Hi Joe,

Using the data from your Astromart forum posting I laid the design out in Zemax, and the system performance is good as expected. Although this is a reflective system I weighted the spot diagram below for the dark adapted (scoptopic) eye to get the Ary disk about the right size. There are geometric spot traces for on axis, 0.25” off axis and 0.5” off axis. This is the radial distance from the field center. As expected coma is the dominant aberration. Performance is good on axis and this will make a very good imaging instrument for planets, double stars, etc. as it is nearly diffraction limited over a 0.5” diameter full field. I get better performance with the primary secondary spacing of 45.900” and the modeling represents that very minor change…

For figuring this mirror, as you know it will be less correction than a parabola by a factor equal to the conic constant (K). For a parabola each knife edge position is calculated, K=-1 or 1.0*( r2 / R) where r is the zonal radius from the center of the mirror and R is the radius of curvature of the mirror. If moving both the knife edge and source use K*(r2/2R). For your scopes elliptical primary the conic constant K is -0.7578 and you’ll need to calculate the zones as 0.7578*(r2/R).

Conic Constant -0.7578

Radius of Curvature 116

Zonal increment 0.5

Parabola r^2/R

0 0

0.5 0.00163319

1 0.006532759

1.5 0.014698707

2 0.026131034

2.5 0.040829741

3 0.058794828

3.5 0.080026293

4 0.104524138

4.5 0.132288362

5 0.163318966

5.5 0.197615948

6 0.23517931

6.5 0.276009052

I looked at using the 2 foci of the ellipse as a null test but as expected the long focus (conjugate) of the ellipse is 895” almost 75 feet away with the short focus of the ellipse being 62” or 5 feet away. Thus if you placed a slit or point source 895” away and a knife edge 62” away you would cut the light uniformly across the aperture, i.e. it will null.

Well the images didn't make the posting and I couldn't attache a pdf so if you are interested send me an email to the address below and I'll forward the pdf to you...

Hope this helps…

Dave Erickson

[email protected]www.hbastro.com