NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Testing sextant arc error
From: Brad Morris
Date: 2017 Jan 17, 13:24 -0500
From: Brad Morris
Date: 2017 Jan 17, 13:24 -0500
Hello David
Calibrating the arc of a sextant has been the topic of much discussion here.
The concept of using star to star distances does, of course, fit the bill nicely. You would start off with SHA and declination, and yes, do the spherical trig. Beyond that computation is the essential clearing of the distance, much like a lunar. Refraction comes into play as part of that clearing. That will provide a known angular distance to measure your arc against.
A simple one time measurement of the distance won't be sufficient to determine fine arc error however. There should be a statistically sufficient number of independent measurements of that angle. Some measurements should be starting with a larger angle on the sextant and others with a smaller angle on the sextant, such that the targeted angle is approached bidirectionally, in sufficient numbers, as to get that relevant figure of merit.
Further, you really cannot limit yourself to the 57 navigational stars. That will not provide sufficient coverage of angles, unless you were to hop about the globe. What does provide a sufficient number of stars is to use the Astronomical Almanac's "Bright Stars" list. That provides a rich set of stars to work with. Naturally, identifying those bright stars in the sky requires some degree of confidence that the star you want in the sky is the one you are measuring. Celestron's Sky Scout proved up to that task.
So there you have it. A sufficiently rich set of stars. A proper method to identify those "Bright stars" in the sky. A statistically sufficient number of measurements to provide a confident figure of merit. The proper clearing of the measured star to star difference.
I have yet to see anyone perform this task and document the results. It's been talked about and talked about. I tried it and found the task unmanageable. Perhaps you will be the first to complete this task.
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The Nikon Autocollimator and Ultradex in my personal tool kit is clearly up to the task, yet I would have to add in the appropriate granite surface plate, tooling and support slab. I've examined the cost of the additional parts and it would be ~$3000. There simply isn't enough financial interest on NavList (and the general public) to support my investment in it to justify the expenditure. The Nikon Autocollimator and Ultradex pair are about $50K new, so it's not something a casual user is going to run out and buy.
Bill Morris gives a wonderful review of his measurement of sextant arc error and micrometer drum error on his publicly available blogs. The method he provides does work but again, specialized equipment and techniques.
Neither Bill's method or my method are for the faint of heart or those who are, shall we say, economically timid.
Brad
On Jan 17, 2017 11:55 AM, "David Pike" <NoReply_DavidPike@fer3.com> wrote:
I’ve been thinking how one might calibrate the arc of a DIY sextant while living inland without specialised equipment. Is there anywhere on the www a list of the angular difference on the celestial sphere between the 57 navigational stars, or would you have to start off with up-to-date SHA and declination and do the spherical trig? Not my favourite cup of tea. I am aware that you would have to choose your stars to minimise differences in refraction.
Another possible solution would be to compare horizontal angles of ground features with those measured with a real sextant of known good quality, but the features would need to be quite a way away for repeatability. DaveP
