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Bill,

> I assume it is important to use a flat-screen CRT or LCD monitor so distance
> does not change due to curvature of a standard CRT.

Sure. I use a flat-screen, LCD monitor of my laptop. The distance changes
anyway, due to the inequality tan(a) not exactly equal to a,
but this seems to be negligible, by experiment and by calculations.

> I also assume you want the screen perpendicular to the line of sight on both
> the horizontal and vertical to avoid convergence.

Sure. This is adjusted by hand, and the errors introduced are
also negligible.

> Is the line if sight in this case considered to be from the scope, rather
> than the index mirror?

That's a potential important issue, but what is REALLY important is only
that your sextant does not MOVE during the experiment.
(Moving the sextant few milimeters on its place (table) destroys
everything. Because of the parallax between the index and horizon mirror).
Maybe the sextant has to be clamped somehow on the table, it is very hard
to be sure it did not mopre a little when you rotate the drum.)

On the other hand, strict perpendicularity of the line
of sight to the screen is not very essential. The main thing about
this experiment (as Frank noticed correctly) is that the angles increase
LINEARLY,
as a function of the distance on the screen (I can elaborate on this
when we meet for a beer next time).

> When you wrote of distance from the monitor, what point on the sextant is
> used as the reference point?

This is not essential, and I measured the distance with a usual
tape measure, from one table to another, essentially
the length of my room. This distance never enters in the calculation
of the sextant errors.

> Does this mean that the
> ends of the line segments do not overlap, so you are butting one line
> segment up to the other when you align them?  Would the target be similar to
> below? (Connect the dots vertically.)

Various patterns can be used. My lines do not overlap.
 But I experimented with slightly overlapping lines, it does not make any
differences. All lines are strictly vertical.
One line (say the upper one) is fixed. And it is important that it
stays on the same place on the screen on ALL pictures you use.
The other line, also vertical is different of different pictures
(as Frank suggests) but in my case one picture contains 10 lines
"below" and I use only 3 pictures.
Because I am lazy to go across the room every time to change the picture
and
my computer does not have a remote control:-(

> linear increase that I would predict if the sextant has no micrometer
> error."  How did you predict the values?

By the deviation of the pattern from the linear pattern.

Here are my results BTW.
a) excentricity 0
b) backlash 0.1'
c) my ability to align two lines on the screen one time mean
quadratic error: 0.2'


Excentricity is obtained by graphing several test results,
space between the observations was about 2.3', 30 observations each.
Backlash was obtained by averaging the results of one run forward
and one run backwards. "My ability" comes from the mean quadratic error
of the difference between two adjacent observations and the average
of this difference 2.3'.

Alex.

   

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