4
Extension Rings
Included with your RC telescope are three thread-on exten-
sion rings (
Figure 3). These extension rings are provided to
allow multiple visual or photographic accessories to reach
focus, depending on their backfocus requirements. They
are designed for installation individually or in combination
between the optical tube and the focuser to take up unneeded
backfocus. Refer to the Specifications at the end of this man-
ual for the native backfocus distance of each Ritchey-Chretien
model.
If the focuser drawtube is fully extended and you are still
unable to achieve focus you will need to install one or more-
extension rings. To do so you must first remove the focuser
from the optical tube by rotating the focuser attachment collar
counterclockwise (refer to
Figure 2). Once you have thread-
ed on the desired number of extension rings onto the male
threads on the telescope tube, re-attach the focuser by align-
ing the silver attachment collar over the exposed extension ring
threads and tighten by carefully turning clockwise (
Figure 4). It
may be useful to experiment with different combinations dur-
ing the day before heading out into the field. Choose a target
over ½ mile away to ensure you are simulating infinity focus.
The goal is to reach focus with as little extension of the fo-
cuser drawtube as possible, to avoid drawtube flexure.
Depending on what equipment you use to observe or image
with, you may need to add one or more of the included exten-
sion rings. The examples listed and pictured in
Figure 5 are
Coarse focus knob
Fine focus knob
Focus attachment collar
1" extension ring
2" extension ring
1.25" adapter
Drawtube lock knob
Drawtube
Drawtube tension
adjustment knob
Thumbscrew locks
2" accessary collar
Linear bearing
Figure 2.
The linear bearing Crayford focuser on the 8" and 10"
RCs provide extra drawtube rigidity to support heavy camera loads.
(8" RC shown)
Figure 4.
Extension rings are installed between the telescope’s
rear cell and the focuser, to take up unneeded backfocus.
Figure 3.
Each RC comes with one 2" and two 1"
extension rings.
of common scenarios. So for instance, for DSLR cameras and
most visual applications you will need 3" of extension (one 1"
ring and one 2" ring) between the OTA and the focuser. Larger
2" eyepieces may require only 2" of extension. Some setups
may require combinations different from those shown. The
backfocus distance of your camera’s sensor as well as the
other components in your imaging train, e.g., focal reducer
or flattener, filter wheel, or off-axis guider, will influence how
much extension you will need to add between the OTA and
the focuser in order to reach focus.
