Re: ATM Wynne corrector

[Aplanatic@aol.com]

>     Ok, Dave...  enough teasing.
>   Are you going to post the prescription...?
>        Cheers,
>           Clive.


Several people have asked for the prescription for this astrograph.  As it 
turns out there are many different optical configurations that work very 
well, all based on the 3-element Wynne corrector.  The main difference is the 
final focal ratio, which varies strongly for different glass types.  I will 
only present one design because I have to transcribe it from my own ray 
tracing program.

The following uses two lenses made from F2 and one made from SK6.  The design 
was adapted from a design presented by Dean Ketelsen.  He used BK7 for the 
last element, which causes a substantially shorter effective focal length.  
His design also used a hyperbolic primary, which I changed to parabolic so 
that the scope could be used visually without the corrector.  Slightly better 
images are possible if the primary is hyperbolic, but it works so well with a 
parabolic primary that I see no reason to give up the flexibility.  Many 
other choices for the glass types are possible.  Some will likely give better 
results than the ones that I have explored.

The corrector requires very accurate fabrication.  Even small deviations from 
the given lens thickness cause the images to suffer.  Luckily, the individual 
lenses can be optimized once the final center thickness has been determined.  
Obviously, anyone that wants to make one of these must be a) crazy, b) well 
versed in using a ray tracing program and a spherometer, and c) very patient 
and meticulous.

A 16", f/4.78 medium format astrograph that covers an 80 mm image circle:

All dimensions in millimeters.

Primary Mirror: Diameter = 406 
RoC = -4000 SC = -1

Spacing, Primary to Lens1: -1760

Lens1: Material = F2, Diameter = 115
RoC1 = -103.9, RoC2 = -125.8, Thickness = 7.5

Spacing, Lens1 to Lens2: -57.5

Lens2: Material = F2, Diameter = 95
RoC1 = -207, RoC2 = -87.6, Thickness = 5

Spacing, Lens2 to Lens3: -100.5

Lens3: Material = SK6, Diameter = 100
RoC1 = -151, RoC2 = 8000, Thickness = 14

Spacing, Lens3 to Focal Plane: -65.34

Dave Rowe.