SEST - System Performance
Telescope Pointing

SEST pointing model

The basic model used at SEST is the standard alt-az model plus ``tube'' flexure to allow gravitational bending.

where A' and E' are the raw telescope azimuth and elevation, IA and IE the azimuth and elevation zero point offsets, CA collimation error, NPAE the nonperpendicularity of the A and E axes, AN and AW the azimuth axis misalignment North-South and East-West, respectively and TF the ``tube flexure''.

Mid 1996 the azimuth encoder had to be temporalily removed because of work done in the cable winder and after this a systematic, cosine like feature appeared in the azimuth residuals. This could be satisfactorily removed by introducing an extra cosine term into the azimuth correction term.

Pointing runs

The typical SEST pointing run consists of aprrox. 12 hours (usually night time) or 24 hours of observations. Stellar sio v=1 J=2-1 (86 GHz) maser sources are observed at elevations between 25 and 80 degrees. Due to the SEST 50 degree sun avoidance limit the number of day time sources is limitted in the midsummer as most of the sources are within the limit. The aim is to update the pointing model parameters once every two weeks but weather conditions (mainly the wind) may at times extend this time interval to a month.
The long time record of the SEST pointing rms is shown in figure 1. Besides the total pointing error also the errors in azimuth and elevation are plotted. The pointing can be devided in to two periods. Up to 1995 the pointing was done using using a shcotky reciever and there after with a SiS receiver. The better sensitivity of the latter receiver explains the improvement in the SEST pointing performance from 4.5''-6.5'' to 4''-5''. As the same spectrum rms can be reached in one quarter of the time with the SiS receiver as compared to the shcotky the number of suitable pointing sources has increased considerably as fainter sources can also be included. This leads to a much better sky coverage (which is essential in defining a good telescope model).Experience has shown that provided the weather conditions stay constant the same pointing model can be used for a longer time. Changes in the outdoor temperature and cloudines (i.e. the illumination of the dish by the sun) may change the pointing characteristics of the telescope within few hours.Daytime anomalous seeing is not uncommon during the warm and humid summer time. Pointing may wonder around up to 20'' within minutes due to the unstable atmosphere. Therefore pointing critical observations are not recomended daytime in the summer.
Time series of the outside temperature and local humidity (in grams per cubic meter) produced from the SEST pointing runs from 1989 to present are shown in figures 2 and 3. The 24 hour average temperature at La Silla through the years has been approx. 13C with a a dip of few degrees every winter. The seasonal variation in the humidity is more pronounced being between 2 and 3 g/cm-3 in the winter and between 5 to 8 g/cm-3 in the summer. Extreme weather conditions are excluded from the plot as no pointing was made in bad weather.The wind limit enforced at SEST is 18 m/s in gusts and 14 in average. Wind being higher than this the telescope is automatically parked. The pointing accuracy of the telescope degardes however long before these limits are reached. Gusts of 15m/s in an 10m/s average wind are enough to move the telescope up to 10'' around the tracked position thus efectively smearing the beam.