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5 replies to this topic

### #1 Aquarius Of The Night

Aquarius Of The Night

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Posted 21 November 2017 - 11:19 PM

Hello all!  I am wondering what exactly it means and how to figure out what focal length is based on the interior radius.  If you don't understand what I am saying probably because I am not competent on the topic Gordon Waite is referring to it in this video.

Thanks!

-Lance

### #2 J A VOLK

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Posted 21 November 2017 - 11:42 PM

Didn't watch the video, but the focal length = (D*D)/(16*s), where D is the mirror diameter and s is the sagitta, or depth of the mirror. Also, s = D/(16*f#).
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### #3 BGRE

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Posted 21 November 2017 - 11:47 PM

The mirror blank in question had been generated and was not spherical with the radius of curvature varying all over the surface.
By surveying the mirror with the spherometer an idea of in which direction various parts of the mirror surface deviate from the desired sphere can be obtained.

### #4 JamesMStephens

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Posted 22 November 2017 - 12:08 AM

For a spherical mirror the focal length is half the radius of curvature.  The mirror is a 20" f/3.5, so the focal length is 70", and the radius of curvature is 140".

Edited by JamesMStephens, 22 November 2017 - 12:45 AM.

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### #5 Aquarius Of The Night

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Posted 22 November 2017 - 08:43 AM

Thanks all!  I was just wondering how the heck I would do that If I ever wanted to be exact.

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### #6 perfessor

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Posted 28 November 2017 - 11:05 AM

Didn't watch the video, but the focal length = (D*D)/(16*s), where D is the mirror diameter and s is the sagitta, or depth of the mirror. Also, s = D/(16*f#).

I made my spherometer with tripod feet on a 4" dia. circle.  If you plug 4" into the above, you get fl = 1/s, which keeps things nice and simple.

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