NavList:

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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