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    SNO-M arc accuracy -- errors from one degree to the next?
    From: Richard Reed
    Date: 2010 May 4, 12:39 -0700

    I just tried Frank Reed's 2005 'test pattern' method of measuring drum eccentricity on my SNO-M as here:

    http://www.fer3.com/arc/m2.aspx?i=026884&y=200512

    Apart from being a little disappointed to see 0.4-minute errors in both directions -- a sine-like pattern with 0.8 minutes peak-to-peak -- there also seemed to be substantial errors from one degree to the next. I had assumed that arc/scale errors would have a longer period of variation.

    Frank suggested a pattern with two degrees divided by 24 steps, so this gives two whole turns of the micrometer. I would have expected to see fairly close agreement between the errors of both turns, but it looks like there are extra errors adding and subtracting over this small interval.

    For example, at micrometer setting 52' in the first degree/turn the data curve was about 0.15' below the trend line, but at 52' in the second degree, it was 0.5' below the trend line. This was the worst variation between the turns, but in the 'up' direction the disagreement went from 0.1' up to 0.5' and back over about 20'.

    There also seemed to be about 0.5' backlash, which wasn't constant either, so I'm thinking the machining on the SNO-M arc is a little ragged around 0 degrees, and maybe elsewhere.

    Richard

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