[APML] Well corrected Newtonian, was[Vacuum Pump]

[Aplanatic@aol.com]

Hi Bobby,

As some of you know, I have been pursuing the design of a large (16" 
aperture), well-corrected, wide-field astrograph for several years.  Has it 
really been 4 years now?  Yikes.

I've been through countless designs including Wynne variants, Schmidt 
variants, Maksutov variants, corrected Cassegrains, etc.  For each type, one 
can achieve excellent performance (with the use of focal plane correctors, in 
some cases) but each has its own difficulties and complexities.  The Wynne 
family requires a very large 3- or 4-element corrector with exceptionally 
tight manufacturing tolerances, the Schmidt and Maksutov designs require a 
very large and heavy full-aperture corrector, and the Cassegrains require 
complex focal plane correctors and have difficult collimation issues. 

Anyway, to make a long story short, I've found several designs in the good 
old hyperbolic Newtonian family that use a 2-element corrector near the focal 
plane.  The corrector is relatively easy to build, having realizable 
manufacturing tolerances.  The back focal length is nice (about 75 mm) 
allowing an off-axis guider, and the lenses are not too big to be 
impractical.  Of course, the corrector housing / focuser / OAG /  film holder 
assembly must be custom machined, and it will not be cheap.

The basic design consists of a 16" hyperbolic primary at about f/6 with a 
deformation constant of about -1.45 (not too onerous to figure), a 5.5" 
diagonal and a 2-element corrector consisting of BK7 and F2 glasses.  The 
glass is garden variety.  The elements are about 5" and 4.5" in diameter, 
respectively.  The performance of the basic design is quite good over an 
image circle 3" in diameter, and respectable out to 3.5".  Notably, the image 
characteristics are excellent inside an image square of the largest 
imaginable (to me) CCD camera out there, namely, the AP16.  Maybe someday.... 
<g>

I also have an enhanced design that uses FK51 glass.  It has excellent 
performance over a 3.5" image circle and is respectable out to the edge of a 
4" field, but the glass for the corrector is very hard to find and will cost 
a pound of flesh, I think.  I'm still working on obtaining a piece of this 
stuff.

With the corrector, both designs have a focal ratio of about f/5.1.  Taking 
as losses the big secondary (12%), the primary mirror's reflective losses 
(12%), the secondary mirror's losses (8%) and the losses in the coated 
corrector (4%), the transmission ratio (t-stop) will be about f/6.2.  The 
focal length will be about 2070 mm.

Dave Rowe





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