NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Artificial horizon question
From: George Huxtable
Date: 2009 Apr 20, 18:45 +0100
From: George Huxtable
Date: 2009 Apr 20, 18:45 +0100
I wonder whether the root cause of JKP's doubled images, sideways, is the result of sextant misalignments, as Brad has suggested. I tentatively offer an alternative possibilty. He is seeing two reflections, one from the surface, and one from the shiny bottom of the oil-pan. That's the reason for using an opaque liquid, such as treacle (molasses to Americans) or old engine oil. Or why, if a glass reflector is used, black glass is preferred. Otherwise, John could try introducing a non-shiny surface to sit on the bottom of the tray, such as fine sandpaper, fine sand, cloth, matt black paint. The aim is to ensure that any reflection comes off the top surface, and isn't confused with another from the bottom. John has no call to apologise in describing his experiences. He has described well what he sees, and we can all learn by trying to interpret them. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From:To: Sent: Monday, April 20, 2009 3:59 PM Subject: [NavList 7998] Re: Artificial horizon question Since the topic of artificial horizons has returned, let me bore you all with a newbie's experience in learning to use one. Last night I made my first attempt at a star sight with the Davis artificial horizon. The pan was filled with mineral oil and sat on a table, uncovered. I used no "house" as the weather was absolutely calm. I found the star's reflections were pretty faint (I am in a city) but could be picked up by bringing my eye very close to the surface of the pool. I then had to slowly back away and simultaneously straighten up while keeping my eye on the reflected image of the star in the pool until I was standing confortably, some six or seven feet back from the table, and could still see the reflection. WIth the sextant set at zero I looked through the scope at the star and brought the instrurment down while swinging the arm so as to keep the star in the index mirror until I was again looking into the artificial horizon. Sirius and Arcturus were bright enough for this method, but Betelgeuse's reflection just could not be seen once I began to back away from the table. I remembered reading some postings in this list's archives about black glass plates, etc., so I took the dark blue shade that comes with the Davis A.H. and dropped it into the bottom of the pan of mineral oil. It helped somewhat, but not really enough. I may go out looking for some black glass this week. The biggest problem I had, and in fact always have with the A.H., is that I can't seem to bring the two images very close together laterally, despite adjusting the mirrors of my Davis Mark 15 beforehand. It is hard to tell whether the images are level with one another when they are so separated. The lateral space varies suddenly and without apparent reason; that is I can't figure out what I am doing that causes them to converge and separate. Moving my eye from side to side behind the telescope doesn't help; also the images seem to move towards and away from one another as I move the index arm, but not in a predictable manner. Can anyone suggest why I might be having this problem and what I could do about it? -John P. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---