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Old 10-08-2020, 03:19 PM
Afro Boy (Carl)
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Filter sizes to suit ASI 183MM Pro

Hi everyone,

Am going to buy a monochrome camera so wanting some filters to go with it. Not sure if I can get away with the 1.25" or if I need something larger like the 36mm?

I'm using a Stellarvue SV105 f/6.2, with an ASI 183MM Pro, so wide field. Sensor diagonal is 15.9mm.

I know I can calibrate out any vignetting but want to avoid as much of it as possible in the optical train.

Keen for any thoughts/advice.

Thanks.
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Old 10-08-2020, 04:16 PM
sunslayr (David)
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According to Zwo when using their EFW "The distance between the filter and sensor is around 10mm when you connect it to ASI1600.

So 1.25″ filters won’t have vignetting up to F5 focal ratio scope and 31mm filter won’t have vignetting up to F2 lens". So I imagine you should be safe going for 1.25" filters with the 183mm's smaller sensor even if you pick up a reducer later.

edit: This is the the equation for calculating vignetting

Filter distance from sensor / Filter clear aperture - sensor diagonal (26mm for 1.25, 28mm for 31mm)

10mm/(26-15.9) = F0.9 Vignette ratio

Last edited by sunslayr; 10-08-2020 at 04:28 PM.
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Old 10-08-2020, 07:42 PM
Afro Boy (Carl)
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Thanks for that.

What's the relationship between the filter size and f-ratio? I'm also going to be getting a RedCat51 (f/4.9) so want to ensure 1.25" are suitable for that too.

Thanks again
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Old 10-08-2020, 07:52 PM
Afro Boy (Carl)
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Nevermind, I found the formula ...

Minimum Filter Size = CCDdiag*(FL-ChipDistance)/FL + (ChipDistance/f)

So in both of my set-ups, around 17.2mm is the smallest filter size. Looks like I can safely go the cheaper size then
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