10" F/5.5 Truss-Tube Dobsonian
This telescope is the brother of my 10" F/8.8 - the two telescopes have
exactly
the same mirror box, mirror cell, and secondary cage. Instead of
just
building one box and cage, I made two when I built the F/8.8, and just
set
the extras aside until I had time to finish the F/5.5, which was late
in
2005. Here are some images of the whole telescope.
Features that differentiate it from the F/8.8 include larger side
bearings,
a lower rocker box, shorter, black-anodized truss poles, and an
obviously
different color of wood stain. The truss poles attach in the same
manner,
with six spring-loaded draw catches. This scope has a
FeatherTouch
focuser with 1.5" of travel, and a 2.14" secondary mirror, both
different
from the F/8.8. It has a wire spider.
The cooling system is the same too, two 75mm fans mounted in the top of
the
mirror box draw in cool air around the mirror. The controls are
slightly
different - a double-pole-double-throw (DPDT) switch with a center off
position
controls whether the fans are in series or parallel, which provides
each
fan with either 6V or 12V, corresponding to slow and fast speeds,
respectively.
The center off position is very convenient - I only had to wire
this
one switch to get two fans speeds and an off switch. It works
well.
Here's a photo of the switch panel and one of the bottom blocks
and
latch, and a view down the tube.
Finally, here's a photo of the mirror cell and a closeup of the side
bearings.
They are 1/2" thick baltic birch, with plastic pegs on the side
of
the mirror box to help locate them properly in the dark so the knobs
can
be threaded in and tightened. They are removeable for transport.
I am looking forward to getting a chance to use this scope more.
It's light
weight, and the shorter truss poles make it easy to manage in transport
and
assembly. I've been busy with other scopes and many other things
lately,
but I am eager to see how the performance compares to my longer focus
10"
F/8.8, which I already know to be a winner. I don't expect this
scope
to disappoint.
UPDATE: Well, this telescope is a winner. On the evening of
April 12th, I set it up in my driveway on a pleasantly warm early
evening. After collimating with a laser, I popped in a low power
eyepiece and found the beehive cluster, and then Saturn. Starting
at 50X, I kept increasing the magnification until I finally got to my
5mm eyepiece, 275X. Saturn was tack sharp, showing lots of very
crisp ring detail. It was so good I went for the 1.8X barlow to
get to 500X - and I was not disappointed. With the jury-rigged
ground board there was a little vibration, but after that settled down
and I got it focused, the Cassini division was crisp, and hints of the
Encke minima were evident in moments of good seeing. Clearly the
mirror was as good as it tested, and the scope moved well and was easy
to set up and tear down. And all this without using the
ventilation fans! When I get some place darker I will have to put
in a 41mm eyepiece and enjoy some nice wide-field views with less coma
than the F/4-4.5 scopes I am used to using. I think I will enjoy
that very much.