John's Astronomy - 15" Dob project journal

Aug 3, 2001 - Done!

OK, telescopes are never done. Now I need a new observing chair, I need to put an eyepiece rack on, etc, etc. But it's usable!

This page will document my experiences building a 15" dobsonian telescope using the general design and construction techniques outlined in Dave Kriege and Richard Berry's "The Dobsonian Telescope" book.

September, 2000 - Bought the book. Don't do this if you expect to be able to resist aperture fever. However, if you are going to build a medium-to-large scope, then buy this book. Don't argue, just do it.

October 10, 2000 - Ordered 15" f/4.5 x 1.625" thick mirror from Swayze Optical.

October 22, 2000

Today was the first day of doing actual construction. I found a local source this week for small amounts of steel and plastic, and got everything I need for the cell.

I decided to buy a welder. I've always wanted to learn to weld; I grew up on a farm, but my dad and brothers did all the welding, and I just never learned. Although I'd never seen a wire-feed welder before, all the literature said they were easier to use and very suitable to light projects, which I expect will be mostly what I'll be doing.

I was looking at cheap $200 welders from various places, but I sprung for an extra $100 to go up to a Lincoln Electric Weld-Pak 100 for three reasons; Brand, because I have more confidence that I'll be able to get parts for the Lincoln in 25 years than for the house brands. Also, like other welders in the $300 range, it has more adjustment range, and so can be used on lighter metals (it says down to 26 gauge steel) and it can be outfitted for MIG welding should I ever want to weld stainless or aluminum.

Update - The welder has MORE than paid for itself in the last year since I wrote this. It saves me $100 every time I use it to fix something. Very worthwile investment. (and fun too!)

Here is the raw metal, cut but not yet drilled. The flat plate of 304 stainless will be cut to be the floatation triangles.

This is the welding jig before clamping. Basically it's just a piece of plywood with two pieces of wood screwed to it, carefully squared with a carpenter's framing square. Then there are two pieces of scrap plywood cut to the correct size on the table saw for the between-the-bars distance, and another piece of wood to spread the clamp pressure to on the other side.

Welding jig for mirror cell Here's the jig, assembled with two quickclamps. The holes are not yet drilled, but don't worry, I didn't forget.
One note: I decided that although it's definitely a good idea to drill the holes before welding (as Kriege mentions in the book) because otherwise it'll be difficult or impossible to get the finished product on the drill press, I didn't see any reason to tap the threads into the collimation holes ahead of time, expecially since they might have gotten welding spatter into the threads, so I left this for later.

My kids showing off after the welding is (pretty much) done. My daughter thinks welding is pretty cool, especially after watching Junkyard Wars (Scrapheap to those in the UK) on TV this year.

Collimation bolts For some reason I couldn't find the leveling jacks that Kriege recommends, so I made these out of some threaded and non-threaded rod. See, I'm already finding new uses for that welder I bought "just for this project"!

Completed mirror cell Here's the completed cell. Some notes:
Although it is very hard, I was able to cut the stainless steel with a jigsaw and 24 TPI metal-cutting blades. To do so, you must have a very solid anchor for the material (I C-clamped the material to the cast iron top of my table saw), you must go slow (no more than 5 strokes per second, I'd say) or the blade will immediately overheat and rip the teeth off. Also, you must cool the blade. I used a glass of water and a sponge, generously dripping water down the blade every 30 seconds or so. Still, it took a couple of hours to cut the triangles and grind the corners round.
The circle is actually cut from a flexible kitchen cutting board. It's what I had in the house Sunday evening, and I wanted to finish this.
I still need to get the side pins in. I bought 3/8 bolts and 1" plastic dowel according to the parts list in the book, but later on it says "Oh, for the 15" use 1/4" bolts and 1/2 and 3/4" dowels." I guess I should be more meticulous when I read. At least I read that part before I drilled the holes!

November 2, 2000 - I couldn't find an easy source for the little rosette insert nuts, so I just got some T-nuts and inserted them into some plugs I cut with a wing cutter, and used some epoxy. WARNING if you do this; go light on the epoxy. I wound up with kind of a mess when the epoxy got into the threads.

July 11, 2001 - "The Book" makes a point of having the pivot bolt drilled out for the encoder on a lathe so that you can be sure the hole is centered. However, I had very good luck with this method. Chuck the bolt and clamp the drill bit in a vise, use a square to make sure the drill bit is square, and carefully drill this way. You must go very slowly at first to make sure the drill is centered, but it's easy to tell since if it's not it'll wobble.
I actually drilled several bolts because of some experimenting I did, and they were all very well centered.

May 8, 2001

Well, that was a long break from the project. Actually I did a little undocumented work last fall, cutting out the spacer tubes for the secondary cage, and one of the rings. That's where it stayed until a couple of days ago, when I cleaned up a bit and cut out the other ring.

Last night I glued up some wood (red oak) into the raw material for the lower tube clamps, and I built them today. No problems other than one tearout while routing, but I made two extra in case of problems like that, so I just set that one aside.

BTW, I know that red oak is not the best choice here. It was what I could get where I was at though, and I hand-picked the pieces with the tightest grain I could find.

Here's a photo of the end result of 3 hours work (OK, I'm a little slow. I did have to build a couple of jigs in the process)

Picture of split
block truss tube clamps Yes, there are 4 left and 4 right sides, I didn't forget :-)

July 20,2001

I have not been a good boy about documenting this project. We have a two-week vacation planned in the UP of Michigan, where it's really quite dark, and I decided that I WILL BY GOD finish this scope to take on the trip. As a result, I've been in blitz build mode for the last two weeks, working as fast as possible in the time I have, working until midnight or 1 AM many nights. I have learned a lot, but unfortunately I didn't take a lot of pictures on the way. I will try to take pictures of the finished scope and add narration before I forget all the stuff I screwed up :-)

Balancing scope July 15, 2001 Out on the deck, balancing the scope. I wound up with a pretty deep mirror box, because I only ordered a 1.5" thick primary. I'm kind of bugged that the book doesn't say why they suggest a 2" thick primary until late in the book, when they're talking about balancing the scope, where they say "See, now you know why; that thick mirror makes the base heavy and you can make your mirror box shorter." Gee thanks, too bad I didn't read every sentence in the book 10 months ago before ordering that $1500 mirror.
Anyway, you can see from the bag of flour that I plan on up to 5 pounds of accessories hanging on the cage.

	Here is the raw metal, cut but not yet drilled. The flat plate of 304 stainless will be cut to be the floatation triangles.
	This is the welding jig before clamping. Basically it's just a piece of plywood with two pieces of wood screwed to it, carefully squared with a carpenter's framing square. Then there are two pieces of scrap plywood cut to the correct size on the table saw for the between-the-bars distance, and another piece of wood to spread the clamp pressure to on the other side.
	Here's the jig, assembled with two quickclamps. The holes are not yet drilled, but don't worry, I didn't forget. One note: I decided that although it's definitely a good idea to drill the holes before welding (as Kriege mentions in the book) because otherwise it'll be difficult or impossible to get the finished product on the drill press, I didn't see any reason to tap the threads into the collimation holes ahead of time, expecially since they might have gotten welding spatter into the threads, so I left this for later.
	My kids showing off after the welding is (pretty much) done. My daughter thinks welding is pretty cool, especially after watching Junkyard Wars (Scrapheap to those in the UK) on TV this year.
	For some reason I couldn't find the leveling jacks that Kriege recommends, so I made these out of some threaded and non-threaded rod. See, I'm already finding new uses for that welder I bought "just for this project"!
	Here's the completed cell. Some notes: Although it is very hard, I was able to cut the stainless steel with a jigsaw and 24 TPI metal-cutting blades. To do so, you must have a very solid anchor for the material (I C-clamped the material to the cast iron top of my table saw), you must go slow (no more than 5 strokes per second, I'd say) or the blade will immediately overheat and rip the teeth off. Also, you must cool the blade. I used a glass of water and a sponge, generously dripping water down the blade every 30 seconds or so. Still, it took a couple of hours to cut the triangles and grind the corners round. The circle is actually cut from a flexible kitchen cutting board. It's what I had in the house Sunday evening, and I wanted to finish this. I still need to get the side pins in. I bought 3/8 bolts and 1" plastic dowel according to the parts list in the book, but later on it says "Oh, for the 15" use 1/4" bolts and 1/2 and 3/4" dowels." I guess I should be more meticulous when I read. At least I read that part before I drilled the holes!