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    Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Alexandre Eremenko
    Date: 2013 Jan 18, 19:01 -0500

    Thanks I will try it.
    What is is supposed to do ? center your eye against the eyepiece?
    What is the size of the central hole?
    > 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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    > Attached File: http://fer3.com/arc/img/122002.f1-img_1609.jpg
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