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You will find the Freiberger mirror here
http://www.fpm.de/index.php?c=1&s=khorizont
As the total diameter is 125 mm I guess the mirror itself is 100 mm.

Yesterday around noon I had a quite frustrating experience with my mirror. I 
was in fact unable to adjust it to horizontal. First I tried hard to get the 
level accurately adjusted but finally I came to the conclusion that the sun's 
heat radiation made the screws grow.
When adjusting them I gave them shade with my sleeve. The mirror was set to horizontal.
As soon as I moved my arm away the I could watch the bubble rise towards the sunny side ;o)
And back when shade was given again.
Though it was a relieved to find the reason for my futile efforts I went home with no observation made.
Liquid mirrors definitely have their advandages...

Any ideas how to make a reliable solid horizon mirror?

Wolfgang




-----Urspr�ngliche Nachricht-----
Von: "Bruce Hamilton" 
Gesendet: 25.09.09 19:30:14
An: navlist@fer3.com
Betreff: [NavList 9922] Artificial horizon glass selection


The most recent Edmund optics catalog just got delivered to me, and I started 
looking at the first surface mirrors. Depending on the grade, they can be 
quite reasonable. A 75mm diameter� with a surface accuracy of 4-6 lambda per 
inch is $24.00 USD.� 

The Freiberger seems to use about a 70 or 80 mm diameter mirror, but i am just 
guessing by looking at the pictures. If anyone knows the actual diameter , 
please let me know. Would round or square be better? 
 
What would be the lowest surface accuracy one could use to have the necessary 
accuracy while keeping things cheap? I am guessing that first surface mirrors 
are best, but let me know if anything else would be better. If you can think 
of an optical item, the Edmund catalog has it.  I remember someone was 
looking for a penta-prism, and they have these, and the prices are quite 
good.  They have a web page with a sales area so it looks like you can buy 
direct.

BTW Thank you for the discussion on GPS time. I was unaware of the difference 
between GPS time and UCT, and the single occasion this summer when I thought 
I would use it as my time source, I had a frustrating night where all my 
sights from my A 12 were off by 25 miles no matter how many times I tried. I 
took a lot of sights this summer while camping and learned things like never 
trust a sight that is taken through too many leaves or too close to the top 
of a hill. All sorts of odd refraction takes place.

Cheers

Bruce

Thanks





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