The last tracking exercise revealed clearly the signature of a 'race defect' in the central bearing. The amplitude of the effect was as much as +2" at elevation 58 (the first of the 4 features). Correcting the amplitude of each of the 4 observed features to the amplitude at the horizon suggests a value of 3.3" + 0.4"; considerably smaller than the amplitudes seen in 1997-1999. Since the amplitude of the defect is larger at lower elevations this latest exercise involved tracking Uranus through (az,el) = (125,42) so that a better estimate of the amplitude (at the horizon) might be made.

The data were taken using RxB and the fives method, with the transit step correction left enabled:

Formal rms residuals through the 2 hours of the experiment are (0.8", 1.1") in (daz,del), respectively.

The defect is revealed as expected at (az,el) = (125.4,43). (The azimuth is quoted after correcting for the delay in logging to the mid-point of each fivepoint, and may be accurate to 0.1 or 0.2 degrees). The observed amplitude of the defect is +2.9", giving an amplitude at the horizon of +4.0". A second defect feature is seen at (az,el) = (142.0,53); the observed amplitude is +1.8", suggesting an amplitude at the horizon of +3.0".

The plot below shows the data together with a crudely scaled and adjusted version of the April 1999 empirical correction (tailored to the az=125 event only):

Conclusion
The amplitude (at the horizon) of the current defect is no more than +4". This is less than half the amplitude measured last time although it has the same general 'P Cygni' (double-gaussian) profile and is of approximately the same width.

This will have have (had) limited impact upon observing. At 40 degrees elevation the excursion in pointing is less than 3", although this must be weighed by the time taken to cross the couple of degrees of maximum excursion in each direction.

We should be able to resurrect the empirical correction formulae as an interim measure while a mechanical solution (repair/refurbishment) is contemplated and scheduled. It must be anticipated however that the defect will evolve in shape and amplitude and so will need regular monitoring.