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I can provide the following brief account regarding distorted mirrors.  Some years
ago I sent my metal sextant to the Original Equipment Manufacturer (OEM) for
certification.  The OEM later informed me the mirrors were distorted due to overly
strong springs they originally installed years earlier.  I recall the OEM stating the
sextant was okay for angles less than 70-80 degrees but the pinpoint star images
would deteriorate (blur)  quickly for the higher angles making back-sights impossible.
For a fee of course, new mirrors and new design springs were installed, the sextant
was then remounted on the collimator and passed certification.  The mirrors are
rather thick optical plates and I was very surprised that little springs could cause such
distortions.

I am always impressed with the high precision measurement a good sextant is
capable of making.  Since your instrument passed certification by not one, but two,
recognized experts I doubt your mirrors are the problem.  When a star is viewed
without the telescope (mirrors are in the optical path of course) is the distortion
perceptible to the naked eye?  Well maybe not naked, eyeglasses are okay too.

Frank J.
Rochester, NY

Date sent:      	Thu, 4 May 2006 11:22:34 -0400 (EDT)
From:           	Alexandre E Eremenko 
To:             	NavList@fer3.com
Subject:        	[NavList 120] Mirror problem
Send reply to:  	NavList@fer3.com

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Can anyone give an explanation/advise
on the following problem I encountered.

1. When I focus my telescope on a star,
a weak star really looks like a point,
but a very bright star looks like a little
spot, approximately round. This is OK.

2. But the image of the star reflected in the
two mirrors does not look like this.
It looks sometimes as a little dash, sometimes
a like a comma, or a little spot of irregular shape,
or a little disk with a little tail sticking from it.
My estimate of the size of this irregularity is about
0'.5. So when taking star distances I have to aim
at the "middle of the star":-)
At the same time, the direct image of the star is fine.

The problem is evidently in the mirrors.
But what is this exactly? Some microscopic scratches?
Not exactly flat surface? Double reflection from
the horizon mirror?
The image becomes somewhat worse at large angles,
but it is not very good even at small angles.

My index mirror is front-silvered, and the horizon mirror
is back silvered, as on most of modern sextants.

I've read that the mirror's flat surface can be a bit bent
because of the pressure of the adjusting screw...

Alex




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