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