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    Re: Star - Star Observations
    From: George Huxtable
    Date: 2010 Mar 12, 10:40 -0000

    A few notes about making star observations..
    I've seen statements, on this list, that measuring distances between two
    stars is less precise that those using Sun and Moon, and wonder why that
    should be. And that a star is a difficult object to use for checking index
    error. As a star should produce the smallest image of all, this is a bit of
    a surprise. Why should it be?
    My own old eyes are long past the point where I can contribute to this,
    Let's concentrate on index error checking, which seems to me to be the most
    difficult case. The image of a star will be some sort of circular blur. The
    aim is to superimpose two such blurs, one via each path, direct and
    reflected. The problem seems to be this: that when the two blurs get
    sufficiently close, they coalesce, merging into one. Once that has happened,
    it isn't clear which way to turn the micrometer drum to bring their centres
    closer, because those centres can no longer be distinguished; you can't tell
    which is which..
    Might it help to introduce a bit of side-error, so that instead of the two
    star-images merging, turning the knob causes one image to slide past the
    other, the aim being to put them at the same level; and nearly, but not
    quite, in contact, in the sideways direction. Does that actually improve the
    operation, in practice? For this purpose, a lit crosswire in the focal plane
    of the telescope could be helpful.
    How else, without side-error, can we distinguish one image from another? If
    we trust our shades to be optically undeviating, would it help to put a red
    shade in one path, and a green shade in the other? Then when misaligned, the
    combined image would show a red fringe at the top and a green one at the
    bottom, or vice versa. The aim would be to adjust until they matched,
    What about introducing a weakly cylindrical lens in one path, not in the
    other, so that one image becomes a horizontal streak, that has to be aligned
    with the disc of the other image. That would require a simple astigmatiser,
    that used to be a regular fitting on many sextants, but then, it was to fit
    to the telescope, to smear both objects. Here, I'm suggesting it goes into
    one path, beyond the horizon mirror; so it would have to be a somewhat
    larger lens, and precisely undeviating on one direction.
    What about a shutter, a simple vane that can be flipped into and out of one
    path, by a lightly-acting trigger on the handle? That would allow the user
    to align a fixed image with an occulting one. Better, in some circumstances,
    would be two alternating shutters, one in each path, which would produce
    alternating images. That would work when a sextant was used on land, with
    little motion. But at sea, when the two images are zooming around the field
    of view, it seems to me it would be necessary to see simultaneous views, at
    least part of the time.
    Unlike the use of a star for checking index error, when measuring star-star
    distances, rocking the sextant will shift the two images in different ways,
    which in itself allows for the two images to be identified. In addition, the
    two stars may have different magnitudes, or different colours. If so, does
    this ease the operation of measuring the distance between them?
    These are no more than notions or talking-points. I'm not really proposing
    that they are worth implementing, for the occasional case when star-only
    images are being observed. I'm really just asking this question: if star
    images could be separated in such a way, would their use in observations
    then be as accurate as other sky-objects?
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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