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This question was discussed a lot in October 2004,
and I want to add something.

Recently I made many experiments with Davis art horizon
sold by Celestaire.

I tried various combination of Caro syrup and oil,
Caro syrup alone, and oil alone.
(Caro syrup was recommended on this list).

The best "combination"
seems to be pure vegetable oil, without any syrup.
I really don't understand why this sirup was proposed.
(To make the bottom darker? What for? And if this is
indeed useful, why not to paint the bottom of the horizon
vessel black with ordinary paint?

The syrup hardens when the weather is cold (it was 23 F last
weekend), always has impurities and causes double reflection
unless the oil layes is extremally thin. Such thin layer of
oil is usually not enough to smoothen the surface of sirup.

I Have not tried the dark (mahine) oil yet, but why it is
considered useful to have dark substance in general?

Alex.

P.S. I DO understand why Bauer and others recommend sextant
mirrors painted black on the back side (to prevent second
reflection from that back side) but with liquid filled art horizon
with plastic botom it is different: there is no reflection
from the bottom.

   

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