication.
> 
>   I used my free version of OSLO LT to design this lens.  The tolerances
> for the curves are not too tight for an amateur to make - in my
> opinion.   The cost and performance of this design falls in-between a
> conventional long focus achromat and an apochromat or flourite design.
> 
>   Here is my design:
> 
> 7" diameter      f/16.8      focal length 2985mm
> 
> 
> Radius of Curvature        Thickness        Diameter        Glass Type
> 
> 1930mm                        22mm                178mm        BK7
> -640mm                          0
> 
> -640mm                        13mm                178mm        KzFN2
>  640mm                          0
> 
>    640mm                       21mm                178mm        K5
> -5300mm
> 
> 
>   The space between the lenses is 0mm.  I plan to use a drop of
> immersion oil between the lenses.  KzFN2 is a relatively stable glass
> which has a bulk cost of about three times that of BK7.  The cost of
> this triplet should be about twice that of an achromatic pair made from
> BK7 and F2 glass.
> 
>   The images from this triplet should be better than the BK7/F2 achromat
> because it has better color correction.  I know that some have commented
> that better color correction does not improve the images very much.
> Many people are willing to buy apochromatic and flourite telescopes.  I
> believe that shorter tube lengths are a major reason for that, but they
> also give good images with very good color correction.  Some of us may
> be willing to use a long focal ratio to get very good images.  I see
> this design as being well suited to amateur telescope makers who want
> great images from a refractor that they can make and afford.
> 
>   I dream of one day having an observatory with a large reflector and a
> good 6" or 7" refractor.  I plan to make this lens my next project.  I
> intend to fold it with one flat mirror and a diagonal mirror.  This
> telescope could then be mounted on my existing Dobsonian mount, and
> should be as comfortable to use as my Newtonian reflector - as well as
> portable.
> 
>   This kind of refractor design could be made with a shorter focal
> ratio, but it would result in steeper curves.  Substituting BAK4 as the
> third element can give better images, but again requires steeper
> curves.  I would much rather figure four surfaces which are f/2 than try
> my hand at four f/1 curves.  I just thought that some folks might
> appreciate this information.
> 
> May God bless you,    Steve Fejes
> 
>