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Re: AH: windproof shades material
From: Modris Fersters
Date: 2021 Mar 20, 07:57 -0700

Hello, Igor again!

You wrote that you are going to use motor oil as a liquid surface of the artifical horizon. I would like to say some words about my adventures with this stuff. I poured used motor oil in small basin (approximate dimensions 10cmx15cm). Then covered this with wind shade.  Everything seemed to be perfect. But the reasults of the measurments showed unexpected random errors. It took a long time untill I found what caused these random errors. The problem was MOTOR OIL! It does not form perfectly horizontal surface in basin of above mentioned dimensions. The surface is slightly curved (surface tension effec). Therefore the measured angle will depend on the location of the reflected image in the basin.

Of course, all these things are actual only in case if you are working with accuracy of angles of fractions of minute.

How to check the quality of the liquid surface? It can be easily done during Sun culmination when its altitude does not change for a while. During these some minutes you make reflected and directed limbs of the Sun in contact and at the same time slowly move sextant in such a way that reflected image change its position with reference to basin. If during this procedure the both limbs move away one from other (or contrary: overlap a little bit), then it can be concluded that the surface of the liquid is not perfectly horizontal.  

Contrary to motor oil water makes perfectly horizontal surface. But it has one disadvantage: water vapour condensates on the wind shade. Of course it is more actual during long observations. But even in such cases I prefer water (but it is question of individual taste).       

Another disadvantage of motor oil: during long and frequent observations (it was my case) I accidentally managed to overturn the basin with oil several times. And the mess with this stuff is much more than with spilled water.

PS

I attached photo where my homemade wind shade can be seen (with food film covering). In the picture it is tilted to one side.   

Modris