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Greg,
I think I checked all this many times, as well as I could.
And I know that my vision deteriorates with age, and I do not
see stars very clearly without a telescope.
But vision deterioration would give larger dispection, scattering,
not the systematic, constant bias that I observe.

Conserinng the sextant itself it was checked by Freiberger,
and never traveled much since.
I never noticed any changes in the perpendicularity or index correction.
The bias existed before and after Freiberger's check.
I did not tell this to Freiberger
people because the sextant was almost new then, I had little experience,
and not so much statistics as I have now.

Freiberger people did not adjust it, except putting some grease
in the night scope eyepiece, because it was slightly shaking.
And they issued a certificate with a table of corrections
in seconds (!), for every 10 degrees,
and the largest correction in this table is 7" :-)
I could never detect any correlation between this table of correction
and observations, and I never use these corrections.

I agree that the error is small, and can
be easily dealt with because it is constant. But still I was looking for
explanation
and could not find it.

Alex.


> 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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