A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A-12 Experiences
From: Geoffrey Kolbe
Date: 2020 Feb 9, 09:47 -0800
Ed Popko commented on the 'cons' he found with the A-12. I agree that this is a great little instrument and pretty much concur with Ed's pros. However, these are my comments on his cons:
- Requires a steady platform for good results, don't know how those air navigators did it (hats off to them) The bubble chamber is curved so that the bubble follows the celestial body as the instrument is tilted. It is 'tilt invarient'. If xylene is used as the bubble medium, it should not be too sluggish and certainly no worse in this respect than any other bubble sextant.
- Narrow field of view requires pre-calculation of Hs! Well, the sun is pretty easy to find and the narrow field of view when used in that format is not a problem. However, when viewing stars the instrument is not used the same way as when viewing the sun. When viewing stars, you look right at the star *through* the index "mirror" (plain piece of glass) and you see the illuminated bubble *in reflection* off the index mirror. When used that way, the field of view is pretty wide - tens of degrees - and acquiring stars is not a problem. Just look at the star and lift the sextant to your eye.
- Vernier and averaging drum night-light not effective Do you have the right bulb in there? Should be a small bottle shaped bulb with a magnifying front which then fits neatly into the perspect 'light pipe' that carries the light to the vernier.
- Vernier very difficult to read, very fine markings. That is probably a particular symptom of that particular instrument....
- Only bright stars are usable, bubble chamber viewing window frost obscures dim ones, Venus, Sirius, Aldebaran, Pollux OK See my comments above on how to view stars. Used the way it should be, you can take sights on all but the dimmest stars.
- Difficult to determinme IE. Polaris is not very visible. I have taken many sights on Polaris with my A-12. It is not a problem, believe me.
- No cross hairs to center bubble!! No, because the instrument is tilt invarient. If you tilt it, the bubble follows the image, so you do not need cross hairs...
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