HOW TO DETERMINE THE CORRECT SECONDARY SCREW LENGTH

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 Knobs
Distance A
                 Screw Length

Less than 22 mm                25 mm
Between 22 and 27 mm         30 mm
More than 27 mm               35 mm

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