JCMT Pointing - general

Pointing with JCMT involves the following process :

• The astronomical coordinates are made known to the telescope computer;
• These are converted from their natural reference frame into azimuth and elevation, taking into account the observer's latitude and longitude, the time, refraction , aberration, precession, and other astronomical corrections. For a useful description of all these effects and other terms of astrometry, see the document Explanation and Examples - which is part of Pat Wallace's (Starlink's) Positional Astronomy Library.
• the current azimuth and elevation are read by on-axis encoders , taking into account their zero-point differences - i.e. the readings the encoders give when the antenna is at (az,el) = (0,0), which, by convention, is on the horizon in the north;
• the telescope drives servo is commanded so as to close the gap between the demanded and current positions.
• The calculation is repeated every second by the TEL task, but the servo cycles every 50 milliseconds, using appropriate interpolations on the TEL analysis.

Additionally :

• the elevation is corrected for the temperature difference between the front and back legs . This correction is of the order 6"/^oC of difference. The temperature of the dish suffers little in the way of gradients and so does not impact telescope pointing.

• The misalignment of the tertiary mirror or flexure in the cassegrain cabin are possible additional terms that could be considered. These are coded as terms called u3 & u4, and have definitions that are dependent upon the focus station. As yet, keeping their values set to zero has not limited the performance of the model.

• After establishing a pointing model with one receiver/instrument/FrontEnd it will be found that other FEs do not point in the same direction. These pointing offsets are called collimation offsets. It makes life easy to maintain the model using one FE (the primary-FE) and to hold its collimation offsets to (0,0) (in azimuth and elevation).

The values of the (usually 7) parameters are determined by analyzing pointing data. If the model is correct, any source should arrive precisely on-axis when the antenna is pointed at it. In general, this is not the case, and errors in any parameter produce systematic errors in the pointing residuals. When these are analysed as functions of azimuth and elevation (currently by a least-squares method), a more optimal value-set of the parameters may be determined.

A good model obviously requires good coverage of (az,el) space. Previous to the arrival of SCUBA in the summer of 1996, it proved difficult to obtain sufficient data in any dedicated pointing run to do this. Resulting model updates were probably often biased by the selection of pointing sources in that run. Since then, however, the pointing model has remained reassuringly stable, and has allowed us to identify and quantify other factors that affect the pointing, such as central bearing problems, antenna loading problems , etc..

Of course, there are other complications, such as

• the uneven nature of the track, and the need to measure the track profile using Inclinometry.
• seeing - as measured by the 11GHz CfA phase monitor, maybe like the one in use at OVRO - reflects fluctuations in the atmospheric refractive index due to moisture passing through the beam. The pointing model ought to be derived in, and applied during, the absence of such effects.

• It is also assumed that the instrument/detector used in the pointing experiment is aligned properly - see how.

• A small correction is made for the friction and hysteresis in the elevation encoder housing -- a phenomenon misleadingly called the transit effect, although it occurs whenever the elevation motion is reversed. For a full description of the effect and the efforts to correct it please go here, (item 4).

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