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I think you are spot-on with the 30" level.  The Freiberger artificial
horizon apparently ships with two 30" levels (at least the level reads
30" on the end cap in the pictures I have seen). For comparison, I
understand that a Wild T1 (in its day, the workhorse surveyors
theodolite for low-order  triangulation, etc) typically came with a
30" bubble.  A Wild T2 "Universal" theodolite shipped with a 20"
bubble, while the Wild T3 "Precision" has a 6.5" bubble. The T2 is
called a "Universal"  theodolite because it is well-suited to every
type of surveying task short of 1st and 2nd order triangulation, which
was the forte of the T3.

I thus have assumed that a 30" bubble is plenty accurate for leveling
an artificial horizon used with a handheld sextant.

best
-p

On Aug 21, 8:04�pm, engineer  wrote:
> You need only one level if there are three levelling feet to your
> mirror. First set it parallel to a line joining two feet and level.
> Then set it at right angles to that line and use the third foot to
> level up. Recheck the first pair and so on.
>
> A little while ago, I constructed an artificial horizon out of 6 mm
> float mirror glass. It sits in a backed hardwood frame upon a bed of
> thin felt and is held down at three points by springy bits of brass.
>
> At these points, levelling screws pass through threaded bushes in the
> frame to a thick wooden sub-base. The levelling screws end in ball
> bearings. One of them sits on a plane , one in a conical depression
> and one in a vee groove machined in bits of brass let into the
> sub=frame. The whole sits firmly bolted to a surveyor's tripod.
>
> The level tube is home-made with the interior ground to give a
> sensitivity of 30 seconds per 2.5 mm division. The brass end-caps have
> two lapped flats on them, so the whole sits flat and is of minimum
> weight. Grinding a level tube requires surprisingly little equipment
> and just a little bit of olde craft knowledge.
>
> How does it perform? Well, my excuse is that the weather has been
> terrible lately and I haven't got a round tuit. If I knew how to
> attach a photograph to this, I would gladly do so.
>
> Bill Morris
>
> On Aug 22, 10:56�am, Bruce Hamilton  wrote:
>
> > George:
> > I clipped the information below from an online tool catalogue. �How little 
money would I have to spend to buy two levels accurate enough to calibrate a 
glass sheet (or mirror) horizon?
>
> > Horizontal-Mount Levels
> > N �5/8" Dia. x 3 3/4" Lg.0.001520 sec./2mm0.228"Brass (Black Finish)2160A5$76.30
>
> > N �5/8" Dia. x 3 3/4" Lg.0.0051 min./2mm0.228"Brass (Black Finish)2160A763.04
>
> > N �5/8" Dia. x 4 13/16" Lg.0.0072 min./0.1"0.196"Chrome-Plated Brass2160A196.98
>
> > P �13/16" Dia. x 6 3/8" Lg.0.00036 sec./2mm0.375"Brass (Black Finish)2160A2171.20
>
> > Q 2 15/16" Lg. x �9/16" Wd. x �9/16" Ht.0.03315 min./0.050"0.156"Aluminum2160A327.80
>
> > Q 2 15/16" Lg. x �9/16" Wd. x �9/16" Ht.0.03315 min./0.1"0.156"Polycarbonate (Black)2160A410.30
>
> > Q 2 15/16" Lg. x �9/16" Wd. x �9/16" Ht.0.12245 min./0.1"0.156"Polycarbonate (Black)2160A9*��9.50
>
> > Q 2 31/32" Lg. x �9/16" Wd. x �37/64" Ht.0.12245 min./0.050"0.156"Polycarbonate (Black)3329A46 10.30
>
> > R 2 5/32" Lg. x �29/64" Wd. x �21/64" Ht.0.1851 sec./0.1"0.079"Brass (Black Finish)2160A3611.95
>
> > R 2 5/32" Lg. x �29/64" Wd. x �21/64" Ht.0.1851 sec./0.1"0.079"Brass (Chrome Finish)2160A3711.95
>
> > R 2 1/2" Lg. x �1/2" Wd. x �5/8" Ht.0.0051 min./2mm0.125"Brass (Black Finish)2160A1177.20
>
> > R 3 23/64" Lg. x �5/8" Wd. x �3/4" Ht.0.0051 min./2mm0.125"Brass (Black Finish)2160A672.92
>
> > ----- Original Message -----
> > From: George Huxtable 
> > Date: Thursday, August 21, 2008 1:51 pm
> > Subject: [NavList 6181] Re: accuracy of glass artificial horizon figure
> > To: NavList@fer3.com
>
> > > Gary LaPook wrote-
> > > |
> > > | Well, the ultimate limit is the 1/8th wavelength of light "Raleigh
> > > | limit" since anything more perfect is not detectable due to
> > > the wave
> > > | character of light. This is the standard for telescope
> > > mirrors. But,
> > > | since you will not be trying see the moons of Saturn when
> > > doing celnav,
> > > | the artificial horizon doesn't need to be that perfect. Since
> > > the angle
> > > | of incidence equals the angle or reflection any error in the
> > > shape of
> > > | the mirror is doubled in the reflected ray. So, the answer to your
> > > | question is that it must be accurate to 1/2 the accuracy limit
> > > you are
> > > | trying to achieve. If you only want sight accurate to one
> > > minute of arc
> > > | then the mirror must be accurate to 1/2 of a minute. If
> > > working for one
> > > | tenth of a minute accuracy then the mirror must be accurate to one
> > > | twentieth of a minute.
> > > |
> > > | gl
> > > |
> > > | pls wrote:
> > > | > Does anyone know how accurate (i.e., level) the surface
> > > figure of a
> > > | > sheet of black glass must be to serve as an artificial
> > > horizon? �In
> > > | > particular I am trying to determine the point beyond which
> > > additional| > accuracy is irrelevant in terms of the result,
> > > given the other
> > > | > variables in a sighting with a hand-held sextant.
> > > ======================
>
> > > I wonder whether Gary has that right, or if he is a factor of 2 out?
>
> > > Yes, if the mirror angle is half a minute out, then the angle
> > > between the
> > > incident and reflected light becomes a whole minute out. But
> > > then, to arrive
> > > at a measured altitude, you have to divide that resulting angle
> > > by two. So I
> > > suggest that, if there were no other sources of error, the
> > > accuracy in
> > > measuring altitude will be no better than the accuracy achieved
> > > in levelling
> > > the mirror, and there is no such factor-of-two to apply.
>
> > > Maybe "pls" is concentrating on the wrong question. "Surface
> > > figure" refers
> > > to flatness, in a plane, not level-ness. The difficult bit is
> > > not getting
> > > the surface figure of the glass right; any decent glass flat
> > > will be good
> > > enough. It's getting a sufficiently rigid mounting, that can be
> > > finely
> > > adjusted, and tried with sufficiently sensitive levels, so that
> > > it can be
> > > got level, and will stay level, throughout a measurement. It
> > > calls for a
> > > sensitive spirit-level that's sufficiently light in weight so
> > > that its
> > > weight shifting on the glass causes negligible deflection. It
> > > requires firm
> > > ground so that no observable shift occurs as the observer moves
> > > his weight
> > > around.
>
> > > If these requirements can be met (and they can be bypassed, in
> > > the right
> > > conditions, by using a mercury reflecting surface) then
> > > altitudes can be
> > > measured with much greater accuracy than is possible at sea
> > > using a natural
> > > horizon. Besides the factor of two reduction in instrument
> > > errors caused by
> > > the doubling of the measured angle, and the firm footing on land
> > > compared
> > > with a vessel, all the problems inherent in the natural horizon
> > > disappear,
> > > particularly the unpredictable refractive component of the dip.
> > > As long as
> > > the observed body, preferably a star, isn't too low down (but it
> > > can't be
> > > above 60�, of course), then I would expect altitudes to be
> > > measurable, with
> > > care, to around 0.3 arc-minutes, or so, as long as the glass
> > > plate can be
> > > levelled with corresponding accuracy. And that becomes the
> > > difficult bit.
>
> > > George.
>
> > > contact George Huxtable at geo...@huxtable.u-net.com
> > > or at +44 1865 820222 (from UK, 01865 820222)
> > > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.- Hide quoted text -
>
> > - Show quoted text -

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