We offer Newtonian secondary knobs with several different screw lengths to fit a variety of telescopes. It's easy to determine the correct screw length for your application. Measure the distance A between the top surface of the secondary holder 1 and the top of the secondary mirror mounting plate 2 at the end of the factory collimation screw. Use the table below to determine the correct screw length for your Bob's Knobs.
Bob's KnobsDistance A
Screw LengthLess than 22 mm
25 mmBetween 22 and 27 mm
30 mmBetween 27 and 32 mm
35 mmBetween 32 and 37 mm
40 mmBetween 37 and 42 mm
45 mm