Old 20-11-2016, 12:37 PM
AstroApprentice (Jason)
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Location: Melbourne, Australia
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Filter size calculator

Hi all,
I want to buy one of the Asi1600 MM-C kits from ZWO including camera, filter wheel & filters. I can chose 1.25", 31 or 36 mm filters. Does anyone know if there is a calculator available to help determine the best filter size for given focal length, sensor diagonal, and filter spacing?
Thanks for your help.
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Old 22-11-2016, 05:30 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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might be able to use simple geometry to find the filter size at which vignetting begins.
back of the envelope for ~f4 or slower comes out as:
required filter diameter = d/fNo + diag
where d is the distance between the filter and the chip, and diag is the chip diagonal.

In practice, some vignettting will be OK, so you could use slightly smaller filters than calculated without much penalty. I am using 1.25 filters with smallest possible spacing on an f4 system and the 1600 is illuminated just well enough (some vignetting, but still OK).
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