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    Re: Star to Star Distances taken on a Second Hand Sextant
    From: Brad Morris
    Date: 2019 Dec 6, 17:11 -0500

    From my point of view, there is no need to rush.  I have what amounts to permanent interest in this topic.  

    Being within a few minutes of Cmdr Bauer is exactly what is expected.  You got a reasonably close value, therefore the sextant is okay (after the theoretical drop!).  

    An actual calibration, in which you define your arc error, is trickier.  In my experience with industry, for statistical relevance, you should obtain 6 observations of any given star pair.  That is, assume the datum to be 50°.  Start at 53°, or so, and get 1 observation.  Move the sextant back to 53° and repeat.  If you approach from the low side, instead of the high, that's okay too.  Which ever is your habit.


    On Fri, Dec 6, 2019, 3:48 PM David Pike <NoReply_DavidPike@fer3.com> wrote:

    Well there was only one thing to do; go out and try it.  Wednesday, Thursday, and Friday we were down in the low light pollution area of Norfolk, so with great fortitude and determination and the temperature down to 2 degrees Centigrade, Mrs P and I drove the car onto Kelling Heath (N52.92789 E1.13489 on Wednesday evening 4th Dec, taking along ‘Navigator’ on a Netbook, and my Hughes Three Circle Mates Sextant No 25410.  First star pair was Deneb at roughly Hs= 48.55 and Az= 286 and Vega at Hc= 27.17 and Az= 299 and a tilt of about 30 off the horizontal. 

    First thing I found is, with bifocal eyeglasses and only a 2.5x telescope, checking index error against a nice bright star like Vega is nothing like as easy as checking against the sea horizon or a roofline many blocks away in daylight.  All I could say was that index error didn’t seem too far off the sextant’s historic index error of zero.  Next came how to hold the sextant.  I ended up holding it handle side up at a tilt of about 30 degrees to the horizontal.  I found this rather awkward.  No one ever tells you that your carefully attached lanyard will get in the way of just about everything, and to be prepared for an index shade to fall under gravity and leave you wondering where the second star has disappeared to. 

    After that comes bringing one star onto the other from zero.  Well that might be OK for the ‘Experten’, but for yours truly it explains why I was never any good at ball games.  The ‘wimp’ way to do it is to set the sextant about half a degree lower than what you’re expecting; point at the brightest star though the horizon glass and look for the other star in the mirror.  The half degree was to prove to myself that if, I could only see one star they weren’t, though some sort of fluke, on top of each other.  There turned out to be little chance of that I can assure you.  In fact, I found finding the second star so hard that in the end I unscrewed the telescope and just looked through the hole, even then, I had to cheat a bit at times by keeping both eyes open.

    After about 15 minutes practicing like this, I felt confident enough to put the telescope back in and do the job properly.  I found the best method was to rock the sextant ever so slightly like rolling the Sun about the sea horizon and gradually increasing Hs until the reflected star crashed though the index glass star.  I then increased Hs further until the stars separated again.  Going back onto crashing into each other, the best I could manage with the 2.5x scope and my not very special eyesight were values within a couple of minutes of Cdr Bauer’s values.

    After all that messing around with Vega and Deneb, Orion and the Twins were becoming nicely visible and with Betelgeuse at roughly Hc= 17.54 Az= 102 and Pollux at Hc=17.42, Az=067, I had two stars horizontal and at a low Hc.  (I’m not sure if this is the worst case or not.  True there’s a fair amount of refraction, but the XP lines are almost parallel at low altitudes, so that might cancel out the refraction effect a bit.)  This was a lot easier.  With handle down I could get greater support from the car roof and steady the telescope with my left hand.  Also, Betelgeuse is a bit more yellow, so I could see which star was which (In my Universe all major stars would be colour coded).  Once again, with my eyes, spectacles, and sextant, all I could say was that my measurement was within a couple of minutes of Baur’s values.  Therefore, my conclusions so far are that for a rough check it does work, but there must be easier ways of checking a sextant, even inland.  However, you need good eyesight, decent magnification, a strong, steady wrist, and most of all you need to be in good practice.

    My next job is to work out some 2019 values for the stars used and record my results properly, but I wanted to get something written down while this topic was still hot.  DaveP

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