NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2013 Jan 18, 12:35 -0800
Alex,
0.3' is a very small remaining systematic error. Declaring this personal error may be the best way to go here but before you do that check:
1. Index mirror perpendicularity
2. Side error
3. Scope is parallel to frame
4. Scope centered on horizon mirror
5. Lenses clean
6. Focus on Moon craters to get best sharpness
7. Lenses free of condensation
8. Eye centered in ocular
9. Star or Planet split by Moon's limb (not tangent)
10. Sufficient shading in place
What I found that caused an unexplained systematic error was not looking directly through the center of the ocular lens and then directly through the center of the objective lens. Looking from one side of the ocular lens to the far side of the objective lens caused a noticeable shift of the image. A solution for this is to make an ocular peep sight then hold your eye back off of the peep a bit. This forces centering.
Greg Rudzinski
[NavList] Re: Lunar distance measurement in ideal conditions: attainable accuracy.
From: Alexandre Eremenko
Date: 18 Jan 2013 11:02
Dear Doug,
There is still some profound mystery about my Lunars, which I practice for
almost 10 years. Almost all observations are positively biased by about +0'3
in the average. Why this happens, I do not understand at all.
(This was discussed on the list many years ago, but no satisfactory
explanation was ever given),
The Index correction (which I frequently determine with all means at my disposal)
is about -0'3, and stays constant since I bought this sextant, and I always apply it.
The arc error was checked about 10 years ago by Freiberger and Cassens Plath
(and I have 2 certificates issued by them).
Frank once handled my sextant for a week or so, and found no systematic error.
The only remaining explanation remains some sort of "irradiation" or
even more mysterious "personal error".
Indeed, I notices that if I use IC -0'6 (instead of -0'3 that I always find),
then all my observations will be centered (=unbiased, zero average error,
positive and negative errors equally likely).
But this explanation is unsatisfactory for the following reason:
I mostly observe Lunars. If I do something wrong when touching the Moon disc,
then the errors of far limb and near limb must be of opposite sign.
But they are of the SAME sign (always positive).
It is the same for all range of angles, big or small, the amount of data I have
is enormous.
My experience with other sextants is small (for statistics) and It does not
show any similar bias.
I almost always overshoot any Lunar or star distances.
And I have no plausible explanation why can this happen.
Alex.
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