NavList:

Frank,

Sorry I led you astray by not including details about how IC measurements were done.  The overlap data had about 1/10 sun diameter side error, which I think is useful in doing ovelap reading though I have no demonstration of that.  Then before doing edge to edge IC measurements I Adjusted mirrors to remove side error as that would affect measurement of sun semidiameter though not IC.  Thus I have no expectation that the two proceedures will the same value of IC.

The purely mathematical argument concerns a comparison of the distribution of measured values.  The raw data in both methods exhibits a standard deviation(n-1) of about .4 moa.  The eye is doing about as good a job judging ovelap as it is in judging edge contact.  Comparing the two methods with 22 measurements per trial, the overlap IC (average) has a Std Deviation +/- .079 moa compared to the edge to edge IC average measurement Std Dev of +/- .124 moa.  Better precision from overlap measurement than edge to edge for the same work.

Greg:

My measured semidiameter is (32.16 + 30.95)/4 =15.78 moa which at first blush is good agreement with 15.77 from Franks online tool (by way of Bill B.)

The sun was quite high shortly afternoon at 45d N.

As you suggest sunspots will only improve overlap measurements. 

Bill B.:

I think the good agreement in my semidiameter measurement indicates it was not a bad hair day.

The formula involving SD on/off relates the standard deviation in the mean values, ie sqrt( sum( sq var)/(n-1))/n , to the SD of the computed IC value.

Dave F