I have an Astro-Systems 16-inch Telekit Dob that has served me well for
several years. (Completed the Messier list last year.) However, I have
not been pleased with the focuser, which uses two elastic bands to
transmit motion to the eyepiece holder, and I am trying to replace it
with a two-speed JMI unit. Sorry for this rather extended note. -
Although I have mounted focusers on other scopes, there are some
complications with this one that are somewhat difficult to describe.
(I'm aware of the importance of good perpendicular alignment with the
OTA and the axis of the secondary holder, etc.)

The Astro-Systems focuser is mounted on a rectangular box extending
lengthwise alongside the OTA. In the box a flat bar supporting several
filters is (was) slideably mounted, permitting the user to select a
desired filter without removing the eyepiece, by sliding the bar in and
out. (Actually, I found that the sliding "filter bar" was unhandy and
tended to slide over and obstruct the eyepiece opening, so I removed it
from the box and don't use it)

Because of the particular design of the "filter-box," replacing the
focuser is not entirely straightforward. For one thing, the
Astro-Systems focuser has a flat lower base designed to seat on the
outer flat surface of the "filter-box". The JMI focuser, on the other
hand, has a base that's curved to mate with the curved exterior surface
of most OTAs. Second, the Astro-Systems focuser is secured on the
filter-box on four bolts that extend outwardly from the box, but these
four bolts don't match the holes in the base of the JMI focuser. - So,
I'm thinking that I may have to remove them and drill new holes through
the box to seat four new bolts to hold the JMI focuser. However, the
box is adjacent a stiff, cylindrical piece of bakelite-like material
that forms the outer cylindrical skin of the OTA adjacent the secondary,
and this material is brittle and can crack (and has). Another issue is
that it's not clear to me whether the new focuser should be positioned
on the outer surface of the box, or perhaps seated below the surface on
an insert of some kind for achieving a proper focal length. (I can't see
the inner structure of the box clearly, so it's not obvious whether the
box can or should be removed.)

OK, I think I can stumble through this task and eventually get the JMI
mounted. Before I do, however, I thought that perhaps someone else may
have experienced a similar situation, changed the focuser on the same or
a similar scope, and can provide some advice or suggestions based on his
or her experience. If so, did you remove the filter-box altogether, or
just mount the new focuser on top of the box?

Thanks for any suggestions.

Jim Cate