NavList:

How about you put a bulb keel on it? A keel would provide a righting moment and make it behave like a pendulous mirror horizon such as the one in an MA-1 sextant.

gl

hch wrote:

Hi Ken,

Very interesting observations. I have long considered the feasibility of floating a reflective surface, however, the nicety of construction necessary to obtain a truly horizontal floating surface, both longitudinally and transversely, is somewhat daunting to say the least. It simply may not be practically possible to obtain such a surface, given the bulk shown in your photos.

Hydrostatically speaking, if we view your apparatus as a floating barge, it is necessary that the positions of the centers of gravity and buoyancy, both longitudinally and transversely, lie in the same vertical plane to produce a no list/no trim, or truly horizontal, attitude. It might be theoretically possible to construct such a floatation device, although hardly practical - so what do we do: possibly install indicators (bubble levels) and correctors (ballast) to return our mirror barge to the truly horizontal position - and now we are right back to where we started from, with an adjustable reflecting surface.

There are a number of problems associated with an adjustable reflecting surface, i.e., mirror, glass plate, etc., not the least of which is the sophistication of the level indicating devices which must be extremely accurate. Most of the literature I have read seems to support an opinion that a simple liquid or mercury filled tray provides to simplest and best solution.

Regards,

Henry



--- On Fri, 9/12/08, George Huxtable <george@huxtable.u-net.com> wrote:

  

From: George Huxtable <george@huxtable.u-net.com>
Subject: [NavList 6251] Re: Hybrid Artificial Horizon
To: NavList@fer3.com
Date: Friday, September 12, 2008, 3:18 AM
Interesting and inventive notions, there, from Ken Muldrew.

He ended-

"What would really be nice would be some kind of
simple calibration
procedure that could be used to measure the error every
time you set it up
(like measuring index error) but I can´t think of anything
off hand."

If he has something in view with an unchanging altitude,
such as a distant
street-light or a culminating star, why not measure its
altitude, then
rotate the whole gear, raft and tray, by 180º, and
remeasure?

If a trick like that works, then he can correct for small
errors of tilt, as
long as he always keeps the orientation of the raft aligned
(at least
approximately) with the direction of the object being
observed. Perhaps, by
that technique,  he could find and mark the direction on
the raft in which
the tilt was zero, and always align that.

But the important aim would be to ensure stability of the
flotation plane of
the raft, whatever happens. Ken has mentioned the problems
that will ensue
if water drops on the top upset its balance. Any contact
with the sides of
the tray would have to be avoided, or even a near-miss,
because of
surface-tension meniscus effects. To that end, it might be
helpful to gently
tether the raft by gluing light silk threads to hold it
loosely in place, to
keep it roughly central. They would have to be long enough
to let the raft
settle to the bottom when the water was emptied.

But the big problem, as I see it, is to avoid the trapping
of air bubbles at
the under surface, and for that I have no solution.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.





contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Ken Muldrew" <kmuldrew@ucalgary.ca>
To: <navList@fer3.com>
Sent: Thursday, September 11, 2008 9:07 PM
Subject: [NavList 6250] Hybrid Artificial Horizon


For those of us who navigate from our back yard, for fun,
practice, or
just as an excuse to watch the night sky, an artificial
horizon is a
necessary part of our kit. Now that mercury is hard to come
by, and a bit
of a nuisance anyway, most of us use a tray of water to
reflect the object
that we are trying to get an altitude for. This works well
for the sun and
moon, and is OK for bright planets, but it can be a trying
experience to
bring down stars. As an alternative, some people of a
metrological bent
will level a mirror or piece of glass, but this is a real
challenge in
precision, and the trying leveling process has to be
repeated every time
the mirror is moved (some back yards have a lot of trees).
I think that
everyone who tries this in the dark begins to wish they had
a jar of
mercury that they could just pour out and start observing.

I decided to try to combine the two approaches by floating
a mirror on
water. I had no wish to try to build a perfectly balanced,
hollow mirror,
so I thought that I could just mark the surface so that I
would always use
it in the same orientation and the error could be
calibrated, just as is
done for the index error with a sextant. So I bought a
mirror, cut it in
half, and used silicone sealant to glue strips of 1/4"
plexiglass around
the sides. The mirror was 1/8" plate glass (it was
actually a front
surface mirror, but polished on the back, so I used it as a
rear surface
mirror to protect the coating). I used 3 pieces of plexi on
each side to
end up with a sealed box that was about 6" x 3.5"
x 1". I built a tray to
float it that was just a bit bigger so that I could stick
my fingers in on
the sides and remove the mirror from the water. The
attached pictures show
the whole setup.

Last night Jupiter culminated just as twilight was ending
so I used that
to get a calibration. Just a note on the ease of use: it
set up in
seconds, and stopped bobbing in about 5 seconds; a light
breeze didn´t
affect it at all, and you can see the whole night sky
(piecewise ;-) ) as
clearly as if you were looking up. The measured maximum
altitude was
31°9´, the index error 8.1´, giving an apparent altitude
of 15°30.45´.
Refraction (10°C, 1000m above sea level) was 3.14´ for a
meridian altitude
of 15°27.3´. The declination of Jupiter was 23°9.1´ and
my latitude was
51°8.8´ so the altitude should have been 15°42.1´. If I
add 29.6´ to the
doubled angle for a mirror error, then these numbers match,
so I´ll call
that the mirror error.

To test it out, I took altitudes of Arcturus and Alpheratz,
both trivial
to find in the mirror despite it being just after twilight
and all the
lights of a big city adding to the general lack of darkness
(not that it
should be difficult, but if you have ever hunted for a star
in a water
horizon, you´ll know why I mention it). Here is the data:

Arcturus

9h18m46s 51°7.6'
9h21m27s 50°15'
9h22m39s 49°50'
9h23m24s 49°37'
-------- ------
9h21m33s 50°12'24" - 8.1' + 29.6' / 2 =
25°17' - 1.9' refr = 25°15.1'

Almanac gives 25°28.2'for a difference of 13.1'

Alpheratz

9h27m25s 63°19.8'
9h28m30s 63°40.2'
9h29m29s 63°57.6'
9h30m30s 64°15.8'
9h31m24s 64°32.2'
-------- ------
9h29m27s 63°57.12' - 8.1' + 29.6' / 2 =
32°9.3' - 1.4' refr = 32°7.9'

Almanac gives 32°24.8'for a difference of 16.9'

So the results are not very good. Note that I moved the
mirror and spilled
water between each round of sights. Also, when I moved the
setup after
getting Jupiter´s altitude, I noticed that water spilled
over the edge
just as I touched the side of the water box (without
actually moving the
box) so there was a meniscus on one end. I don´t know how
much off-level
the water might have been due to surface forces, but it may
have had some
bearing on the reading. Also, I fear that I slopped some
water drops on
top of the mirror during the moves; that might also have
changed the
balance of the floating box.

I will have to do some more testing to see if better
technique can improve
the results. I hope so, because the ease of use makes this
setup really
attractive. A more careful assembly would probably help (I
spent about an
hour building the whole apparatus). What would really be
nice would be
some kind of simple calibration procedure that could be
used to measure
the error every time you set it up (like measuring index
error) but I
can´t think of anything off hand.

For those interested, the mirror I used was this one:
http://www.surplusshed.com/pages/item/l3757.html
everything else was scrap.


\----------------------------+----------------------------+
  o_,
 O_/ \    Ken Muldrew, PhD      | Voice: (403) 220-5976
     |   <\__/7
 <\__  \  Dept. of Cell Biology | Fax:   (403)
270-0617      |     | /
  "\ L  | University of Calgary |
kmuldrew@ucalgary.ca       |   / /
   <
+-----------------------+----------------------------+ / /
               Morning coffee recapitulate phylogeny
  L/






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