| This 4" adapter replaces the standard 2" rear cell adapter base normally supplied with JMI Event Horizon focusers. The standard adapter base is used to attach the focuser to the 2" diameter 18-thread rear cell used on 6" to 16" catadioptric (Schmidt-Cassegrain and Maksutov-Cassegrain) telescopes.
A 2" rear cell thread is an integral part of the rear cell casting of 6" through 9¼" Celestron and Meade Schmidt-Cassegrain and Maksutov-Cassegrain telescopes. This allows the focuser to be threaded directly onto the rear cells of these scopes. However, on 16" telescopes, the rear cell actually has a 4" thread to accommodate the larger central baffle tube diameter of these scopes without vignetting. A step-down adapter ring is used to bring the 4" rear cell threads of the larger scopes down to the standard 2" diameter.
By replacing an Event Horizon’s standard rear cell adapter base with this optional larger diameter base adapter, you can remove the 4" to 2" step-down adapter ring used on 16" scopes, and connect the focuser directly to the rear cell of the larger scopes. This gives a more rigid connection between focuser and scope, as well as reducing somewhat the weight of the telescope payload that must be balanced.
The existing 2" adapter base is removed simply by undoing the three rotation lock knobs or hex-head screws from the adapter base. The base will slide off the focuser without the rotation lock knobs to hold it in place. Put the larger base on the focuser in place of the one you removed, secure it with the three rotation lock knobs, and the job is done.
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