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Re: Calibrating a sextant: with a sextant and a spotting scope
From: Frank Reed
Date: 2011 Oct 8, 15:49 -0700

Hi Bill.

Yes, there are some collimation issues that complicate matters a bit. But they're manageable. I didn't spell out everything in detail, and I will be curious to see how you work it out. :) You agree, I suppose, that the principle makes sense as long as the collimation and leveling is done right.

You wrote:
"whether your powerful spotting scope does 'make the process really accurate'. It may make the observations more precise, but doesn't the accuracy depend on the known errors of whichever instrument you are assuming to be the standard?"

There are two separate issues here. Addressing the second point first, yes, this is clearly (obviously??) a method that depends on a sextant which has been tested by some other method to serve as the standard. We can transfer the known calibration from the "standard" to another sextant. That second sextant can then become a (slightly lower quality) standard to test other sextants. I wouldn't go more than one step away from the best available standard sextant that I can find. That is, sextant A, of excellent quality and known calibration, can be used to generate a calibration table for sextant B. Sextant B can then be used to calibrate sextant C. But I would not consider using sextant C to calibrate other sextants since noise accumulated at each step.

As for the first issue, the spotting scope is not what I would call "powerful". A magnification of 20x is reasonable for taking the observer's visual acuity out of the equation. The average observer with reasonably good vision (corrected by common eyeglasses or contacts) can resolve angles of one minute of arc. With a magnification of 20x, that resolution is improved (on the target) to 3 seconds of arc. That's probably just a little better than reasonable expectations of accuracy for sextant observations, which is what you want if you're trying to test the sextant at its limits. Certainly going to a magnification of 100x would serve no real purpose. On the other hand, a 3x scope is insufficient to test the sextant at its limiting accuracy. I chose a 20x scope, but anything from 12x to 30x would probably be appropriate.

By the way, setting aside arc error for a moment, using a spotting scope stands as the best method I have found, after years of experimentation, for measuring index error. It is reliable and repeatable to the nearest tenth of a minute of arc. It is also an excellent and quick way to test micrometer error to the limiting precision of the instrument. You set the good sextant to 0d 5', 0d 10', 0d 15', etc. and then use the same setup I described previously to test the other sextant.

-FER

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