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Telescope focus

The CCD is focussed by making a multiple exposure, stepping both the focus and the telescope between exposures and only reading out the chip at the end of the operation. The multiple images are examined to establish the best focus (accurate to +/ 0.05 mm or so). The exposure is controlled by an automatic focus procedure on the instrumentation computer. Make sure that there are no very bright stars in the field of the chip as these saturate and so are no good for focusing: the plan is to make exposures of > 5 seconds in order to smooth out seeing fluctuations. To start the procedure type:

Adam:> FOCUS

and follow the prompts. The parameters are the number of exposures, the exposure time in seconds and the direction of motion of the image on the chip (N,S,E or W). The focus is stepped by +/0.1 mm between exposures, depending on the sign of the first parameter and starting at

Current focus +/ 0.1(n1) mm,

so that the current focus is in the middle of the range. The telescope is offset (20 arcsec is the default) between shots: make sure that there is enough room by looking at a GLANCE run or on the finder display before you select the direction. Check the exposure time by using PHOTOM on the result of a glance. At the beginning of the night, try seven exposures (+/0.1 mm around the nominal value). (The default increment in focus is 0.1 mm but this can be changed useing the FOCINC command (see II.3.11).An approximate value for focus at a given temperature can be obtained from Figure 8. Use the selector switch on top of the desk to determine the current inside dome temperature. The approximate focus can be read off from the graph. It is intended to introduce automatic focus compensation for temperature in the near future. Fine-tuning during the night can be done with 5 exposures: differences in focus of 0.1 mm are easily distinguishable under good conditions. The telescope and focus will be returned to their starting position at the end of the run. A double step between the first and second image establishes which image corresponds to which focus position. It will normally be obvious which images are closest to focus and there should be measured using the PROFILE option of PHOTOM and the seeing noted from the focused image.