Standard star position errors (Tycho)

From the KeplerE catalog, I took the ecliptic lat/long errors and calculated the average position error (on-the-sky) and the one sigma variation about that average.

Group mean error (arcmin) sigma (arcmin) number of stars

1 1.80 0.90 4

2 1.12 0.34 5

3 2.54 1.72 12

4 2.24 1.18 77

5 2.62 1.54 681

The trend in mean errors should have an increasing slope, as each prior set of standards adds in its errors. Group 2 is better than Group 1 but that might be due to the small number of stars in these two groups.

The first 9 stars are better than the rest, but the remaining 770 stars have an almost constant error. This doesn't look particularly hierarchical. So it remains unclear to me what algorithm Tycho used in forming his catalog.

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