Quote:
Originally Posted by wulfgar
I have a suspicion Vixen source optics from a multitude of makers and perhaps only re-figure at best. There's nothing wrong with a fast Newtonian, other than coma which can be corrected. Or the larger obstruction than reduces contrast and limits them as regard planets and double stars.
But I like planets and double stars, so my 6" F8 Newt has only a 1 inch secondary. That 6" scope certainly splits double stars way better than a 4" Vixen fluorite. But because there is still an obstruction the images are "softer" than in the fluorite.
It is a fact that exceptionally fine optics can make up for some of the loss of contrast by obstruction.
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Indeed. Providing a 100% fully illuminated field of the required size for imaging (usually greater than a 1" diameter) will necessitate a certain secondary size. Not because of optic quality but simply because of physics.
The calculator that James links to in his review of the AT12IN calculates the fully illuminated field size for a given set of telescope dimensions (mirror size, focal length, distance from secondary to the focal plane). "quality of the optics" does come into the equation. He determined that the secondary was too small to provide 100% illumination even at the centre of the focal plane, so he upgraded. I am thinking of doing a similar upgrade to my GSO 200mm f4 (AT8IN) for the same reason.
Anyway, the point im trying to make is that a newt "optimised for photography" will require a larger secondary than would otherwise be necessary for visual use. A larger secondary means lower contrast for visual. Precision optics (Vixen, Orion etc) may help to make that contrast reduction less obvious, but it cant take it away.