The 18" Truss-Tube Dobsonian Project

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In April of 2002 I decided to take on the project of building an 18 inch Truss Tube Dobsonian Telescope. The project took 18 months to complete. This web page documents the creation of the beast. The telescope is the Dave Kriege Obsession Telescope design and was built to the specifications in his book The Dobsonian Telescope. The Telescope is configured as follows:

  • 18 inch f/4.5 OMI Torus Pro Spec Mirror
    • 2 inches thick
    • focal length 81.96 inches (2080mm)
  • 3.1 inch Secondary
  • 18-point flotation mirror cell, 304 stainless, fan cooled
  • Astrosystems spider and heated secondary holder
  • JMI DX1 2-inch focuser
  • Obsession Telescope's 24 inch cast aluminum side bearings and upper truss tube clamps
  • Astrosystems pivot kit and light shroud
  • Telrad finder
  • Baltic birch construction
  • Maple truss pole anchor blocks
  • Many hours of sweat and blood

The views through this telescope are absolutely amazing. Pointing it is a real pleasure, there's absolutely no backlash or flexure whatsoever. It moves like butter. It's one of the nicest telescopes I've ever viewed through. It's really sweet!!! Set up and tear down are a breeze for a scope of this size. It's easier to transport and set up than my 8 inch LX200.

I believe I am cured for a long time from ever building another telescope. This project was long and at times very hard, but the finished product was well worth all the effort.

Project Timeline

I kept a running online logbook on my original website. Below are all the entries I made along the way.

Below is a timeline chronicle of the building of my 18 inch Dobsonian Telescope. Many pictures are included on this page, so if you have a slow connection, be patient.

I have decided to build an 18" Truss-tube Dobsonian telescope from scratch. This page  chronicles the techniques and progress of this project. I chose an 18" because it's the biggest aperture I can build without the need for a ladder to get to the eyepiece (I'm 5' 9" tall). Another big consideration, is that for the price of a 20" mirror, I can build the entire 18" scope.

April 24th, 2002

Today I received The Dobsonian Telescope book by David Kriege. This book is a must for anyone considering building a Truss-tube telescope. It's amazing how much information is packed into this book. You can order it directly from Dave Kriege at Obsession Telescopes.

April 27th, 2002

Today I started on the design of my mirror cell and tailgate. I am going with an 18-point flotation cell. I laid out the triangles and bars for the mirror cell using the dimensions from Kriege's book. I decided to go with 304 stainless steel for the triangles and bars.

May 1st, 2002

Today I finished the layout and drawings of the tailgate assembly for the 18-point mirror cell. 

May 12th, 2002

Today I started preparing a permanent workspace for the telescope. I am building a workbench and setting up all the power tools in a corner of my garage. I purchased a 10-inch Ryobi Drill press, a 10-inch Ryobi Miter saw and metal cutting blade. This completes all the tools I'll need for this project, as I already had a table saw and a router. I also purchased all the metal parts for the tailgate.

May 18th, 2002

Well, I finally made the biggest step of all. I ordered a Torus Pro-Spec 18" f/4.5 primary mirror and a 3.1" secondary from Obsession Telescopes. They had them in stock, so NO WAITING!!!! I also ordered cast aluminum side bearings and a pair of split bolts.

May 29th, 2002

The primary and secondary mirrors, side bearings, and split bolts arrived today. I unpacked and inspected everything. Dave Kriege and the people at Obsession do a great job of packaging things. He sent it via Priority Mail from the Post Office on Tuesday afternoon and it arrived on Wednesday. Torus provides a nice certificate of optical quality with a print out of the interferometer tests right on the certificate. I can't wait to look at the sky with this mirror. The focal length turned out to be 80.96 inches.

June 1st, 2002

The last couple days I spent fabricating the tailgate assembly. I made a cutting jig for my 10" miter saw that allowed me to cut all three rungs for the tailgate to precisely the same length so that the tailgate will come out square. I cut the 1/4 x 1 inch side rails using the same jig.  I drilled all the holes in the rungs with the drill press and tapped them. When I threaded one of the collimation bolts into it's hole, it had way too much play. It was due to the 1 x 1 inch square steel tubing I purchased having 1/16" walls. There was only one thread per wall (total 2 threads) holding the collimation bolts. This was unacceptable to me, so I ordered some new 1 x 1 square tubing with .120 inch walls which should provide 4 to 5 threads. I ordered the metal stock from MetalsDepot.com. They have all the metal you need for the entire mirror cell and no minimum order.

June 5th, 2002

Today I found a great source for PVC rod for the side pins. I also found some great machined knurled edge collimation bolts made of stainless steel. I ordered them from McMaster- Carr. This site has a ton of great items, and no minimum order.

June 12th, 2002

Today I picked up a 4 x 8 ft sheet of .028" thick Kydex 100 for the secondary cage. I found a distributor locally that had it in stock. The sheet cost $65.

June 16th, 2002

I spent four hours yesterday and five hours today cutting out the triangles for the mirror cell out of 3/16" 304 stainless steel. I used my 10" miter saw with an abrasive metal cutoff blade. One thing I learned this weekend , was that "Stainless steel is one tuff material to cut". I used a grinding wheel to round off the triangles to the .5" radius that I scribed in the metal with a compass. Here is a picture of my first triangle. It came out really nice. 

June 17th, 2002

Tonight I finished grinding off the corners of the rest of my triangles. I am so glad all my metal cutting days on this project are over. Cutting stainless steel was a real pain. Much harder than I thought it would be. All I have left to do is drill and weld the tailgate assembly, then put everything together. I also ordered a JMI NGF DX1 focuser today. I ordered it with a 2.5" tube length.

Here is a picture of the triangles and bars laid out on top of the tailgate metal.

June 24th

I've been taking a break for the last few days from the telescope project. My kids were beginning to wonder who that strange man out in the garage was. But seriously, I decided to take a breather and just relax for a while. 

Oct. 1st

My break from the project has been over for a couple weeks.  The mirror cell is now completed. Shown below are the top and bottom views of the mirror cell prior to putting on the furniture pads and the kydex ring and cooling fan.

I am now working on the secondary cage.. 

June 22, 2003

It's been a while since I updated this page. I quit working on the scope from Thanksgiving 2002 until April because of the cold winter. My garage is detached from the house, not insulated, and it was just too cold out there to really work on the project. I have since been working on it really hard and have made some great progress on it in the last few weeks. As of now the secondary cage is complete, the mirror box is together, and I have cut the rocker box pieces and just glued the rocker side panels together tonight. All I have left to fabricate is the ground board and the dust cover for the mirror box. Below are some pictures of my recent progress.

Secondary cage with AstroSystems spider and secondary holder, prior to installing focuser board and kydex.

Splitblock made from maple.

Mirror box - Testing mirror cell fit.

Another view of the mirror box and shameless self-promotion.

Rocker bottom and sides after being cut with mirror box resting inside to check fit.

June 27, 2003

Yesterday I dowelled and glued the rocker box. Today I cut the ground board and the feet for the ground board. I also cut the dust cover for the mirror box. So as of today, I am done fabricating pieces for the scope. From here on out it's sanding, finishing, final assembly, and tweeking. Woohoo!! Below are a couple more pictures of my progress.

Rocker box all dowelled and glued together.

Ground board and feet.

July 10, 2003

After taking a break over the July 4th holiday, I got back to work. Yesterday I put 5 coats of urethane on the ground board. Today I attached the virgin teflon pads and attached the feet. The ground board is completed. It is shown below with the AstroSystems pivot kit installed. AstroSystems makes a very nice pivot kit for Dobsonian telescopes. I highly recommend this pivot if you plan to put digital setting circles on your scope, as it is bored and has a set screw for the encoder. The teflon pads are also from AstroSystems.

Finished ground board with AstroSystems pivot in center hole.

Finished rocker box mounted to ground board with teflon bearings mounted.
Threaded brass inserts installed in the sides for the wheel-barrel handles.
One more piece of the puzzle completed!!!

Oct. 15, 2003

Today I finally finished the mirror box urethane and painted the inside of the mirror box flat black. It came out really nice! It's starting to look like a telescope.

Side view or mirror box. All finished! I really love the grain patterns on this
wood!

View of inside of mirror box. Tomorrow I bolt down the mirror cell, and mount
the side bearings.

Oct. 16, 2003

Today I mounted the side bearings and mirror cell to the mirror box. So of
course I had to mount the finished mirror box in the rocker box. 

While I was at it, I decided to mount the wheels and handles to the rocker and
take it out for a spin. It did zero to sixty in 5.2 seconds :)

Nov. 9, 2003

I put the finishing touches on the secondary cage today. Not too much left to do on this project.

Nov. 14, 2003

Today I mounted the pole seats on the secondary cage, drilled all the holes for the truss pole anchor blocks on the mirror box, mounted the anchor blocks to the mirror box, installed the handles on the mirror box, and installed the magnetic latches for the mirror box dust cover. All that is left to do now is a focal length test and then cut the tubes. FIRST LIGHT IS VERY NEAR!!!! I will be installing the mirrors either tomorrow or the next day, then performing the focal length test to determine the length of the truss poles. I am shooting for a Nov. 19th first light. If the sky clears up before then, it could be sooner.

Mirror box all complete and ready for the mirror!!

Nov. 19, 2003

My unofficial first light occurred tonight!! I performed a focal length test to determine the length of the truss poles. I cut the poles and mounted the secondary cage. Just a couple little details left to finish it.

Nov. 20, 2003

Today I trimmed the truss poles down, cut the latch bolts for the upper truss clamps, and fine tuned everything. It is ready to collimate and use. Of course, I will be in a tweaking mode for a little while to get the truss poles down to their final length and getting the balance right. I couldn't believe how smooth this scope moves. It glides real smoothly and sticks right where you put it. Also the balance came out great. It is slightly tail heavy with the Telrad and my heaviest eyepiece in the focuser, but not enough to make the scope kick up. The weather forecast for tonight is clear until about 2am. So first light will be tonight!!!!!!!! I will post a full observation report tomorrow.

Aperture Fever Cured!!!!

 

Update: November 20th, 2003 - First light occurred at 9:33pm CST!!!!!!!!!!!!

First light went quite well. The first object I pointed the telescope to was M31 - The Andromeda Galaxy. I had never seen it like this before I also saw M110 while I was there. I then looked at M42 - The Orion Nebula. All I can say is WOW!!! I was able to see the two 11th magnitude stars in the trapezium. The cloud structure took on a almost 3D appearance through the eyepiece. My 2 inch 32mm eyepiece wouldn't quite make it to perfect focus, so I will have to trim the truss poles down to their final length.

On Dec. 11th I finally got a chance to take the scope out with the truss poles trimmed to their final length. I spent a little more time getting  the scope collimated. Around 11pm I pointed the scope at Saturn and was completely blown away at the view!! The sky was incredibly steady. I was able to go up to 340X with my 15mm eyepiece and 2.5x Teleview Powermate barlow. WOW!! It was the most incredible view of Saturn I have ever seen. The globe of Saturn was a nice yellowish color, which I never saw in my 8 inch Meade. I could see many  of the bands that are on the globe of Saturn. Truly amazing!!