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As I recall when determining index error using the sun, the point of
contact varies with up and down location in the telescope.

Fred

On Nov 23, 2007, at 5:21 AM, George Huxtable wrote:

>
> Greg Rudzinski asked-
> |
> | What happens to an image of a celestial body when viewed off
> center in
> | a sextant scope? I have a bad habit of using the lower half of the
> | field of view when taking lower limb observations of the sun. I
> | suspect that this increases the observed altitude slightly.
>
> ====================
>
> Response from George-
>
> I think not; as long as its central in the side-to-side field of
> view. It
> doesn't matter if it's central between top and bottom. What follows
> will
> assume the sextant frame is vertical, as when taking an altitude.
> For a
> lunar, it should be read differently.
>
> What's important is that the viewline should be in the plane of the
> sextant's frame, which is why a telescope needs to point in the right
> direction  to avoid collimation error, and why the mirrors need to be
> perpendicular to that plane. Otherwise an out-of-plane error
> ensues, which
> is negligible for small measured altitudes, but matters for large
> ones. If
> you make a contact to one side or the other of the centre-line of
> the view,
> you are creating a bit of skewness. Some telescopes are fitted with
> crosswires to guide the eye to the centre line, where any contact
> should be
> made, but that's not possible for a Galilean; one of its
> disadvantages.
>
> However, as long as everything  IS kept to the same plane,
> symmetrical left
> and right, it doesn't matter a bit whether the viewline is a bit up
> or down,
> the laws of reflection in a plane being quite unaffected. So Greg's
> worries,
> about observing the contact in the bottom of the view, are
> groundless, he
> will be pleased to learn..
>
> These points should become clearer if you experiment a bit with
> extreme skew
> angles, by removing the scope and looking through the mirrors with
> the naked
> eye from various skewed directions, off from the line of the scope.
>
> George.
>
> 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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