4 Comparison of scale and position angle with Harper et al. (1997)

The observations made by Harper et al. of the satellites of Saturn were made with exactly the same instrumentation and should show the same scale and orientation. However Harper et al. made their calibration by fitting to the well-known orbits of the brighter satellites whereas the present calibration is based on star positions. The two are compared in Table 5 where the orientation is relative to the true pole of date. The discrepancies between Harper et al. and the present work in Table 5 are slightly greater than expected and this doubtless arises from the calibration procedures.

  

Table 5: Calibration parameters for the CCD observations

The CCD camera was dismounted from the telescope between the 1990 and 1991 series, so a difference in position angle is to be expected. However the same chip was used for all the observations of 1990 and 1991 and the telescope mirrors are of zero-expansion ceramic. Thus the focal length should be the same for both years and also the scale in arcsec/pixel; no significant difference is found here. The present observations also reveal a correlation between position angle and hour angle as found by Jones (1996). The effect is more difficult to measure in these results for 1990 and 1991 because a smaller chip was used than the one on which the discussion of Jones (1996) is based.