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    Re: Mirror problem
    From: George Huxtable
    Date: 2006 May 6, 00:54 +0100

    Alex wrote-
    | (Someone already complained on the list that your
    | messages are hard to read because they do not contain
    | line breaks. With my mail program I read then easily,
    | but to REPLY a long message from you is a real
    | pain, exactly for the same reason: there are no line breaks,
    | your paragraphs consist of single line of text each).
    Thanks for passing on that information, Alex. I had no idea that was the
    case. Nor have I received that message, complaining about that lack of
    line breaks (another missing-message, perhaps?). If you would kindly
    copy it and resend, I would be grateful.
    Nav-l messages, and perhaps others also arriving here, suffer by their
    lines getting chopped progressively shorter and shorter, each iteration,
    as they bounce to and fro when they are quoted. Line breaks occur at
    unexpected places. Is that a common problem that affects others? It
    didn't seem to happen when using Eudora with my old Mac, but is a real
    nuisance now, with a PC and Outlook Express. It was in a futile attempt
    to improve on it that led me to increase the allowed line-lengths to the
    maximum possible number of characters (132). In response to your
    message, I have since cut that back to 72 characters. Please let me know
    if it makes a difference. If anyone has hints to offer about the matter,
    please say.
    | Back to your message:
    | Your explanation of the index mirror bending
    | is very clear. I checked my sextant: it has ONE
    | adjusting skrew and three springs. One of the springs
    | is opposite to the adjusting screw. The whole configuration
    | looks like this:      :+
    | where the dots and the plus sign mean the springs, and the
    | adjusting screw is behind the + sign.
    | I assume that the sextant is hold with its frame plane vertical.
    | So the design seems sound to me.
    That makes sense. In that case, there's just the one adjustment on that
    mirror, for perpendicularity. Other adjustments, for side error and
    index error, will be made to the horizon mirror.
    Presumably, the two springs that do not oppose an adjusting screw
    instead each face up against a small pip or knuckle, not adjustable,
    providing between them a fixed axis about which the mirror can tilt. If
    the pips and screw exactly face the springs, then they will not impose a
    bending moment on the glass. Especially so, if they are light coil
    springs. But sextants were not always designed that way.
    | > The complaint that Alex makes, about a doubled star image,
    | > with a weaker displaced image or tail as seen in the mirrors,
    | I would not call it "double". I would rather say it is
    | somewhat "blurred", and of irregular shape. It LOOKS
    | like an image in a low quality scope.
    | But the OTHER image, through the horizon glass looks
    | like an image in a good quality scope.
    | I can certainly test the spare mirror that I have.
    | And your idea of putting a diaphragm on the mirror,
    | decreasing its effective size is great.
    | If the reason is the mirror deformation, then the diaphragm
    | should evidently decrease it.
    | It is probably also possible to design an experiment that
    | will show which mirror is really responsible
    | for my problem: I am just
    | thinking of fixing the sextant somehow and looking
    | into the index mirror through the detached telescope
    | (which I can hold in my hand) under different angles.
    I think that will be difficult, depending on what you fix the sextant
    to, because of relative movement; especially with a wobbly floor I
    suggest that you try a scheme with a detached telescope, hand-held,
    cobbled to a single detached mirror. The mirror should be held at an
    angle in the view of the telescope, close in front of the objective,
    with some arrangement of cardboard and sticky-tape. It calls for a bit
    of ingenuity and improvisation, the sort of skill that was once expected
    of physics research students, in the days before they devoted themselves
    entirely to computer screens. That's an under-appreciated gift, and I
    bet you have it.
    | (My main technical problem with all such experiments
    | is the shaking of the wooden balcony floor as I move; this
    | also almost prevents me from using the art horizon.
    | But I will invent something to overcome this:-)
    | BTW, do you think it safe for the eye to use a laser pointer
    | in the experiments? Together with Sun filters, of course:-)
    | It looks much less bright than the Sun.
    Will you see anything at all, using such a laser pointer, seen through a
    Sun filter? I'm not sure exactly what you are proposing.
    Without such a filter, however, I would hesitate. Laser pointers are
    supposed to be sufficiently limited in power so as to avoid eye damage,
    but I doubt  it was envisaged for them to be viewed through a telescope!
    I would not wish to pontificate about that.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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