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I'll throw in my two bits' worth based on over 25 years' experience using
these finicky things.

First off, I agree with an earlier poster about mercury being ideal for
stars. That being said, the stuff is hard to find, is highly toxic and
therefore not recommended.

Of all the AHs I've used, the Frieberger black glass is the best of the
bunch. C.Plath used to make them and I have been trying to find one but they
are virtually non-existent.

Ok, let's cut to the chase: the best way to view stars is to get your
eyeball/sextant as close to the artificial horizon as you can. What I have
done with pretty good success is to raise the AH on a platform till it is
about at your chest level (sitting down or standing up). In this way, you
can more easily discern the reflected star image. Simple as that.

I hasten to add that this works well for 1st magnitude stars and planets but
not so well for 2nd and 3rd magnitude bodies. The black glass tends to suck
up the light. This is where mercury has its advantages.

Oh yes, I agree that edge to edge images are the way to go. Trying to
superimpose is difficult and not very precise.

Good luck and good hunting.

Robert
----- Original Message -----
From: 
To: 
Sent: Monday, April 20, 2009 7:45 PM
Subject: [NavList 8006] Re: Artificial horizon question



George, GL and Peter,
Thank you all for continuing to offer helpful advice.  To answer some of
George's questions about the horizon I am using, it is the inexpensivce
Davis model, and has a sloped-roof type of cover, with solid plastic sides
("gables" if you will) and clear or colored plastic panes.  The panes appear
to be quite flat as far as one can tell by looking.  No doubt they are cut
from large sheets after emerging from a pair of rollers set to a precise
tolerance.  Perhaps in cooling or handling they acquire some irregularity.
Moreover, the image is passing through the panes more or less normal to the
surface.  It may be refracted a bit going through, but I don't think I'm
seeing and extra reflection, as I might if I were reflecting the sun off a
stack of horizontal plates.  Again, it may be reflecting from the top and
bottom of the pan, so I will try some light-deadening measure on the bottom.

When I see the reflected sun in the pan of mineral oil, it is very distinct
and solid, and depending on the cover I have over it, may be bright yellow.
As I bring the sun down from the sky, again viewing through various filters,
it will appear to the right of the yellow reflected image and will be,
perhaps bright green or blue (depends on the filter).  There is no mistaking
which is which, and no particular impression that either image is double.
They will be separated horizontally anywhere from two to four or five
diameters apart, but as I squirm and squint, fiddle with the telescope focus
and the micrometer, and try to hold my toes just the right way, they drift
slowly together, then apart again. It's much like watching the little
floating debris on the surface of your eye; you can sort of chase it in one
direction by eye-rolling or will power, but it always wants to slide back.
But I digress.

I'll give your suggestions a try next time I'm out.  I'm especially grateful
that some of you pointed out the advantage of getting the images
edge-to-edge rather than totally coincident.  Let's see if this improves my
results!

-John






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