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I not sure what values you used for your newt analysis but I will tell you that a 70mm secondary should not produce a 11mm 100% illumination in your scope.
The factors that change the illumination are the diagonal size, the tube inside diameter, the tube wall thickness, the focuser minimum height, the spare in travel, and the additional height for camera.
The diagonal is the variable. The tube inside diameter and the wall thickness aren't that important but you must make the sum of both of them equal to double the distance from the center of the scope to the focuser baseplate. My moonlite has a minimum height of 65mm yours might be slightly shorter depending on drawtube options. The spare focuser in travel is the amount of drawtube you want to be out when the camera is focused, mine is 10mm so that it is fairly close to the scope but with room to move for different filters and temperatures. The last variable is additional height for camera and generally this will be the corrector distance because that is the distance you will have to stick too to get good correction. In this case I used 55mm as this is what most correctors need.
These are the numbers I get with estimated values.
diameter 250mm
ratio 4
diagonal 70mm
inside diameter 265mm
wall thickness 1mm
minimum height 65mm
spare in travel 10mm
additional height for camera 55mm
100% illumination 5.6mm
Generally you want to get the 100% illumination to be larger than the diagonal of your CCD, for the kaf8300 this is 22.5mm.
Just changing the secondary size until the 100% illumination covers the 23mm I get a secondary size of 83mm. Looking at the orion optics AG10 they use a 100mm secondary, obviously these scopes are built for larger sensors but you can see that 83mm does not seem silly.
Your setup looks great by the way, I have been contemplating building myself a larger faster scope and reducing it with an asa reducer to be a superfast astrograph and I love the idea of making a truss scope with carbon rods.
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