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Re: How flat do sextant mirrors need to be?
From: Geoffrey Kolbe
Date: 2009 Jan 28, 15:02 +0000

```Ok Bill, as no-one else is prepared to give you a definite number, I will

Suppose you have index and horizon mirrors 25mm square. Suppose you are
just using a 'peep tube' with no telescope. The resolution of the human eye
is about one MOA, so what mirror flatness is required to give this resolution?

To rotate a 25mm mirror by one MOA, one edge of the mirror needs to be
moved 25/3400mm (approximately) while the opposing edge stays where it is.
This is about 6 microns or 12 wavelengths of light.

From this, we can see that if the mirror is slightly saucer shaped, so
that the middle is about 6 wavelengths lower than the edges, our 25mm
mirror will cause sufficient blurring of the image to be just noticeable by
the naked eye. To ensure that two mirrors in the optic train give blurring
of the image below the one MOA level, the mirrors should each be flat to
better than 3 wavelengths of light.

How flat the mirror needs to be then depends on the power of the telescope
being used. With a 3 power scope, you need the mirror to be flat to one
wavelength of light. When using a 25 power scope - such as the one I made
for my Husun - you can see that I had to have the mirror polished to better
than an eighth of a wavelength of flatness.

Geoffrey Kolbe

At 06:20 27/01/2009, you wrote:
>
>
>I recently checked some old sextant mirrors against a tenth wave
>optical flat prior to resilvering them and found that the greatest
>deviation was a half wavelength. 3 mm modern mirror float glass
>compared favourably, suggesting a cheap solution to replacing
>deteriorated mirrors in old instruments.
>
>I have posted some images in my blog on my website www.sextantbooks.com
>
>Can one of our erudite members suggest how flat the mirrors need to be on
>theoretical grounds? I have vague memories that people aimed for a quarter
>wave when making the flats of amateur Newtonian telescopes, but this may
>well be irrelevant.
>
>Bill Morris
>
>
>
>
>

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```
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