The Polaris Plus protractor

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The Polaris Plus stylus alignment gauge by Martin Bastin in England was mentioned recently. This is a variation of the Geo-Disc sight-line principle, which depends on you being able to sight the arm pivot axis at a distance. The Polaris Plus is made of cardboard, and it says "standard pattern", whatever is meant by that.
There is one point and grid, and two spindle holes. This way, you can use both the inner and the outer point by switching spindle hole. The sighting arrow is, naturally, located between the spindle holes. The grid offers a line for aligning the cantilever. This line is located ever so slightly to the left of the centre of the circle for the stylus, so that you can see if the cantilever lies above and *along* the line.
The primary alignment is done at the inner null point (unlike the Geo-
Disc, but like the Dennesen Soundtractor). The inner null point is located at slightly less than 64 mm from the spindle, which is closer to the label than where the average LP ends.
After aligning the cart at the inner null point (using hole A), you can, according to the instructions: "Confirm the setting, if you wish, using hole C instead of hole A, but note that the alignment using hole A is both more accurate and far more critical."
Measuring the distance from the spindle with an ordinary ruler, I find that the inner null point is located at a little less than 64 mm (maybe 63.8 or 63.9), while the outer point is located at about 120.5 mm. The distance between the sight line and the stylus point is slightly less than 92 mm (maybe 91.8 or 91.9). I haven't checked properly if the placement of the inner null point and the sight line correspond to some Baerwald formula, but I think a minimum radius of something like 58.3 might be assumed. The inner null point is placed further inwards than where inmate Klaus found that some 230 of his pop/rock LPs ended on the average (66.8 mm).
The problem:
First, if some kind of standard geometry were used, the sight line should, as far as I understand, be placed exactly in the middle between the two null points. Right? Assuming null points of 63.9 and 120.5, the line should be placed at 92.2, and assuming an inner null point at 63.9 and the line at 91.9, the outer null point should be placed at 119.9. Also, the line is not placed exactly in the middle between the two spindle holes. As can be expected on the basis of the above, it's closer to A (inner point) than C (outer point).
Second, I've tried several times to check the alignment by using hole C, and every time the stylus is correctly located left-right, but too far forward. (Have any other users been more successful?)
Third, what could be meant by the statement that the outer null point is less accurate? Is there any deeper meaning behind the inaccuracy?
These matters probably don't matter for the alignment using the inner null point. But the outer point is confusing.

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Topic - The Polaris Plus protractor - Helge Gundersen 04:21:57 01/17/01 (5)

The Polaris Plus/Geodisk protractors (Long as usual) - bkearns 01:31:10 01/18/01 (1)

Re: The Polaris Plus/Geodisk protractors (Long as usual) - Helge Gundersen 13:20:28 01/18/01 (0)

Re: The Polaris Plus protractor - sober1 13:41:06 01/17/01 (2)

Re: The Polaris Plus protractor - Helge Gundersen 00:04:02 01/18/01 (1)

Didn't run w/OL/RB250......... - sober1 13:34:48 01/19/01 (0)

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