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.

Side view of scope Side view #2

Front and side view Top view

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.

Cage view Cage view 2

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.

Switch and latch 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.

Mirror cell from back Bearings and knobs

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.

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