Focussing the spectrograph is a fairly straightforward exercise involving taking short arc exposures, measuring the width of arc lines over the whole chip, and making small adjustments to the focus. The latter is done manually at present by turning a micrometer on the detector stage. Experience shows that even when the dewar is taken off the GIRAFFE table, the new focus position is little changed. At best focus, typical line widths are 2-2.5 pixels FWHM. Usually there is some degradation towards the edges, and for the `RED' position the image quality deteriorates for the far red orders (i.e. near the top of the chip), where the widths are closer to 3.5 pixels FWHM. For the `BLUE' position, it is easier to achieve a uniform focus of 2.5 pixels over the whole CCD.
It's possible to get a focus position which is good for both `BLUE' and `RED', but observers intending to use one position almost all the time will obviously obtain better performance if the focus is optimised for that position.
Eventually the focussing operation will be controlled through the computer, rather than being a manual process. Once focus is achieved, it will remain good for the whole run, provided the detector mount is not bumped.