The following chart is a scatter diagram of astrometry of comet C2002 E2. The astrometry was carried out between 20 March 2002 10:07 UT and 20 March 2002 12:04 UT. The first observed position was at 19 04 37.65 +08 24 11.6. The scatter diagram illustrates the comet's motion in RA and Dec over the period of observation, as well as scatter and systematic effects caused by star involvement, etc. A total of 76 positions are included in the chart.
The chart is meant to illustrate the noise and measurement error in a series of observations of an object with moderately rapid motion, an asymmetrical centroid, and motion through a field crowded with stars.
The effects on centroiding of the involvement of nearby stars can be seen at several places in this chart, since this comet was at approximately 1 degree galactic latitude at the time of observation. In particular, the curl in the chart at about 40.5 arcseconds RA is caused by the close involvement of a fairly bright star (SNR 250 in the image). There is a similar curl at about 41.8 arcseconds RA.
Postfit residuals (i.e., comparing observed positions with positions for the same time, computed from the comet's orbit after incorporating this astrometry) can be illustrative of certain kinds of measurement error. In particular, it can reveal systematic errors in the positions (especially at angles to the lov). This scatter diagram illustrates a different phenomenon.
Click Here to download a large version of the above chart which incorporates a linear fit of the series.
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