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Hi, Igor!

According to my own experience normal glass does not has both surfaces absolutely paralel each other. When I want to achieve maximum accuracy using artfical horizon I use home made wind shade. The frame  is made of carton (but it may be made of all available materials)  and the cover is made  from ordinary food film. This membran is very, very thin and therefore the rays passing throug the film are not disturbed. Of course, the film should be streched and then fixed in place by adhesive tape. I made my shade not in shape of prism (as it can be seen in your attached pitcure) , but insted of it I used curved upper surface.

Modris