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    Re: Inverting Scope
    From: Alexandre Eremenko
    Date: 2004 Oct 9, 23:11 -0500

    Dear Bruce,
    I am glad you purchased an SNO-T inverting scope,
    so I can discuss some specific points with you
    (maybe we should do it off-the-list, I am not sure).
    It seems to me that my inverting scope has the following problems:
    1. It won't simultaneously focus on a remote object
    (a star) and the cross hair. From what I read about Kepler
    telescopes of this type I conclude that the cross hair assembly
    should be adjustible by changing its distance from
    the objective lens. This does not seem to be the case
    with my scope, but actually I am not sure. Do you think the black
    part of the tube which holds the cross hair can be moved
    somehow with respect to the objective lens?
    It has two holes which look like the holes for an
    "optical spanner wrench" but on the other hand, I afraid that
    this part is glued to the main tube, so I'm afraid to break it
    by trying to adjust.
    By the way, the cross hairs are barely visible or invisible
    when it is really dark... and I suspect they were not designed
    for Lunar distances.
    2. Bright objects like stars and sometimes the sun are surrounded
    by some spots ("coronas") of light which look like hair growing from
    the object. If the object is exactly in the center of the field
    of view, and my eye is exactly on the optical axis,
    the star is visible in the middle of this
    "corona" of light. When I shift a little my eye or the sextant,
    the corona becomes excentric, and sometimes shifts completely
    to one  side (so that the "hair of light grow" only
    on one side of the star). A professional astronomer explained
    that this kind of aberration is called "coma", and it is typical
    for Kepler scopes. Do you notice anything of this sort in your
    inverting scope? I found that this aberration does not influence
    much my star observations; the star at the vertex of its "coma"
    is sharp enough, but sometimes it obstructs my sun observations.
    (That's why I said once that the inverting scope
    seems to work better at night (I mean star-to-star distances)
    while the
    Galileo one is
    better for the sun, which is exactly contrary to the Russian
    SNO-T manual: "to use the inverting one in daytime and the
    Galileo one at night".
    I agree that the star scope (Galileo) gathers more light.
    But the 7x30 inverting one, despite its coma aberration seems to have
    better resolution and better
    permits to tell the moment when the images
    of two stars collide).
    > The wires are nearly 2 degrees apart in the field.
    > Two vertical,
    > two horizontal. Why the horizontal ones, I don't know.
    > But they do make it easy
    > to measure the angular distance between wires.
    Apparently these wires were fitted not for lunar distances
    but to help the observer to hold the objects in the middle
    of the point of view. That's why there are 4 of them
    making a square rather than a cross in the middle.
    > or do accurate
    > work with an artificial horizon, you'll find the
    > inverting scope invaluable.
    Could you explain more about how the cross hair help with
    artificial horizon?
    Do you mean a Davis-type or bubble art horizon?
    > My guess is, a right-side-up scope with a reticule
    > would be far more
    > expensive.
    And more importantly, it would let less light through.
    As I understand at least two extra lenses or prisms are
    required to make the image straight.
    (Contrary to what many people say, I experience no inconvenience
    with this inverting scope. Maybe because I never used a sextant
    and started learning with both types of scopes simultaneously.
    This shows that this "inconvenience" is just a question of habit.)

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