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The Baader Sliding T-2 Focuser functions as a variable length T-thread adapter as well as a T-thread to 1.25" eyepiece holder. The adapter consists of two parts. The inner part has a female T-thread on one side and a 1.25" eyepiece holder/barrel with a single thumbscrew on the other. The total length of this part is 29 mm/1.14". The outer part is an adapter that cannot be used by itself, but slides over the inner part.
This combination functions as a variable T-thread spacer with a male T-thread on one side and a female T-thread on the other. The total length of this combined adapter can be varied between 31.5 mm/1.24" and 42.5 mm/1.67". The outer, sliding sleeve clamps down on the inner adapter with another thumbscrew.