NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Adding beta Capricorni to the web-based lunars calculator?
From: Peter Monta
Date: 2013 Jul 28, 08:38 -0700
From: Peter Monta
Date: 2013 Jul 28, 08:38 -0700
> Of course. Please do. A positive bias like that suggests a simple error in > index correction. How do you normally test index error? I don't recall > --what type of sextant do you use? I'm using an Astra IIIB, purchased about ten years ago from Celestaire, along with a porro-prism monocular from them, which is marked "Cassens & Plath 6x30". I recently discovered that it was out of collimation by about 2 degrees relative to the sextant plane with no visible damage or problems with the fork or attachment points, so I've shimmed it pending a more permanent repair. I'm kind of evolving on index error measurement; there are many good suggestions in the archives. Unfortunately it's not a constant 0.5' bias; from session to session it moves around from close to zero to close to 1', but fairly tight within the session. My current thoughts are: - I don't like IE measurement with the horizon or single stars. With the horizon (a treeline about 10 km away, so sextant parallax is a negligible 0.02 arcmin), contrast is poor because of the superimposed images, and I'm concerned about possible bias when approaching coincidence always with increasing angle, as recommended to avoid backlash. As for single stars, it's even worse: the horns of the dilemma are, on the one hand, with no side error the superimposed images are aberrated mush even with good focus, and with some side error, judging when the stars are closest is difficult with no wire or reticle telling me when the images are in a line normal to the sextant plane. - IE measurement with the solar or lunar disk is better, but this still gives information only near zero degrees on the arc, and I'm starting to worry about arc errors and micrometer-worm-eccentricity errors. - So, recently I've been measuring IE with star-star sights, three or four spread across the arc from 10 degrees to maybe 90 degrees. For me, the swinging motion of the star in the index-mirror path really helps. Just having some motion when the stars collide makes it seem like I have higher precision---the relative paths are trackable by eye. I suppose I'd want the distances uniformly spread modulo 1 degree to try to average out worm eccentricity, but I haven't done the planning for that yet. > It's good you're using a 6x30 scope. I'm coming to the conclusion that I really need a reticle. I think some of my remaining error is still collimation, because with a 6-degree FOV in the scope, it's hard to touch exactly in the center without a guide. Chauvenet mentions 7 arcmin as a maximum angle between touch and collimation axis! That's only 2% of my FOV. Even if I loosen that to 15 arcmin, it's still challenging with no wires. > With that magnification, there's every reason to expect you'll be able to get > your mean error down to about a third of a minute of arc on individual > lunars, with no bias. I'd be happy with that. I'll try to post my plots later today; random errors seem to be about that large, but all that's showing is my tangency repeatability. The systematics are the hard part. Of course, at some point one has to just stop, given the diminishing returns, or else get a better instrument (repeating circle perhaps). Cheers, Peter