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Hi John

The lateral displacement of the reflected and direct images has to do with the 
plane of the arc and the angle it forms with the two images.  That is, the 
plane that contains the arc of your sextant does not also contain the two 
images.

You can see this quite readily if you can get to a location where you can see 
two objects far away, when the angle is about 90 degrees on your sextant.  If 
you wiggle the sextant, you will see the two objects brush each other as they 
move in your scope.

You can actually test the parallelism of your scope relative to the sextant 
arc by this method. By wiggling (Bowditch called this a "vibratory motion") 
the sextant, you should be able to see the objects brush each other on either 
side of the scope.  If they touch only on one side and not the other, then 
the scope is not parallel to the arc.  It takes a bit of practice but is well 
worth the effort when done.  In particular, since artificial horizons yield a 
doubling of the angle, and the parallelism becomes more significant as the 
angle grows, you should minimize this error.

Best Regards
Brad


-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of JKP@obec.com
Sent: Monday, April 20, 2009 10:59 AM
To: NavList@fer3.com
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.




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