Quote:
Originally Posted by Astro78
Thanks for your reply Geoff, very much appreciated.
I downloaded that software and now am more confused? It's basically saying i'm getting "Vignetting of 75% ray at front aperture". I have no idea what this is? But reducing the secondary size to 45mm (from 55mm) all good. It doesn't seem to say the actual size of the focal plane either?
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What this means is that the ota tube geometry would be the cause of vignetting given the parameters you have entered. ie the limiting factor in your widest field viewing. I wonder if you have entered the dimensions properly. The focal plane for example should be some distance above the top of the focuser when it is racked fully in by something like 15mm.
What you are trading off is illuminated field size vs contrast improvement by using a smaller secondary. If your scope is not intended to be a dedicated planetary scope
and using premium grade optics, i really wouldn't suggest you bother. What 75% illuminated field size does the software tell you in each case?