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    Re: Sextant scopes and an old-bold empiricist
    From: Bill B
    Date: 2006 May 1, 15:29 -0500

    The below has been gathering dust in my "drafts" folder for  about 10 days
    now.  Sorry for the delay.
    Thank you George
    I suspect I am pretty much on track, and like the "tracing paper" suggestion
    for determining power.  It only just occurred to that the diameter of the
    exit pupil will be the diameter of the objective lens /power. I am a bit
    fuzzy on how far to place the tracing paper from the ocular lens.  My best
    guess is to let the built in eye relief serve as a guide.
    > | 1. How many lens elements should I expect to find in 3X to 4X power
    > | "straight" scopes?
    > My plastic Ebbco sextant (3x) appears to have just two simple lenses in a
    > Galilean (opera-glass) configuration. That means it has a
    > normal convex objective, and a concave (diverging, negative-power) eyepiece.
    > The advantage of that simple arrangement is that it
    > results in a short telescope, and a non-inverting view. One disadvantage is
    > that there is no internal focus-point within the
    > telescope, so there's no way to instal a crosswire, to centre your sightline
    > on.
    Interesting.  Field glasses (long binoculars--no Porro or roof prisms) seem
    to have a third (center) lens set instead of a prism pair to keep the image
    from being inverted.
    > There's another way to get a rough figure for magnification. Look at something
    > like the strakes along a garden fence, with both eyes
    > open; one through the scope and one not. Estimate how many of one will fit one
    > of the other. I am assured it can be done, though not
    > by me, as since childhood I have a "lazy eye", that I've never bothered to
    > focus. Luckily, it's my left eye, not the sextant eye.
    This seems like an interesting experiment, as the brain will very quickly
    start to adjust and merge images (or just ignore one eye)--which goes back
    to an unanswered "dominant eye" post many moons ago.  Odd as it may seem, my
    right eye is dominant and has better vision than my left eye, but for some
    unknown reason I get better results (lower standard deviation when averaging
    and better agreement between Ho and Hc at a known location) with the sextant
    using the left eye.  Go figure?
    > I'm not sure what question Bill is asking here. Yes, the objective lens itself
    > has a certain f-number, but not a telescope as a
    > whole, as I see it. That's because the telescope does not have a focal length,
    > as such. It takes in parallel light and puts out
    > parallel (or very nearly parallel) light. So the notion of f-number doesn't
    > fit with "prism optics" in the way Bill implies.
    I am sorry if not clear.  In a camera lens objective diameter related to
    focal length determines f-number (a "Reader's Digest" version of our
    exchange on theoretical f-number vs t-number for a lens).  As I read it (and
    I am on very shaky rope/line here, hence my desire to tight-rope walk with
    the list as a net) a telescope's objective lens has both an aperture
    (diameter) and a focal length.  How "nearly parallel" the rays are is
    determined by the objective lens focal length.
    From Sky and Telescope:
    "If a telescope's aperture is its most important spec, its focal length
    comes next. Say you have two telescopes with the same aperture but different
    focal lengths. The one with the longer focus (that is, a higher-numbered
    f/ratio) will generally lend itself better to high-magnification viewing.
    (The f/ratio is just the focal length divided by the aperture.) One reason:
    you can stick with longer-focus eyepieces, which are easier to use,
    especially for eyeglass wearers. Another reason: "fast" objectives, those
    with low f/ratios, are harder to manufacture well, and thus they tend to
    make fuzzier images unless you've paid a premium for top-quality optics."
    Thanks also for the fence-post trick for magnification.  That put me in the
    right ballpark with the least amount of effort.

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