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

[swalters@att.net]

Dave,

Well, when you turn this puppy out, make two please!

Steve...
> 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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