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Author
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Topic: Should the index mirror image move as well as shrink?
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Lance
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posted March 28, 2020 03:40 PM
Hi,
I've been trying to find the source of a systematic error of possibly 12 minutes in my sextant observations (using an artificial horizon) and I'm trying to eliminate faulty assembly as a possibility. (The first sextant sold to me had no horizon mirror, so faulty assembly seems conceivable.) With the index arm at 0.0 degrees, I get a combination of images from both mirrors in the telescope but if I move the index arm, the image of the index mirror gets smaller (as I expected) and moves upward so that at high angles it occupies only the top 3/8 of the field of view. Is this typical behavior of a correctly functioning sextant?
From: Hobart, Australia
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David Burch
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posted March 28, 2020 03:43 PM
What brand of sextant is it? Did you buy a mirror and add it yourself.
Also it might be easier to speak in terms of direct and reflected images, rather than reference to a mirror or glass.
From: Starpath, Seattle, WA
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Lance
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posted March 28, 2020 04:15 PM
Thanks, David,
It's a Davis Mark 25 with the whole horizon glass for us beginners to use. I exchanged the sextant for one with a horizon mirror. The boating shop were very good about it - although as astonished as I was by the missing mirror!
The direct image of the horizon seems OK. Looking at the reflected image, I notice that it is close to full field of view at 0° on the scale but shrinks as I go to higher altitudes. This seems reasonable, as the apparent size of the index mirror as viewed from the horizon mirror gets smaller. I'm surprised that it migrates away from the field of view, though. I fondly imagined that it would stay in the centre, getting smaller.
From: Hobart, Australia
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David Burch
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posted March 28, 2020 05:31 PM
Sounds like we are not talking artifical horizons any more. right? you just have the sextant set to 0º 0' and are looking direct toward something on your local horizon.
I will have to look at this on monday, but when you increase the angle the reflected view will move and indeed get increasingly cut off. Maybe this is what you mean by shrinking?
what kind of artificial horizon are you using?
Sounds like you have taken sights before and they are always off by 12' or so. is that the case.
From: Starpath, Seattle, WA
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Lance
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posted March 28, 2020 07:59 PM
Yes, that's right. Stuck here and home in isolation and trying to understand what is happening, I guessed at 12 minutes because I was getting position lines about 7 or 8 miles away from my GPS position, using a Davis artificial horizon. I think I'm just seeing what you described - the reflected view is getting cut off (sliding towards the top of the field of view as it shrinks). So I think it was just my inexperience in not knowing that it's normal.
I managed to get AsNav to give me a probable position within 3 nm of a GPS fix during my recent cruise, but I think maybe it was just that the GPS fix was in the centre of a large error area that ASNAV was computing. A 2 body sun and moon fix later that morning was more like 30 miles in error, using the NAO tables.
So in retrospect, I think maybe I was something like 6 to 10 miles the whole time I was on the last cruise of the Voyager of the Seas (hope that's not prophetic). When my positions were very close it was probably more by accident.
Assuming there's nothing pathologically wrong with the sextant, and that's probably the way to bet , I think I need to pay more attention to things like index error. I suspect collimation error is unlikely to explain that much error.
I have your book (Using Plastic Sextants) on order, so when it comes I will work through it and see if I can improve my sights.
Again,many thanks for your kind assistance.
From: Hobart, Australia
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David Burch
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posted March 28, 2020 08:05 PM
That book will answer a lot of questions about what you are doing... or at least provide some tips.
From: Starpath, Seattle, WA
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