Several years ago i invented the Bris minisextant as a navigational toy and back up to the GPS. There is a video on youtube – search Yrvind.
Yesterday I got a thelephone call asking if the surfaces where coated .
There is no coating. Not on the production type nor on any experimental prototypes. It would not work on this kind of sextant.
I use ordinary flat panel glass. When I was a small child living on the windward side of a small island I noticed, and was intriged by the fact that the windows in our house was transparent in the daytime and mirrors at night when it was dark outside. I remember that I asked my grandmouther about about that very interesting and curious fact but she could not give me an satisfactory ansver. I never forget it and thougt that one day I may have use of it. Now I now that windows are beamsplitters. 90% transparent and 10 % reflective.
In the 3-glass sextant one can see eight images of the sun. (in the 4-glass sextant many more). One of the glasses is a filter to reduce the strenght of the sun. It is very balanced not to bright not to dark. To enable the observer to see the 3 bright suns created by dubble reflections and the 5 dim suns created by quadrupple reflections at the same time. People have fat on their fingers. The fat creates fingerprints. If fat or dirt gets on the sextants optical surfaces it may reduce the transparency and only the bright suns may be seen.
If this happens the sextant should be cleaned. I use ordinary disch washing detergent. After rinsing I dry the surfaces with compressed air – oil-free compressed air. Like what people use for diving or cleaning negatives. If you do not have acces to clean compressed air try to centrifuge away the water use the cord attached to the sextant, attatch a little weight to it to gove more inertia, swing swiftly, do not hit anything.
The outer surfaces are easy. For the inner ones one can use a dentalfloss-houlder and cotton thread – do not use dental floss as it is waxed.
The sextant is glued with a specially formulated NM-epoxy that chemically reacts with the glass. You can clean with anything that does not harm epoxy and glass.