The Borg Helical Focuser M is 'standard issue' for the Borg 71FL and 77ED series.
In addition to being a standard component on Borg scopes, a common use for a Borg helical focuser is to add it on to an SCT or any other scope with a coarse focuser such as a rack-and-pinion focuser. Especially in the case of SCT's this allows for fine focus positioning without the annoying image shift which is often large enough to move a target off a small imaging or guiding CCD chip.
Helical focusers work by turning a ring to focus as we do when using a conventional camera (at least in manual focus mode). A true helical focuser works like a camera - when you turn the focusing ring, neither the lens nor the camera (or eyepiece and diagonal) rotate. The only motion is in and out. Some manufacturers implement a "pseudo-helical" focuser which results in a rotating camera or eyepiece, but Borg focusers are true helical focusers.
The advantage of helical focusers is the fine adjustment possible, in part due to the fine pitch of the focusing ring's thread, and in part due to the large diameter of the ring. In the case of the Borg helical focusers, virtually backlash-free movement and graduated markings on the focusers allow repeatable return to the focus point simply by noting the focus position reading.
For use with an SCT telescope: Please select the Borg SCT to Helical Focuser Adapter 7428. Specification
2-inch eyepiece holders: Please select the Borg 2" Eyepiece Holder SII 7504 for visual use or the Borg 2" Eyepiece Holder S 7508 for a mix of visual and imaging (the 7508 accepts the Borg M57 to T Adapter 7522 which enables you to attach T-threaded cameras and accessories).
Index : 80 microns
Length : 63 to 82 mm (19 mm travel)
Weight : 320 g
Outer Diameter : 76mm
Inner Diameter : 56mm
Front interface : M68.8 P0.75 Male
Rear interface : M60 P0.75 Male / M57 P0.75 Female