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helical focuser

 

For my 8" travel dobson I needed a very lightweight low-profile focuser for 1 1/4" barrels. Fortunately, I could get a defect 50mm SLR lens with still intact focusing mechanics.

The focuser's height is only about 30 mm, with 12 mm travel. If this travel should not be sufficient, e.g. with eyepieces having extreme back focus, a coarse focusing can be done by sliding the eyepiece in the focuser sleeve.

This focuser has an astonishingly good handling. Focusing is very smooth, still you can sweep through the whole focusing range with a single 270° turn.

Optics and diaphragm of the lens could be removed easily, such that only the focusing mechanics were left.

The sleeve, in which you insert the eyepiece barrel, is made of a 25 mm long brass tube, that was fitted to 1.25" inner diameter on a lathe. The finished focuser was inserted into a small plywood board and a stray light shelter was glued to the back of this board.

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