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Dear list members,
I already addressed this problem in the past,
but I am still at a complete loss.
Maybe someone can help.

I have a misterious systematic error in my SNO-T.
The essence is this: no mater how carefully I determine
the Index error, I get systematic substantial overshots
more or less independent of the angle.

Here is a typical result, of yesterday. I can send or post
complete data.
IC was determined by 14 measurements, before and after
observations, using various stars. It is DEFINITELY between
-0.2' and -0.5', the average is -0'.3 and sigma is 0.15.

All checks (perpendicularity, of both mirrors, collimation etc.
were made as carefully as possible before).

Then I measured star distances, under ideal conditions,
using 9-10 pairs of stars, approximately at distances
10d, 20d, .... ro 100d.
Each distance was measured 3-5 times, as carefully as possible.
THEY ARE ALL OVERSHOTS by almost 1 minute! (No index correction made).
More precisely, the average error is -0.8, with sigma only 0.22.
On the previous day there were some clouds and conditions were
not perfect. The average error in the same experiment was -0.9
and sigma 0.3.

What can this be???
I tried Bill's Astra (with my scope) and the star distances were OK.
Bill tried my SNO with his scope and was -1' off.

The only explanation I can imagine is that the arc is deformed near 0.
(Worn after so many index checks, as Bill joked:-)
Inspection of the arc and teeth with powerful magnifying glass
shows no defect.

Alex

   

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