AAO Newsletter October 1996 - Page 8
Figure 5: The number of stars in one field with proper motions greater than 30 mas/y plotted as the fraction of all stars as a function of magnitude.
90 mas/y level (5 year old plates) about 1% of (11 < mR < 16) stars move so there is no problem choosing guide stars; and the 150 mas/y level (3 year old plates) is even better with less than 0.5% of stars moving.
We then compared the results in the 4 different fields (at Galactic latitudes b = -60,37,58,60 degrees) to look for any variation with the position of the field. We calculated the percentage of faint (13 < mR < 16) stars with measurable proper motions ( µ > 30 mas/y): these ranged from 3 - 9% with the lowest value for the field at b = 37°. The rate may be higher at higher Galactic latitudes because local group stars are a higher fraction than in fields towards the Galactic bulge.
In Figure 6 we plot colour-magnitude diagrams of the moving ( µ > 30 mas/y) and normal stars in one field. This shows a clear colour term: fainter than mRapprox 14 the majority of the moving stars are redder than BJ óR = 0.7 whereas the majority of the normal (non-moving) stars are bluer than this limit. Hence a colour criterion of BJ óR < 0.7 will virtually eliminate all high-proper
Figure 6: Colour-magnitude diagram of field F858 showing stars with proper motions greater than 30 mas/y compared to normal stars in the field.