The telescope focus varies with temperature, primarily as a result of thermal expansion in the Serrurier trusses. This dependence has been determined empirically by fitting a simple linear model to the focus position as function of temperature, and the focus on the INT is now temperature compensated. Temperature sensors are positioned on the steel work of the tube; their analogue output is digitised in order to be read by the PE3220. The temperature of the structure is calculated by assuming a linear relation between the ADC output value and the temperature:
Temperature = ATEL + BTEL ADC reading.
The calibration constants ATEL and BTEL should be checked occasionally.
The assumed focus-temperature relation is of the form:
Focus position = FOCT0 + TEMP2F temperature
The constants are TSKCOM variables. For Cassegrain mode, FOCT0=0.5 and TEMP2F=0.07335; for Prime TEMP2F=-0.0854. The coefficients for the two focii have opposite sign because the encoders are mounted in opposite directions. Their values merely reflect the coefficient of expansion of the steel work, and once determined should never again have to be measured.
The system checks that the focus is within a margin FOCERR of its demanded value, provided that the parameter FTRACK is set to either 1 or 2; if FTRACK is set to 0, the temperature compensation is disabled. FOCERR approximately equals the value of the stopping window FOCSTP. Similarly, the maximum allowed change of temperature before the focus is moved is set by a parameter TERR (currently 0.5).