What are helical focusers and why does
Borg use them?
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. The graduation
spacings for some Borg focusers are:
- #8167 (large focuser) - 80 microns
- #7857 (M57 focuser) - 100 microns
- #7835 (M focuser) - 80 microns
- #7315 (S focuser) - 50 microns
A complaint sometimes heard about
helical focusers is that they have
insufficient focus travel. In the case
of Borg scope systems, a draw-tube is an
option which allows more than enough
extra travel to accomodate accessories.
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.