In practice, the smallest meaningful focus-offset is about 20-30 TCS units.
The best telescope focus is dependent on the thickness of the filter, wavelength and type of glass.
The telescope focus offset is calibrated to be +19 TCS units per the thickness of the filter in mm. Luckily the 100mm broad band filters (#104-112, #120) have all almost the same thickness (5 mm), so the expected focus offset between these filters is small.
There is an indication of a slight difference between the focus offset between the CCDs.
|U#104||B#105||V#106||R#107||I_int#108||SDSS g#110||SDSS r #110||SDSS i #111|