Borg Helical Focusers.

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) - 75 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.


4317
This is Borg's most economical helical focuser and the only model with a rotating movement.
This model is useful as a supplemental visual focuser to a coarse rack pinion focuser or an
SCT focuser which causes image shifts. Attachment is via a metric M36.4 P1.0 male thread
or M35.8 P0.75 female thread, but with the addition of Borg adapter 7316 it may be inserted
directly into a 1-1/4" eyepiece sleeve. The 7316 adapter also includes threads to hold an
eyepiece filter.

Index Marks - None
Length - 38mm to 52mm (14mm travel)
Weight - 60g
Front interface - M36.4 P1.0 Male, M35.8 P0.75 Female
Rear interface 31.7mm (1 1/4" eyepiece) Sleeve


7315
This is a deluxe version of the 4317. This non-rotating helical focuser includes index marks.
This focuser works well for fine focusing for CCD cameras, digital still cameras and video
cameras as well as for visual use. The front interface is a metric M36.4 P1.0 Male thread, or
a
1-1/4" eyepiece interface with the use of adapter 7316 (includes threads for 31.7mm eyepiece
filters). The rear interface includes a T-thread so that it may be connected directly to SBIG's
CCD cameras.

Index Marks - 50 microns
Length - 50 to 60 mm (10 mm travel)
Weight - 80 g
Front interface - M36.4 P1.0 Male
Rear interface - 31.7mm (1 1/4" eyepiece) Sleeve, M42 P0.75 Male


7757
This is Borg's shortest helical focuser and includes a variety of adapters providing a very wide
range of interface options. Its short length is useful in accomodating accessories with long
focus requirements such as diagonals and binoviewers. This focuser is recommended only for
visual use because the short focuser does not include a locking screw.

Index - None
Length - 22 to 32 mm (10 mm travel)
Weight - 50 g
Front interface - M57 P0.75 Male
Rear interface - M57 P0.75 Female


7835
This focuser is provided as the standard focuser for a Series 80 scope. Borg adapter 7428
allows this focuser to be attached to an SCT.

Index Marks - 80 microns
Length - 63 to 82 mm (19 mm travel)
Weight - 320 g
Front interface - M68.8 P0.75 Male
Rear interface - M60 P0.75 Male, M57 P0.75 Female


7837
This is a heavy-duty version of the 7835 focuser. It has three locking screws and operation is
smoother even when supporting the attachment of heavy accessories such as cameras or
binoviewers.

Index Marks - 80 microns
Length - 63 to 82 mm (19 mm travel)
Weight - 320 g
Front interface - M68.8 P0.75 Male
Rear interface - M60 P0.75 Male, M57 P0.75 Female


7841
This focuser is basically designed to be used with 42mm screw mount lenses and cameras.
But since it is relatively short compared to other focusers, it is also useful with telescopes
when other focusers don't allow for sufficient inward travel.

Index Marks - None
Length - 27mm to 47mm
Weight - 110g
Front interface - M42P1.0 female
Rear interface - M42P1.0 male


7842
This focuser is basically designed to be used with 42mm screw mount lenses and cameras.
But since it is relatively short compared to other focusers, it is also useful with telescopes
when other focusers don't allow for sufficient inward travel.

Index - None
Length - 15mm to 25mm
Weight - 80g
Front interface - M42P1.0 female
Rear interface - M42P1.0 male

7840
Lowest profile M42 focuser with shortest travel (7mm)

Index - None
Length - 11mm to 18mm
Weight - 60g
Front interface - M42P1.0 female
Rear interface - M42P1.0 male

7857
This focuser has a versatile front end interface (M57 P0.75 Male) which allows connection to
a wide variety of Borg accessories including adapters (7901, 7910, 7911 and 7912) for
third-party scopes. This allows connection with Vixen and Takahashi focuser sections. The
internal diameter (47mm) is wide enough to cover 35mm format film.

Index Marks - 100 microns
Length - 49 to 65 mm (16 mm travel)
Weight - 165g
Front interface - M57 P0.75 Male
Rear interface - M57 P0.75 Female, M60 P0.75 Male


8167
This is the largest focuser in the Borg helical focuser family, fully covering up to a 6x9 format
film frame. It is recommended for use with Series 115 and 140 scopes.

Index Marks - 80 microns
Length - 115 to 146 mm (31 mm travel)
Weight - 830 g
Front interface - M95 P1.0 Female
Rear interface - M89 P1.0 Male


Focuser accessories:

7754 - Drawtube with mm graduated scale makes it easy to easily set rough focus position.
7838 - Focuser wheel goes over focuser rings for smoother focus adjustments and easier
gripping when using gloves.

Prices:

4317 - £34.00
7315 - £63.00
7316 - £17.00
7757 - £117.00
7835 - £141.00
7837 - £196.00
7841 - £94.00
7842 - £63.00
7840 - £78.00
7857 - £134.00

A suitable Helical Focuser plus adaptor combination for a Meade or Celestron SCT would be:

SCT Visual Back + 7424 + 7522 + 7315 = £109.00

For Focusers that accept 1.25" Eyepieces use:

7315 + 7316 = £83.00

The Astronomy Shop

1 Tone Hill. Wellington

Somerset TA21 0AU

United Kingdom

Telephone: +44 (0)1823 665510