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Old 20-10-2013, 11:28 AM
stevous67 (Steve M)
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??How to calculate System F Ratio, and CFZ per filter

Hi all,

Wondering if anyone can help with correctly calculating my systems actual F ratio? I'm ultimately interested in working out my critical focus zone per filter.

I've seen the below calculation, but I'm unsure how to work out the affect of the AOL and filters on this F ratio number:


CFZ (in microns) = 2.2 x (Ft)2

where Ft is the true focal ratio (focal length / aperture) of the optical system (i.e. including the affects of focal reducers, barlows, filters, etc.)

http://astro.azurewebsites.net/Artic...icleContent/11

My system is F8, I use the AOL and Astronomik CCD filters. CCDAP suggests the following:

CFZ [Red filter]: 66.6micron
CFZ [Green filter]: 56.3micron
CFZ [Blue filter]: 46.1micron

How does that work?


Thanks!

Steve
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Old 20-10-2013, 11:49 AM
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RickS (Rick)
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Steve,

One good way to find out your true focal length is with plate solving. The focal length specs for most scopes are a rough approximation.

The effects of an AO or filters should be negligible (these should be designed to have zero power) but flatteners, reducers and barlows will certainly have a significant effect.

You'll find a formula for the CFZ and a calculator here:
http://www.wilmslowastro.com/software/formulae.htm

The CFZ is dependent on wavelength (which is why CCDAP gives you different values).

Cheers,
Rick.
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Old 22-10-2013, 03:52 PM
stevous67 (Steve M)
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Thanks for the assitance again Rick, I'll do that next night I'm out - good idea!

Regards,

Steve
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