NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sextant Error and Whole Horizon Mirrors
From: Chuck Griffiths
Date: 2002 Jul 9, 14:43 -0400
From: Chuck Griffiths
Date: 2002 Jul 9, 14:43 -0400
Bruce Stark's recent posting in which he mentions the need for lunarians to pay close attention to mirror adjustment brings up a problem with errors I've run in to. While I've been measuring lunar distances, I've noticed that I have an error in my sextant that I can't remove or account for. This is easiest to see measuring interstellar distances because the angular distance doesn't change with time. If I move the images from one side of my whole horizon mirror to the other it appears that I have a mirror error. This error manifests itself by making the measured angle smaller when the images are on the area of the mirror closer to the frame and larger when the images are farther from the frame. Just to be perfectly clear I'll try to describe the problem one more time. If the image of two stars are brought into coincidence in the middle of the field of view of the horizon mirror (while carefully rocking the sextant to swing the arcs of course) the images will move apart or closer together if the images are moved from the center of the field of view to the edge of the field of view. Watching for this is a challenge as other lunarians will testify. I've had the best luck reclining in a chair with something to brace my arms on. I thought, at first, that this was due to the index mirror not being perpendicular to the frame. I've adjusted my sextant several times using both the image of the arc and using small cylinders placed on the arc (Celestair sells a product for this, but I just used something similar). After each adjustment I removed the resulting side error. But this apparent movement of the images from one side of the mirror to the other persists. I haven't taken enough measurements at different angles to say how consistent it is over the whole arc but it's usually in the neighborhood of about .2 arc minutes. I imagine that the problem is related to the fact that I adjust the index mirror at one point on the arc and as the index arm moves the pivot must not be perfectly parallel with the arc. I would be very appreciative if other list members with whole horizon mirrors on their sextants could comment on whether their sextants also demonstrate this error and whether they were able to remove it by adjusting the index mirror. Chuck Griffiths ********************************************************************** This e-mail and any files transmitted with it are confidential and may be legally privileged or otherwise exempt from disclosure under applicable law. This e-mail and its files are intended solely for the individual or entity to whom they are addressed and their content is the property of Smiths Aerospace. If you are not the intended recipient, please do not read, copy, use or disclose this communication. If you have received this e-mail in error please notify the e-mail administrator at postmaster@si.com and then delete this e-mail, its files and any copies. This footnote also confirms that this e-mail message has been scanned for the presence of known computer viruses. ***********************************************************************