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    Re: How does the AstraIIIb split mirror work?
    From: Jim Thompson
    Date: 2004 Apr 24, 12:03 -0300

    > -----Original Message-----
    > From: Jim Thompson [mailto:jim2@jimthompson.net]
    > > From: Joel Jacobs
    > > A question for Jim T., is one or both of your mirrors front or back
    > > silvered.
    > Index mirror is front-silvered.
    > Horizon mirror is back-silvered.
    > I checked my sextant, but see also
    > http://celestaire.com/catalog/Marine_Sextants/Astra_IIIB/
    1. I should add that on closer inspection the horizon mirror seems to be
    constructed this way on the mirror side:
    - The thick clear glass (Joel guessed 4 mm, and that seems about right)
    extends across the whole horizon mirror frame.
    - The mirror behind that glass (toward the celestial bodies) is a thin
    material with silvering toward the glass, and a grey backing.
    2. About the front/back clear glass reflection issue:
    I took the telescope off the sextant and set the sextant on my desk in my
    office with a banker's light off to my right and a little above behind me.
    The sextant was looking away from the light at a "zenith angle" of about 110
    degrees, so the light was coming from a little above and a little behind to
    the right.  Then I held a pencil between the horizon mirror and index mirror
    to look at its reflection in the horizon glass and mirror.  Then I held the
    pencil a foot in front of the index mirror in the direction a  celestial
    body would lie, so that its image was relected onto the horizon glass/mirror
    via the index mirror.
    I can definitely see a ghost image about 4 mm away from the brighter image,
    all the way across the field of view, whether viewing a direct image, or an
    image reflected from the index mirror.
    Obviously the ghost is a reflection from the back of the glass, offset from
    the main image coming from the front of the glass. This is true whether I
    hold the pencil in front of the horizon mirror/glass and view its direct
    reflection, or if I view the image double-relfected via the index mirror.
    The 4 components of the image order from brightest to dimmest this way:
    1. Front glass over the mirror (the main image).
    2. Front glass over the clear window (the main image).
    3. Back glass over the clear window (the ghost).
    4. Back glass over the mirror (the ghost).
    I still do not understand why I do not see a ghost image of distance objects
    with the telescopes.  But then I have not yet properly digested everyone's

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