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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Perpendicularity check
From: Frank Reed CT
Date: 2004 Sep 22, 23:22 EDT
From: Frank Reed CT
Date: 2004 Sep 22, 23:22 EDT
Fred H wrote:
"My understanding is that observed lunar distances will be too large if
the sextant is not properly oriented. If anybody could speak to this,
it would help me solve a current problem I am having with lunar
observations; my telescope does not have cross hairs."
The main thing that you need to know about this issue, Fred, is that you can SEE it if it's a real problem for your sextant. Next time you shoot a lunar, bring the two objects into "perfect" contact as nearly as you can. Then "walk" the two objects around the field of view (you've got 12 seconds before the Moon moves a tenth of a minute of arc). If they remain in "perfect" contact at various points across the field of view of your sextant's telescope, you don't need to worry about this issue. Also, consider trying this with star-star sights since the apparent distance changes much more slowly (due to changing refraction as the stars move across the sky). IF your sextant's telescope is parallel to the sextant frame, you should find no change in contact unless the telescope has a very low magnification in which case the images will separate very slightly on the left and right sides of the field of view. If the telescope is NOT parallel to the frame, you may see a substantial increasing monotonically from one side of the field of view to the other. If you see that, you will want to correct the telescope's alignment.
I think you mentioned a couple of days ago that you were getting errors in observed distances larger than a minute of arc. Can you post the actual sight data? Have you tried any star-star sights in this distance range? Myself, I'm skeptical of the arc error certificates which accompany modern sextants. I wouldn't be surprised if a significant fraction of them do not accurately represent the errors of the instruments.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"My understanding is that observed lunar distances will be too large if
the sextant is not properly oriented. If anybody could speak to this,
it would help me solve a current problem I am having with lunar
observations; my telescope does not have cross hairs."
The main thing that you need to know about this issue, Fred, is that you can SEE it if it's a real problem for your sextant. Next time you shoot a lunar, bring the two objects into "perfect" contact as nearly as you can. Then "walk" the two objects around the field of view (you've got 12 seconds before the Moon moves a tenth of a minute of arc). If they remain in "perfect" contact at various points across the field of view of your sextant's telescope, you don't need to worry about this issue. Also, consider trying this with star-star sights since the apparent distance changes much more slowly (due to changing refraction as the stars move across the sky). IF your sextant's telescope is parallel to the sextant frame, you should find no change in contact unless the telescope has a very low magnification in which case the images will separate very slightly on the left and right sides of the field of view. If the telescope is NOT parallel to the frame, you may see a substantial increasing monotonically from one side of the field of view to the other. If you see that, you will want to correct the telescope's alignment.
I think you mentioned a couple of days ago that you were getting errors in observed distances larger than a minute of arc. Can you post the actual sight data? Have you tried any star-star sights in this distance range? Myself, I'm skeptical of the arc error certificates which accompany modern sextants. I wouldn't be surprised if a significant fraction of them do not accurately represent the errors of the instruments.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois