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George Huxtable wrote:
"However, let me make some guesses. Clearly, for this job, a highly-
sensitive
level is called for. The one at the top of Bruce's list, if I follow
his
numbers right, at 20 sec per 2mm shift, appears to fill that bill, at
$76.30. But there are other matters to consider, such as how
accurately the
tube has been set into the base, and how uniformly it's been ground,
for
smooth movement of the bubble, and whether the glass plate will be
big
enough to accomodate the span between its feet.."

There is no point in having a level that is too sensitive. The cost
goes up as the sensitivity and very sensitive levels, say, less than
10 seconds per division, not only take a long time to settle down,
they are very susceptible to minor and subtle temperature changes that
cause swirling in the liquid and instability of the bubble. The plate
levels of theodolites are usually of 20 to 30 seconds sensitivity.
The accuracy with which it is set into the base doesn't really
matter(see below). As for uniformity of grinding, this was a problem
apparently in the post WWII years in Britain and a few years ago I
corresponded with someone who had worked on the problem at the
National Physical Laboratory. Sixty years on, it is unlikely still to
be a problem. He imparted to me the trade secret of getting the inside
of the vial ground to a segment of a circle(and not some other shape).
I am happy to share the secret(off list, as it is likely to be of very
limited general interest).

George also wrote:
"Bill Morris rightly asked  why  two levels would be called for. What
he
didn't point out, because it's so obvious, is that you should always
try a
level both ways round, and take the average. "

This is correct, if you are using a calibrated level to measure slope.
More usually, you want to know that your surface is level, in which
case you adjust until you get the same reading of the level each way
round. It is of course a great convenience if the vial has been
adjusted in the base so you don't have to do this and can just centre
the bubble.

And George also wrote:
"I'm a bit uneasy about the way his mirror sits on "a bed of thin
felt" . Is he quite certain that when he lifts off the level, after
setting
the plate horizontal, the felt doesn't spring back, just a touch, when
its
weight comes off?"

I'm sure there's no problem because on checking I find that I must
have considered this and have used three pads of cork, not felt, about
50 x 50 mm at the hold-down points so the frame could move
independently of the mirror. I've perhaps reached an age where I
should keep a proper workshop notebook! But George's question is still
valid, so I have done the experiment. I made the level to be of
minimum weight, just the vial and two brass end caps with flats for
feet; and it weighs just 80 grammes. Their is no discernible bubble
movement when a block level of about the same length but weighing 950
G is placed alongside or at either end, nor any movement of the block
level's 10 second bubble when I do the same with my lightweight one.

and George suggested:
"You can alternatively use water, or better, dark oil, or
molasses (which here we call treacle), but those are difficult to use
with
all but very bright stars, and very clear nights.

When I last played seriously with artificial horizons 30 years ago, I
used old sump oil in a flat baking tin and it was very successful, but
if you use cooking oil with dark food dye added, it's nicer to handle
and you can lick your fingers if you get a bit on them.

Bill Morris


On Aug 22, 10:39�pm, "George Huxtable" 
wrote:
> Bruce Hamilton asked-
>
> 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
>
> ===============
>
> Although Bruce addressed the question to me, I doubt if I am the best fellow
> on this list to answer him. To be honest, I've had little experience in
> using levels and artificial horizons.
>
> And I'm unsure about some of the sensitivity numbers Bruce quoted anyway, as
> two columns appear to have run together, and there seems to be little
> correspondence between them. What exactly do those columns represent? I fear
> that, copied once again here, those tables have suffered further mutilation.
>
> However, let me make some guesses. Clearly, for this job, a highly-sensitive
> level is called for. The one at the top of Bruce's list, if I follow his
> numbers right, at 20 sec per 2mm shift, appears to fill that bill, at
> $76.30. But there are other matters to consider, such as how accurately the
> tube has been set into the base, and how uniformly it's been ground, for
> smooth movement of the bubble, and whether the glass plate will be big
> enough to accomodate the span between its feet..
>
> Bill Morris rightly asked �why �two levels would be called for. What he
> didn't point out, because it's so obvious, is that you should always try a
> level both ways round, and take the average.
>
> Bill wrote- "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 thesub=frame." Clearly, and this has become
> inreasingly apparent from his postings, here we have a real expert, with a
> full understanding of kinematic design, and an asset to this list.
>
> However, I'm a bit uneasy about the way his mirror sits on "a bed of thin
> felt" . Is he quite certain that when he lifts off the level, after setting
> the plate horizontal, the felt doesn't spring back, just a touch, when its
> weight comes off? That might be checked by adding a corresponding bit of
> extra weight while keeping the level in position, to see what happens..
>
> ================
>
> And Gary LaPook has added-
>
> "I have been taking series of shots of Jupiter in my artificial horizon
> (since it is favorably placed) and I want to takeseveral more series then I
> will write up what I found."
>
> Gary has a mercury horizon to use and that's undoubtedly the best, although
> there are problems. You can alternatively use water, or better, dark oil, or
> molasses (which here we call treacle), but those are difficult to use with
> all but very bright stars, and very clear nights. However, Jupiter is
> presently very bright, and those liquids could well be usable with Jupiter,
> if anyone wants to do the same as Gary.
>
> 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.
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