Monitoring telescope focus

 The 60-inch/200-inch telescope focus typically increases by about 3-5/0.05-0.08 units for every degree Fahrenheit of temperature drop. This may be noticeable as a vertical lengthening of the guide star image. However, a more accurate means of detecting focus changes is by monitoring the width of stars in each frame, as the frames are taken. Either FIGARO routines on the Microvax or IRAF routines on the Unix machines may be used to measure stellar profiles. In the event that a focus change is needed, a typical amount to change the focus is ~5/0.1 units at the 60-inch/200-inch telescope. Be sure to adjust the focus in the proper direction to avoid backlash. (Since the temperature normally drops in the course of a night, it is recommended that the focus always be adjusted in an increasing direction.)