18" Truss Dob Conversion

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This is an ongoing chronicle of my efforts to build an 18" dobsonian telescope.

My goal is to have this scope completed in time for my annual pilgrimage to the Shingletown Star Party in Northern California.

March 30, 2004 - The Purchase
I ended up bidding on and winning the auction for this behemeth, an 18" dobsonian with optics by Enterprise Optics. The mirror is 18 inch, f4.5, 1-5/8" thick. It has a 3-1/2 secondary, Novak helical focuser, and Novak secondary.

My plan is to use the components from this scope and convert it into a truss-tube dob that will be more easily transportable.

April 2, 2004 - Truss Tubes
I'm trying to do this conversion as cheaply as possible, yet still make a quality scope. I'll have to wait and see if all of the components in this scope will be useable or will require upgrading.

As part of this plan I decided I would go to the local scrap metal shop to see if I could find thin wall tubing for the trusses. They didn't have any 1" or 1-1/4" tubing, but they had some very nice 2" thin wall tubing in 12' lengths. I ended up buying some for $1.75/lb and in the process ended up changing my design while standing there in the lot of the metal shop. The new design is going to be something similar to this Albert Highe design.

The main difference will be the mirror box. Since I want to use the existing mirror cell instead of a new one, I will probably have to go with a square mirror box instead of Albert's round one.

April 6,2004 - Delivery
I met with the guy selling the scope today and took delivery. It is huge and heavy. Built like a tank. The rocker sides are 1-1/2" thick! Here is the weight for this beast.

Definitely some opportunity for improvement. The mirror physically looks like it is in great shape. No marks of any kind on it. The secondary as well. I'm not impressed with the mirror cell, Novak spider or focuser. I'm going to end up replacing all 3 of these. So I basically got a great deal on some optics. The rest is going to be trashed.

Here are some pictures of the beast.

April 7, 2004 - First (and Last) Light
I disassembled the Novak spider/diagnoal today and replaced the adjustment screws with thumbscrews. That basically makes it possible to now collimate the secondary. Prior to this it was damn near impossible, I had to remove the diagonal to access the adjustment screws, make the adjustment, put the diagonal back and see how close I got. Very frustrating. After replacing the thumbscrews I was able to collimate the secondary in 2 minutes. Much better.

It had been cloudy all afternoon but it finally got dark and I was able to checkout the optics. The seeing was a little soft and the transparency was also quite poor, but even so I was impressed. I mainly looked at eye candy. M81/82, M42, M51, M65/66, Jupiter, and Saturn. Every object looks big and bright. I had intended this just a test to checkout the optics before I disassembled everything, but now I'm thinking I will keep it setup for a few days in case I get a really good night.

April 9, 2004 - Design change
I have changed my mind yet again and have decided to go with a traditional dob design. Mainly this is being done for simplicity and expediency. I want to insure that I have a stable working 18" dob come June and I'm not sure I can get there in that time-frame with a non-traditional design. I can always revisit the 3 truss design later.

April 22, 2004 - Another design change
I got word today that the AL casting for the mirror cell won't be available until May 15th. Since I'm hoping to be completely finished by mid June I've decided that, that is too close for comfort. So I changed the design so that I can use either the existing Novak sling mirror cell or the new low-profile cell using the AL casting. So instead of having a dado in the mirror box to support the board holding the cell, I'm going to make it so that the board can be bolted into the box, which will allow me to change it down the road.

April 24, 2004 - Upper Cage
I have completed all of the parts for the upper cage. All that is left is to glue it all together, which I'm going to hold off on until I've completed the mirror box. I have received the Astrosystems spider, and secondary holder. I also have received the Moonlight single rate focuser. So it is all ready to go.

April 29, 2004 - Mirror Box
I have cut the mirror box sides with the new dimensions to accommodate the Novak mirror cell. I'm using walnut strips in the corners as the "joint" for the mirror box.

May 13, 2004 - Mirror Box Assembled
I finally have completed all of the prep work on the mirror box and have glued it all together. It's much bigger than I anticipated.

May 17, 2004 - Wrong Diagonal Size
I am at the point in the project where I'm ready to assemble everything and check the actual focal length using my collection of eyepieces to make sure I cut the truss poles the correct length. So I installed the mirror cell, focuser and the spider. I then went to install the secondary mirror in it's holder and discovered that I have 4" diagonal mirror instead of the 3.5" that I thought I had. Not sure how I screwed this up but as a result my secondary holder is the wrong size. So this effectively stalls the project until I can get the correct size holder. Bummer.

May 27, 2004 - Rocker Box
I decided I couldn't wait to put everything together so I have constructed the rocker box and I came up with a cool laptop table that attaches to it.

June 6, 2004 - Varnish, Varnish, Varnish
It seems like I have been varnishing for days. Oh wait, I have. It is taking about 4 hours to put a coat of varnish on every piece. I was planning on 4-5 coats but now think I will stop at 3. Still no word or sign of the diagonal holder. I put a request out through the local network for either a holder or a 3.5" mirror. I may end up going with a 3.5" mirror just to keep things moving.

So here is what I have done in the past couple of weeks.

• Glued up the UTA
• Built a ground board
• Made a mirror cover with a front fan built-in (pictures coming soon) so I can rapidly cool the mirror during setup
• Made all sorts of provisions for wiring 12v throughout the box. There is a plug for the battery, a jack going up to the laptop stand to power the laptop. Two jacks for the front and rear fans.
   

June 8, 2004 - One Week to Go
I have one week left before I leave for Shingletown. Unfortunately I'm going to be out of town for 3 of those days so I'm doing the final push. Completed the rocker box and laptop stand today.

Here are pictures of the current WIP.

This is the mirror box in it's current state. I still need to install the truss pole clamps (approx where the blue tape is) and then varnish the inside of the box.

This is with the baffle installed.

This is the mirror cover in the "closed" postion. The round hole in the middle is were a fan will be installed. The piece to the left slides to open up the mirror cover so fan can cool the front of the mirror.

Here is a picture with the slide positioned to allow air flow from the fan.

Here is the completed rocker box with laptop stand.

Another picture of the rocker box and laptop stand.

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Closeup of how the table mounts on the poles.

Closeup of bottom supports for the poles. I'm using 2" thin wall aluminum tubing to support the laptop table.

 June 13, 2004 - Second light?

Since the optics used to be in the beast, it's 2nd light for them but 1st light for the new improved scope. Here's a picture of the scope last night. It was a little too dark out. I'll take better pictures later this week.

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