Focus locks for Celestron SCT's are designed to minimize frustrating shifts in focus while imaging by positively locking the focuser in position. The effectiveness of the lock is shown in the table below. Improvement in the shifting of the mirror was measured at the edge of the mirror using a dial gage:

Separate versions are available for the C5, C8, C9.5, and C11 Schmidt-Cassegrain scopes.

The focus lock works by replacing the standard focus knob with a new one which can be locked in position after focus has been achieved by screwing down the axial locking screw. This locks the focuser in position and reduces the play normally found in SCT focusers.

Installation is very simple -- just use the provided Allen wrench to loosen the setscrew on the standard focus knob and remove the knob. Make sure the 4 setscrews on the new knob are retracted as well as the focus lock screw knob, slip the new knob onto the focuser shaft, and tighten all setscrews. To lock the focuser after focusing in the normal manner, tighten the focus lock knob until the focus knob does not rotate. Do not use excessive force!

When imaging, use a Borg focuser in conjunction with the SCT focuser lock. At right, the SCT focus lock and Borg focuser are shown mounted on a C11. A Borg 7428 SCT adapter is used to connect the focuser to the standard SCT threads.

The SCT focuser is used for coarse focus, after which it may be locked in place. The Borg helical focuser may then be used for fine focusing. This avoids the image shift problem of the SCT focuser which is often enough to move the target off a CCD imaging chip.