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    Re: Mirror horizon
    From: Paul Dolkas
    Date: 2023 Sep 8, 08:12 -0700

    I agree on plastic:  Whenever I use my Davis 15 I always zero it out just before shooting, then check the error immediately afterward.  I take half that error and throw it back into the calculations.  It’s never zero.

    But that’s what you get with plastic.

    "How much distorion by heat in plastic, i have not calculated that. But think on the plastic Davis sextants that show this problem you can discover that in the always changing IE. You can work with that but it need always extra care.
    At home we use in the sauna one 10mm thick PE closed cell matt to lay on the hard wooden bench. At 20° C the length is 1800mm. At 85°C the length increases to 1820mm and later at 20°C it again 1800mm long. That reminds me always on the thermal expansion of plastic material.
    So plastic is not good for accurate stabele constructions."

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