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Pointing: central bearing problem

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During February 1997 a severe pointing problem was noticed with the JCMT. The problem manifested itself as large (~10") variations in the azimuth pointing which could not be traced to any of the usual culprits, and without any accompanying errors in elevation. Preliminary investigations revealed that the problem appeared to be occuring at one or two fixed azimuths.

Once the systematic nature of the fault had been realised, we decided to characterise the problem by performing continuous fivepoints on Mars as it transitted from the Eastern to the Western horizon. The top line in the figure shows the measured azimuth offset as a a function of azimuth. The problem is evident as 5" sinusoidal glitches over a ~3 degree interval every 16.4 degrees. The period is very precise and corresponds to 22 excursions per 360 degrees. After some detective work it was realized that this period points to the central bearing which is a thrust ball bearing with 44 balls. Since the top race of the bearing rotates with the telescope, while the bottom race is fixed, the balls move at half the speed of the telescope. A full telescope revolution moves 22 balls past a fixed azimuth, corresponding exactly to the period of the observed azimuth glitches.

The sign of the glitches point to a depression in the bottom race, but the effect is complicated. The reason is that the azimuth encoder is effectively fixed to the top race of the bearing, but the azimuth motors drive rollers which connect to the central-bearing pillar via a fairly long X frame. The mounting of the dish comes down near the wheels and the elevation axis closely follows their motion.

Our working hypothesis is that, upon encountering a "hole", a bearing ball "falls in" and the azimuth axis tries to speed up. Thus the az drives must apply backward torque to maintain proper azimuth and velocity. This causes a minute bend of the X frame which connects the wheels and elevation axis with the central bearing. The bend will yaw the elevation axis behind the source which means that a fivepoint will show a positive pointing correction. On the other side of the hole, more forward torque by the az drive is required for the ball to climb out, causing the elevation axis to yaw ahead of the source, requiring a negative pointing correction. First a positive, then a negative correction is what we should see, and this is what we do see (figure). The amplitude of the glitch is constant in encoder units, but scales with cos(elevation) on the sky. This is because the observed pointing offsets are measured in arcseconds on the sky rather than arcseconds at the encoder.

The full explanation of the effect was provided by Firmin Oliveira who has carried out a detailed analysis of the situation.

The shape of the glitch is very well modeled with a double gaussian and empirical pointing corrections have been derived. The model is shown as the solid line in the figure, the residuals as the lower series of dots. Note that during the course of this work, we checked the loading on the central bearing but had not made the small corrections required to the seven-parameter pointing model when the Mars data were taken - this explains some of the large scale variation seen in the raw data and fitted by the short-dashed line. The empirical corrections will remain a standard component of the pointing model until such time that the central bearing can be fixed.

We will be implementing a standard program of monitoring the pointing performance at these problem azimuths, to ensure that the magnitude and zero-point azimuth of the empirical correction remains valid.

However, observers should be aware of these problems and be on-guard for larger than expected azimuth pointing errors. If these occur, the zeropoint of the empirical model will need to be adjusted.

The engineering staff are currently reviewing possible options for fixing the central bearing. A spare bearing is available, but the current races have been heat-shrunk into the mount and were not designed to be removed. Since replacing the central bearing will thus be a delicate procedure involving down-time of the telescope, such project needs careful planning to ensure a minimal impact upon operations.

A more complete analysis of the problem can be found on the next page.

Please address any comments, suggestions or requests to:

Remo Tilanus