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Old 09-12-2019, 08:54 PM
FrancoRodriguez (Franco)
Franco

FrancoRodriguez is offline
 
Join Date: Mar 2019
Location: Ramsay, SE QLD
Posts: 83
backfocus distance with filters. How to calculate?

Greetings people! I have a CLS CCD filter. It's 7.52mm thick including the housing. The filter itself is 1.1mm +- 0.002mm according to the manufacturer, with a refractive index of 1.5. Does anyone know how long the equivalent extension tube would be for backfocus measurements? I vaguely remember Snell's law from way back yonder but not sure about anything these days!
I saw on cloudy nights that astrodon filters (which I also have on another system) the equivalent distance is 1/3. IE, a 3mm filter adds (or subtracts?) 1mm of backfocus distance. I'm pretty skeptical about that. Any thoughts would be greatly appreciated! PS--Experimentally determining the correct backfocus distance has been hard for me because I seem to have introduced complex distortion/astigmatism due to an unknown factor, possibly an imaging train issue.
Thanks in advance!
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