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    Re: Sextant calibration accessory
    From: Peter Monta
    Date: 2021 Feb 25, 16:44 -0800
    Frank writes:

    In short, you can use a sextant to calibrate a sextant.


    Yes, but an auxiliary sextant is just as mechanically complex as the first.  It's like someone with two clocks: you don't know what time it is.  The prism, on the other hand, is unimpeachable.  Its angle will not change unless the prism is destroyed with a hammer.  It's also more compact.

    Fabrication is a bit of a speed bump, yes, if low cost is a goal.  Someone like an amateur telescope maker would have no trouble making a blank, grinding, polishing, and holding the angle to reasonable tolerance, but the prisms would have to be sent out for aluminizing unless workshop chemical silvering is an option (or the new spray-silvering process [1]).  (You mention "optical qualities": that would apply to a partially refractive device like a pentaprism, but a prism with just aluminized external faces could get away with any reasonable glass, or even an opaque material like granite, provided surface quality is good and it takes a polish.)

    you could use a pair of mirrors in a solid mounting

    That could work, but it seems to me just a little less reliable than a solid prism.  If the mirrors are bonded to a small plate with epoxy (or to each other), how good is the angle?  The epoxy will not be a thermal match to glass.  (Same concern with the Bris.)

    Cheers,
    Peter


       
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