10" F/8.75 truss-tube Dobsonian

A while back I traded some refiguring work for a couple of 10" mirrors, both of which were figured, but not polished out!  I spent quite a few hours polishing and figuring both an F/5.5 and an F/8.75 mirror, and then when they were finished they proptly were wrapped up, placed in a container and banished to a closet while I finished up other projects (like a 12.5" Cassegrain, a 16.25" Cass., and many other refigures).  This summer I decided I would finish this long-focus dob in time for Mars, and I set a self-imposed deadline of Sept. 7th, the day before the start of Astrofest 2005 (for a report, click here).

Well, starting at the beginning of August I got to work.  I decided to build the major parts of both the F/5.5 and the F/8.75 scopes at the same time, thereby saving time.  I made the mirror box, mirror cell, and diagonal cage for both scopes early in the month.  First light for the F/8.75 was achieved on Sept. 6th when I viewed some brighter stars in the northern sky from my driveway.  Through haze, the star test looked excellent, and the focal plane was found to be at a spot where all my eyepieces could come to focus.  For once I didn't have to trim the truss poles!

Anyway, Astrofest 2005 was good for planetary viewing, and I enjoyed some excellent views of Mars in the early morning hours.  Those views made all the work of the scope worthwhile, and I now have a great appreciation of the images that can be deliverd by such a long-focus Newtonian.  The scope has excellent contrast on deep-sky objects, even with the truss tube construction and no shroud.  The cage is fairly well baffled, and will be even better soon.

Construction

The mirror box for the scopes consisted of 1/2" Baltic Birch (BB) plywood sides, and two 1/4" BB baffles, one near the front of the box and one nearer the mirror.  The parts for both boxes are shown below in the photo at left.  The right photo shows the holes cut for ventilation fans in the top of the box and the dados (slots) cut to receive the baffles.  Biscuit joinery was used to assemble the box sides, and no fasteners were needed, only glue.

box parts box parts 2

The box was assembled, and the mirror cells and diagonal cages were also put together.  The cages and mirror cells were assembled with glue only first, and then reinforced with a single stainless screw into each brace.  The rings of the cage are 1/4" BB, and the braces and focuser board are 1/2" BB.  By the way, the glue, TiteBond II, has water cleanup and is stronger than the wood when properly used.  You don't need to use Gorilla Glue to get high strength.  (If you try to pull a joint apart, you will pull a layer of the plywood off!)

cell assembly glue cage

After adding various other parts, the mirror boxes and cages were stained and painted.  The fans were installed in the box and wired up.  A plastic cover was added to the back of the mirror cell, to block stray light and allow air to only enter from the bottom, encouraging air flow up and around the mirror.  Below is a photo of the front of the mirror box with the mirror cover off, and the back of the mirror with the back cover removed and installed.

box top box backbox back

Here's a large photo of the mirror cell without mirror, showing the ball swivel and the two adjustmend bolts and their associated springs, etc.

Mirror cell

Here is a photo of the front of the box with the cover on.  Note the dowels and latches that locate the truss poles.  The second photo is a view of the name plate that I made for the scope from a transparency printout sandwiched between a piece of white plastic and a piece of plexiglass.  Yes, I made it.

box front

tag and fans

Next, the 6-pole truss assembly was made.  This assembly collapses like a folding camp chair into a compact clump of poles.  When expanded, holes or dowels (depending on which end you look at) locate the wooden "hinges" , to which the poles are attached, on both the rocker and diagonal cage.  Then six stainless, spring-loaded latches are snapped and the scope is assembled.  Here are photos of the poles and closeups of the "hinges".

The hinges are made of mainly 1/2" BB.  First, a long length of "U" channel is made from the BB, and then pieces are cut, with one end of each piece cut at a 30 angle.  When two pieces are glued together, this forms the 60 angle that is required for the poles to fold up.  This angle can be see in the left photo in the second row below.  The dowel pins are also visible, and these slip into holes in the secondary cage.

poles

poles poles

The cage is pictured below.  Two of the attachment points for the trusses and the attached "clips" for the latches are visible at the bottom left and right of the cage.  Focuser is a MoonLite single speed crayford.

cage

Yes, that's a wire spider.  To string the spider, the central hub was bolted to a piece of plywood that was temporarily clampted to the top of the cage.  Below is a photo of the central hub and the jig clamped in place.

hub

jig

With the scope able to be assembled, I put it together and made sure the truss poles were as close to the ideal length as possible, placing the focal plane about 1/4" above the top of the focuser when the focuser is fully retracted.  Then I assembled the scope and balanced it with a 41mm Panoptic in the focuser.  This gave me the highest possible balance point, which was something like 26" above the bottom of the rocker box.  This is excellent for an F/8.75 scope.  The "wings" were then made to attach the bearings to.  The "wings" extend the sides of the 10" tall rocker box up to the balance point, and are removeable for easier transport.  The wings are attached to the mirror box with plastic knobs with stainless bolts inserted and locked in place, forming handles with stainless threaded shanks.  Below is a photo of the two wings with the bearings attached, one turned upside down.  Note the black paint where it is exposed near the optical path.  The next two photos are of the wings installed on the mirror box with three knobs apiece.

wings

box and wings box and wings

With the wings attached, the scope was re-balanced, and the balance point moved downward another 3"!!  The wings were trimmed and the bearings moved down.  Now the rocker box could be constructed, and I wanted it to come apart for easier handling.  (My house is rapidly filling with telescopes, so I need things to break down and be compact!)  The base of the rocker was made from 3/4" BB and four strips of 3/4" BB were glued and screwed in a square on top of it to reinfoce it.  These strips also serve as attachment points for the rocker sides, and the T-nuts that the knobs screw into can be seen in the first photo.  The rocker sides were glued together from ~4" strips of BB, and the curved sides were attached with screws.  With only two knobs, the sides are still fairly stiff.  However, later I will probably add a front brace/battery shelf to stiffen it up a bit.  In any event, without a brace tracking at 450x is no problem.

rocker base rocker side
rocker side 2

Below is a photo of the assembled rocker, with a couple of knobs pictured as well.

rocker box

At last, it was time to assemble the scope.  Here's a photo of it set up in my house.  It is tall enough to require a good step-stool (zenith eyepiece height 7'+), but there is something strangely satisfying about finally having a scope that requires more than just a few inches for me to view through it.  (My 16" F/4 requires about a 6" step when I point it at the zenith when it's sitting on my equatorial platform.)  In the photo the white sheet is covering the 12.5" Cassegrain parts.  Clearly I have too many scopes.

scope

All parts (excluding bearings) are stained with Minwax "Golden Oak" stain.  The bearings are a "dark walnut" stain by another company whose name eludes me.  All wood parts are finished with two or three coats of Minwax fast-drying gloss polyurethane.

Here are a couple of photos (courtesy of Mike Conron) of the scope set up at Astrofest 2005.  The second photo shows the QuickFinder installed on the secondary cage, the yellow flashlight counterweight that is attached when light eyepieces are being used, and the central baffle/vibration dampener.  (The 30" F/3.8 is in the trailer behind the scope in the photo.)

scope

scope

For my thoughts on the scope, see my Astrofest 2005 report.  What I will say here is this - a friend and I compared his 12.5" F/5 Starmaster with excellent Zambuto mirror with my 10" scope while viewing Mars.  Both scopes had equilibrated and were in good collimation.  Comparing the view, both yielded excellent high-power images, but my scope had a bit more contrast and showed a bit more detail on the Martian surface.  Additionally, the edge of the planet seemed a bit sharper to me.  I attribute these differences mainly to the wire spider and smaller obstruction.

This telescope has altered my plans as to what to build next (besides finishing the 10" F/5.5).  While I had planned to try an unobstructed design, I think I might shelve that project in lieu of making a larger moderate focal-length scope.  By moderate I mean F/5.  By bigger, I mean 20" or 22", and still manageable by one person.

Back to Telescopes....