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What I consider my best instrument is a Cassens & Plath sextant that has gray 
filters and polarizing filters for both the index mirror and the horizon 
mirror. Because of the recent conversations about the errors inherent to the 
polarizing filters, I put this instrument in a table top stand and cycled 
though the range of adjustment for both polarizing filters while focused on a 
roof top over a mile away. I must admit that I cannot see any displacement of 
the image as I turn the filters. If there are any errors, they are smaller 
then my eye can see and of course, that is as precise as the instrument needs 
to be for my use.

I do however now have some ?fog? between or maybe I should say inside one of 
the polarizing filters. This may be caused by keeping the sextant in the car 
overnight to see if the cold stiffened up the adjustments and action of the 
instrument and then bringing it inside to test the errors in the filters. I 
had not considered the use of the sextant in cold weather until cold weather 
was mentioned in a recent posting and my trip to North West Greenland next 
year will mean using the sextant in a colder environment.

First, how do you clean inside a polarizing filter and second because the 
action did stiffen up from the cold how can I ?change? the oil to a winter 
blend?
Robert Gainer

   

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