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    Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Greg Rudzinski
    Date: 2013 Jan 18, 15:33 -0800

    Alex,

    See attached images of scope ocular peep. I used a hole punch through a piece of thin plastic which was cut to fit in front of the ocular lens. File folder material will work for trials. Tape in place for experiment. If it works then make a better one. The peep worked for me on lunars. For conventional observations the peep is removed.

    Greg Rudzinski


    [NavList] Re: [NavList 21998] Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Alexandre Eremenko
    Date: 18 Jan 2013 18:17
    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
    >

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