Now that the dome-induced seeing is under control, the largest contribution to the image size in the telescope focal plane seems to be the telescope focus. In 1.0 arc second seeing, with a 1.0 arc second slit, a deterioration of telescope focus by 0.05 mm decreases the efficiency of the telescope by 10-20 per cent; in 0.6 arc second seeing, it would cost 30 per cent.
Focus in the direct TV mode. First adjust the TV gain to give an image which is easy to see but not saturated. Telescope operators should determine focus by taking an average of at least 3 and preferably 5 independent measurements. Take the mean and write down the result in the TO's log. Once set the telescope tracks with temperature. However if the seeing improves it is wise to focus again later in the night.