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Bill you wrote:
"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."

I think an LCD display would be the most  dependable source. The array of
pixels is created with great  precision.

And:
"I also assume you want the screen perpendicular to  the line of sight on both
the horizontal and vertical to avoid  convergence.  An old photographer
copy-stand trick is placing a small  piece of mirror in the center of the
base, and aligning an single-lens reflex  camera so the lens sees itself in
the mirror.  An assistant could do  that with a monitor [mirror?] in the
center of a
monitor."

Excellent  trick for ensuring perpendicularity. I don't think you need to go
to extremes  here though. It seems to me that even if the display were rotated
five degrees  with respect to the line of sight of the sextant, the angular
distances between  the lines would still be extremely close to a linear
progression, assuming the  sextant is on the order of 25 feet from the display.
Hmmm... but maybe it's the  tilt that you're talking about... I'll have to
experiment more on  this.

And:
"Is the line if sight in this case considered to be from  the scope, rather
than the index mirror?
When you wrote of distance from  the monitor, what point on the sextant is
used as the reference  point?"

The technique is not terribly sensitive to either of these, but  if I had to
pick a reference point, it would be the center of rotation of the  index arm.

And:
"Confused on, "I wrote a very simple  piece of  software that displays two
vertical white lines, one above the other,   which can be separated at
regular pixel intervals (I used 40 pixel  jumps)."  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.)
.
.
.    .
.
."

Yes, exactly. In (x,y) terms where x is  the axis across the display and y
down the display, I'm drawing one line from  (15,0) to (15,300) and then the
other line from (x,301) to (x,600) where x takes  the successive values 20, 60,
100, 140, etc. (starting at 20 is  arbitrary).

And:
"Referring to, "Then I compared two runs of these  measurements with the
linear increase that I would predict if the sextant has  no micrometer
error."  How did you predict the values?"

Sorry I  should have said more on that. You take the highest angle you've
measured,  somewhere around 2 degrees in my case, and the lowest angle you've
measured,  around 0 degrees for me, and you divide that by the number of steps in
between.  Each angular step should be exactly that large if the progression
is linear. So  suppose I took 24 steps to get from 0 to 2 degrees. Then each
step should be 5  arcminutes. So I put my measured angles in one column (of a
spreadsheet or just  on paper) and the sequence 0,5,10,15, etc. (with the actual
step size determined  by the specific extreme angular measurement and the
number of steps) in the next  column. The difference between the values in those
two columns will be a measure  of the micrometer error. If it's just scatter
with a range of 0.1 to maybe 0.2  minutes of arc, the micrometer probably has
no eccentricity error, and that  should be the case with many sextants. A
micrometer with eccentricity will show  some cyclic pattern with larger range.

"BTW, if I can get a handle on the  above, it struck me that instead of using
a monitor I could use graphics/CAD  software and produce target and have
played out at 2400 dpi."

You  could certainly print it, but I find it's much easier to do this with
bright  lines in a darkened room. In bright sunlight, fine black lines on paper
might be  just as good. I don't know, as I haven't tried it.

-FER
42.0N 87.7W,  or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

   

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