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On Feb 7, 2006, at 9:28 AM, Robert Gainer wrote:

> 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
>

I should just add that my experience was with a Simex knockoff, and
my comments and Joel's may apply more accurately to American and
cheap Japanese instruments than to German ones.

You might get an answer to your questions regarding fog and oils
directly from Cassens & Plath possibly via a dealer.

I expect that problems with salt, condensation and freezing of
rotation such as you describe may well have been much more important
in "market rejection" of crossed polarizing filters for shades than
problems with shade errors.

   

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