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John wrote:
"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."

If the lateral space varies, then the images are going in opposite directions. 
 They converge and separate, which describes, at least to me, the exact 
motion you would see when taking a lunar observation and (at the risk of 
confusion, yet one more time) the motion of the objects you would see when 
you check your telescope for parallelism.  In particular, as John manipulates 
his index, the sighted objects move in an unpredictable manner.  Each time he 
touches the index arm, he is disturbing the plane of the arc, which slightly 
moves the optical path, creating this apparent motion of the objects in the 
scope.

Best Regards
Brad




-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of George Huxtable
Sent: Tuesday, April 21, 2009 9:21 AM
To: NavList@fer3.com
Subject: [NavList 8011] Re: Artificial horizon question


Brad Morris wrote-

"Well, I seem to have miscommunicated my answer, based upon the
misinterpretation of it through various postings."

That was a kind way of putting it. I had failed to follow his meaning, which
was my fault, not his.

And with hindsight, I think Brad had it right when he wrote-
.
"I don't think John's problem is parallelism.

I do think his problem is holding the sextant steady.  When you take a
lunar, the moon and the star brush each other as you wobble the scope.  One
goes left, the other goes right.  And hence the reason why I said the plane
of the arc in the same plane as the two objects.  In this case both objects
are the same object, just on a different optical path."

Indeed, yes. When you measure an altitude above a horizon, the limb of the
Sun brushes along the horizon, and you take the point where it comes lowest
as indicating the sextant is vertical, where the reading should be taken.

With an artificial horizon there's no such horizon line. One Sun image
brushes past another, and only when they are aligned, one above the other,
is the sextant truly vertical. So, in a way, that makes it easier, to get it
vertical, and the setup is somewhat forgiving about whether it's exactly
vertical or not.

So when John sees a sideways displacement between the two Sun images, all he
has to do is twist his wrist, a bit, holding the sextant handle, and one
image should slide sideways past the other until they brush when the plane
of the sextant is truly vertical. Does this describe the effect that is
bugging him?

If so, that renders my other other suggestions about his doubled image
superfluous. Anyway, he's explained the cloche-type structure of his
wind-shade, which is the right geometry for the job, and avoids the problem
I had postulated, which could happen with a flat cover.

=============================

By the way, my suggestion, of placing one Sun image above the order, then
reversing their order and averaging, to avoid correcting for semidiameter ,
wasn't very clever. All very well if the Sun is on the meridian, its
altitude unchanging. But if it's rising or falling, it becomes quite
complicated to correct for that movement when you want to average. Better to
follow the procedures suggested by others.

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.




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