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 | |
| Best focus | 23259+-15 | 23248+-10 | 23250+-5 | 23245+-5 | 23240+-7 | 23250+-8 | 23238+-10 | 23238+-8 |