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    Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Alexandre Eremenko
    Date: 2013 Jan 18, 18:17 -0500

    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
    and could not find it.
    > 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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