Using our data base an extensive study was made of the variation in telescope focus determined with ALFOSC. The telescope is automatically corrected for variations as a function of temperature and elevation of the telescope, but a small residual effect was found in the dependence of the temperature of the telescope structure which actually showed up as a seasonal change in the focus. This effect was only at the level of $\sim$3% and normally should not effect observations in a single night, but applying this correct removed any trend in the data over the whole observed range from $\sim$2 to $\sim$18 degrees Celsius. No remaining trend was found in any of the other parameter that were looked at. There is a small caveat in applying this correction generally as the effect might actually be internal to ALFOSC where the change in telescope focus is to compensate for this variations, but the correction will work for ALFOSC which is most often mounted, and the correction is sufficiently small to normally not effect observations for other instruments in a single night.

From our results it also appears that the telescope focus is constant when ALFOSC is mounted, i.e., unmounting and remounting the instrument does not seem to affect this value, and in principle the same focus might be used for ALFOSC at all times. There is still a rather large spread in individual focus measurements and it might be that there are some systematic effects we have not identified and we still remeasure the focus on a nightly basis to be sure, but in case of poor seeing or when ALFOSC observations need to be done by inexperienced users, taking the standard value is a good guess under all circumstances.

Another result from our investigations is that the procedure that calculations the focus corrections from the imaging data does not work optimal, which might explain part of the observed spread around the average value. The procedure has been improved to provide a better estimate of the correction. Due to the design of ALFOSC we also expect a variation in the focus across the field of view and we plan to investigate this in more detail.

The variation in telescope focus we have found as a function of filter thickness for filters mounted in FASU which are in the convergent beam of the telescope has been investigated in some more detail and found to also show a small variation as a function of central wavelength which is likely due to variations in the diffraction index. This will be investigated in some more detail to provide a more accurate correction.

Thomas Augusteijn 2009-01-15