A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2010 Mar 13, 11:57 -0800
George H, you wrote:
"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?"
The problem seems to be minimized by using fainter stars and/or by avoiding dark adaptation. Bill B. (billyrem42) and I have discussed this a few times on NavList.
"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?"
Yes, it does. I've recommended this several times. Leave an arcminute or two of side error and then use it. Side error, after all, is really no error at all. Even so, unless we use very high magnification and take other precautions, it still doesn't seem to produce the results that you would expect. Bill B. has also experimented with this, if I remember correctly.
"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."
Aha. Sort of a modern "Maskelyne flap" turned to a different purpose. It might work, but it seems like it would require some radical surgery on the instrument. Not so good for resale value!
And you wrote:
"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?"
Bill B. has suggested using bright stars with shades. Maybe an orange shade for the star seen through the horizon glass, a green shade for the star seen reflected. The color contrast is then a useful cue.
Finally, I'll just say that magnification is the cure. For index error, just remove the standard telescope and place a common spotting scope in line. It doesn't have to be expensive --just something that will give a magnification of 25x-50x. At that level of magnification, there's no doubt about index error. Differences from factors like clockwise/counter-clockwise rotation of the micrometer are immediately apparent. Results are repeatable to the tenth of a minute level if the sextant is a good one.
By the way, I used to recommend a laser method for checking index error: shoot a laser at the horizon mirror from the position normally occupied by the telescope. It will be split at the horizon glass and one will then bounce off the index mirror. Two beams emerge from the sextant. They will remain parallel if the index error is zero. While this works, it's more work than the spotting scope solution above. With the laser, you need a couple of hundred feet of open space, and there's a lot of fiddling involved to get it set up right. The spotting scope approach is much easier, and you can even do it in dreary weather from inside (looking out an open window) as long as you have a few miles of visibility.
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