At Prime Focus the telescope is focused by making a multiple exposure of a star on the CCD detector, stepping both the focus and the telescope pointing between exposures, and only reading out the chip at the end of the operation. Subsequently, the multiple image is examined to establish the best focus (accurate to about 0.05mm).
The exposure is controlled from ADAM by an automatic procedure on the Instrumentation Computer. Select a faint star with no bright star near it on the TV and move it to the appropriate side of the chip. Saturated images should not be used for focusing. The idea is to make exposures of at least 5 seconds duration in order to smooth out seeing fluctuations.
If you are going to use several filters during the night, you must first run the WHEEL command (see INT Prime Focus Manual section 3.2.4) to tell the software the names and thicknesses of the filters in the filter wheel. This is vital if the automatic focus adjustment for different filters is to be used.
To start the focus procedure, type at the ADAM terminal:
FOCUS
The software will prompt for three parameters to set up the focus procedure:
Current focus postion 0.05(n - 1) mm,
so that the current focus is in the middle of the range. The telescope is offset by 20 arcsec between shots: make sure that there is enough space on the chip by looking at a GLANCE run or on the finder display before you select the direction in which the telescope has to move between exposures. It helps if the rotator is at a cardinal point and the CCD's orientation is indicated on the LEXIDATA display. Check the exposure time by using PHOTOM on the results of the GLANCE. At the beginning of the night, try seven exposures (0.3 mm around the nominal value). 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 will be returned to its starting position at the end of the run but the focus remains at its final value. A double step between the first and second image will help you to establishes which image corresponds to which focus position. It will normally be obvious which images are closest to focus and these should be measured using STARFIT option of PHOTOM in order to determine the seeing. If the calculated focus is out of the range covered by the exposure, you will have to extrapolate and repeat the procedure at another focus position.
The focus setting is effectively determined by the temperature (variation), for which a reliable correction procedure is now implemented (see previous section). In principle only one focus determination at the beginning of each prime focus observing run is necessary. You can carry the focus setting from night to night if you adjust for temperature. In practice, one focus run per night is usually done.