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John wrote-

"I will either get a piece of black glass (and devise a means of leveling it
as I have read in past posteings) or at least try some sort of
non-reflective treatment for the pan bottom."

Beware. Whaver sort of base you fix it to must be smoothly and finely
adjustable in tilt, and very rigidly mounted, right down to ground level.
Sometimes, just bringing your feet, carrying your own weight, close to its
base for reading can affect tilt. And you need a very light and sensitive
spirit level, such that when you apply it to the glass plate, and when you
remove it, there isn't any perceptible change in tilt.

"I don't particularly suspect that I'm getting a double image from the oil
pan, however.  When sighting the sun, I use a colored cover over the
artificial horizon pan, and different colored filters between my eye and the
sun, and I get two distinctly colored images, say green from the sky and
yellow from the pan."

I'm not sure I understand that picture. Are you certain that you are not
getting reflections from the surface of the "colored cover"? What is the
optical quality of that cover?

"It is these that can't seem to be brought very near each other
horizonatally.  That is, until for one luck second they slip right over one
another, and if I'm quick with the micrometer I can get my shot beforfe they
agaion wobble apart."

I can't see what causes this instability with time, unless the liquid
surface is being disturbed . Do you get that instability when looking at the
two images of Sun or star with the index set to zero, in normal view without
an artificial horizon? If you do, then you have a sextant with a loose and
wobbly mirror.

Or does it vary as you look into different parts of the artificial horizon
assembly, as your eye position shifts about, because it's impossible to
stand stock-still? In which case, any shift may be the result of reflections
off different parts of the surface of your "colored cover", if that's not
exactly plane. Normally, any draught-excluding cover has two angled
surfaces, like a gardener's cloche. Is yours like that or is it a single
horizontal sheet?

The trouble with reflecting off water, compared with Mercury, is that only a
small fraction of the light is reflected, so the wanted image is weak.
Because of that, other reflected images, such as that off the cover, or off
the base, can be equally intense.

What's the magnitude of this effect? The Sun is about 30' diameter, how much
are these unstable shifts, compared with that?

"By the way, I had also meant to ask:  Should I ignore altitude correction
when using an artificial horizon?  Clearly no correction for semi-diameter
should be needed, because I am superimposing full discs of the sun and its
reflection, center-over-center, rather than touching the upper or lower limb
to the horizon.  But I guess I can't grasp the principle behind altitude
correction sufficiently to decide whether it counts in this situation."

The way you're doing the job, superimposing the two discs, there's no
correction for semidiameter. However, it's more usual, and gives better
answers, to place the two discs one above the other, so that the upper limb
of one brushes the lower limb of the other If next, you reverse the order,
and average, that corrects for semidiameter..

There is no correction to make for dip. Correct for index error before
halving the answer. Correct for refraction after halving.

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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