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Is central obstruction by diameter or area?
Hi all,
I'm reading about planetary observing and central obstructions as it relates to contrast.
I have an RC 10" and a 400mm newt, so I was interested to see how the central obstruction affects the image.
The book I'm reading ('Planetary Astronomy' various authors) mentions various designs and their central obstructions, including Newts with 20-25% central obstructions, which seems a lot to me.
That only looks right if you do it by diameter, not area which is how I understood % obstruction to be calculated.
For example, my 400mm newt has a 104mm obstruction, which works out to 26% by diameter, but only 6.76% by area.
By contrast, my RC is 254mm with a 112mm obstruction which works out to 44% by diameter, and 19% by area.
The difference is huge. a 19% obstruction is workable for planetary according to the book. 44% is not.
So I'm guessing I had it wrong all along. Can anyone confirm?
Markus
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