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Focus distance

Started by rfic1 , Aug 10 2010 02:32 PM
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#1 rfic1

rfic1

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Posted 10 August 2010 - 02:32 PM

With a f/8 telescope how much change in focus travel would there be focusing a object at 100 yards vs infinity. Is there a formula?

#2 Nils Olof Carlin

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Posted 11 August 2010 - 01:09 AM

call the focal length f, the outward travel (from infinity position d, the distance to the object a.
Simple lens formula : 1/f=1/(f+d) + 1/a, and if d is small compared to f (a is much greater than f):
d~(f^2)/a

if f=30 in, and a=3600 in, then the out-focus is d=1/4 in.

Nils Olof

#3 rfic1

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Posted 11 August 2010 - 06:45 PM

Thanks Nils,
It ended up being close to a inch. I had just completed my 8" f/8 OTA utilizing a low profile focuser that only has 1" of focus travel. After assembly I tried it out but when focusing on tree's about 100 yards away I needed a extra 1" of out focuser travel to achieve focus. I was not to happy needing a extension tube. I confirmed that my measurements for mirror cell,secondary and focuser placement were correct. Last night I was able to achieve focus on stars without the adapter so all is well.
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