NavList:

Quote Richard:
As for the parallax worry, a metallic pointer can be bent closer to the
scales if deemed necessary. The eye can keep the cursor centered in line
with the axis quite well without aid; but a small bit of silver colored
mylar at the base of the cursor will reflect the eye pupil well enough to
allow very repeatable observation positioning.
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The 'problem' is not parallax as the eye can look perpendicularly onto the scales on the tubes quite accurately; it is the difference in scale lengths with different diameters of tube which I would consider a potential problem.

Theoretically they should be exactly the same scale factors and hence same length wrapped around helically.

Different diameters necessarily means there will be slightly different scale lengths if printed with the same scale factor. I have not worked out the errors introduced with a real Bygrave SR, (I'm not sure of the exact diameters involved and the scale lengths themselves) but assume it cannot be too great or it would be noticeable and would have been reported by now.

Douglas Denny.
Chichester. England.
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