Mark Bracewell (MrLunch) July 2005
About a year ago, when I had just been getting into astronomy for a few months, I went to a hilltop nearby where I met a gentleman with a telescope. I hadn't seen anything quite like it before, it was very elegant and clean, and most interesting to me, it looked like something I could build myself.
The gentleman turned out to be Albert Highe, and the telescope many of you will have seen described in Albert's June 2005 S&T article "A Lightweight Composite Dob."
Over the next nine months or so, I became more familiar with Albert's designs. Some of the folks in my observing group had telescopes Albert had built, and a few more people had Plettstone scopes based on Albert's designs. The simplicity of the designs really appealed to me, and one night I got to sneak a peek at Saturn through a 15" Plettstone that just about stopped me breathing. Fever struck, 103 degrees F, I started reading everything I could find on the web about telescope making, especially Albert's own web pages.
Once upon a time I was a professional cabinetmaker, so I felt confident I could do the wood and metalwork required. I have limited tools nowadays, but it didn't seem like I would need anything very special to build something good. Well, nothing special but some cash. This would have to be on a budget, so planning carefully was a must.
If you study Albert's web pages, you'll notice that he's always got his goals listed for the scope he is building, and when I first asked him about making my own, he challenged me right off the bat by saying I must write down my goals and be specific about what I wanted to make.
Good advice. With clearly stated goals, you can make informed decisions about your design. In my case it helped me avoid doing things that looked neat but made no sense, but it also presented me with a few challenges since I had settled on some goals that were conflicting and took a little thought to reconcile.
These were my original goals:In order to design the scope in detail, I had to decide on the main components. The first order of business was parts I couldn't make myself. After purchasing these items I realized I could probably have tried to make the spider myself (see Bruce Sayre's 20" bino scope spiders) but no worries. Next time.... I guess I also could have tried to make the mirror, but well, my excuse is I had only 13 weeks :)
Here are the store bought components:
That adds up to $874.75 - yikes!
Well, I think it's good stuff. But what do I know? I've never built a scope before. I read everything I could and asked a lot of dumb questions, so I think this stuff fit my target of very high quality but not exorbitantly expensive.
I didn't want to follow somebody else's instructions because I wanted to try to understand why people built things the way they did. I found that everything you need you can get on the internet; advice, software, materials and most of all, people. Here on Cloudy Nights there is a huge collection of knowledge, and people very willing to share it. Did I mention I'm a tightwad? I didn't want to spend money on books which might have a fixed point of view. I found I could get a huge amount of information just by carefully studying photographs of telescopes I admired on the web. Unfortunately, this spring was not good weather-wise, so I wasn't able to get out and see many scopes in person.
To help design the scope I found and used Newt - the venerable Newtonian design program, PLOP - for cell design, CadStd - a simple and free cad drafting program, and diagonal size calculators from Woden Optics and Mel Bartels
I decided to use the asymmetrical, parallel truss design Albert used for his 13" travel scope.
The advantages to this design are many...
Of course it's not the perfect design for all situations. In a very cold or dew-prone location a shroud might be needed. Lots of stray light might call for a larger baffle. A 10" scope isn't so heavy that equatorial mounting is ruled out, so a tube or highly rigid truss frame might be the ticket, especially if photography is part of the plan. It gets back to goals again, I found I was constantly revisiting these, and this design seemed to meet mine well.
The main challenge I had was to accommodate my goal of handling a broad range of eyepiece weights. With the scope being so light, just increasing the friction in altitude motion would not work well, it would be too stiff to move easily, so I planned to use a 12 oz. counterweight and a fairly heavy 2" to 1-1/4" eyepiece adapter. With the counterweight and adapter removed, I'd be able to add an extra pound of eyepiece weight and still have the same center of gravity.
For the rocker box, I was reminded by the Composite Dob's lightweight material of something I'd seen built in my cabinet making days called a torsion box. This is a light framework with thin plywood skins glued to it to make a very rigid but light panel. Where the rocker box needed thick material to support the altitude bearings, I decided to make small torsion box panels to save weight.
Once I had the mirror, focuser and secondary sizes, I used Newt to figure out the dimensions of the overall scope, and from that I could make some rough drawings with the CAD software to get the dimensions of parts.
Newt gave me the following dimensions (in inches). I got pretty picky about being exact at this point. I specified a 9.8" mirror for example because the mirror has a small bevel on the edge. The HC-2 focuser has only 1.25" of travel, which is enough to handle almost all of the Televue eyepieces (more wishful thinking), but only if it is placed just right.
| Newt Dimensions | |
|---|---|
| Primary Mirror Diameter: | 9.8 |
| Focal Length: | 54.684 |
| Focal Ratio: | 5.58 |
| Tube Inside Diameter: | 11.5 |
| Focuser Height: | 1.5625 |
| Diagonal Minor Axis: | 1.83 |
| Diagonal Offset: | 0.0819 |
| 100% Illumination Diameter: | 0.5015 |
| 75% Illumination Diameter: | 1.2037 |
| Front Aperture Diameter: | 10.864 |
| Mirror Face to (center of) Focuser Hole: | 46.871 |
With this I made a really terrible drawing, but it was enough to get the dimensions and to illustrate any unforseen problems. It was a radical departure for me, I usually just scribble on a board with a pencil.
| Terrible Drawing |
|---|
|
The tools I needed to make the scope were...
No Table Saw - well, I have a broken one, but it's under 100 tons of stuff in the garage. For cutting the plywood parts I used a jig saw, and finished up with a router and straight edge. That really made me appreciate the small number of parts required.
The mirror box is just 4 pieces of plywood and some Formica rolled into a tube and glued with epoxy. The tricky part here is that the inside surfaces need to be finished before it all gets glued together. Once it was glued up (with Gorilla Glue) I masked off the Formica with paper and finished the outside. 5/8" dowels are glued into holes in the bottom and will have rubber feet on them. They came in handy while spraying finish.
The whole box was sprayed with exterior water based semi-gloss (Varathane) for moisture protection. I used a couple of cans of it, and it's a bit pricey, so for the rocker box and other parts I used the brush on kind. The top I scuffed up with 80 grit sandpaper to remove any sheen, and then sprayed with several coats of Krylon Ultra Flat Black.
There's not much to it. Just a ring of 1/2" ply, very Zen. I drilled holes for the truss tube screws, spider lugs and focuser shelf, and finished it just like the mirror box. The holes for the spider lugs have small brass threaded inserts in them.
The bracket is for the finder scope dovetail to attach to. Just birch ply glued up into a block, 1.5" hole for the tube, brass threaded insert and stainless cap screw. You can see the threaded tubing insert here. I learned that they don't come out.
I found a set of 16 forstner bits - 1/4" through 2-1/8" at a local store for under $50!! I always wanted just one or two but could never justify the cost. Granted these are not Freud bits or anything, but having a full set is awesome. I used the 1/4" bit to make the grooves the dovetail fits into. Now I'm making holes all over the place.
Starting to look like a - box kite?. These parts of the OTA weigh only 9 pounds. The tubes are not yet cut to length. It should wind up about 54" long. What you see here is 62". Each tube is bent inward at the top about 1/4", this makes the whole OTA extremely rigid. Rigidity is important. If you consider that the primary function of the OTA is to hold the optical components in alignment throughout the range of altitude motion, and with a variable load at the upper end due to eyepiece weights (not to mention the effects of wind), a little over building is totally warranted. For these reasons I made the mirror box a bit taller than was absolutely necessary. This made the effective length of the truss tubes shorter and stiffer, and also allowed me a bit more latitude with regard to the altitude bearing placement. I didn't have the spider and secondary yet, so I couldn't get the exact weight to determine the balance point. I also used 1.5" truss tubes, where 1.25" would have been adequate, the larger tubes add additional rigidity. The taller mirror box and larger tube diameter cost almost nothing in weight.
I made the focuser shelf out of some 1/8" aluminum angle and 3/32" plate. I was really proud of it - made it with a jig saw, file and drill press, got really dirty hands. I also made 3 small spider mounting lugs from aluminum angle.
Then I was going to paint it black - but I couldn't get anything to stick. I asked around for painting tips, got some good advice but then I really lucked out and a very generous fellow observer offered to bead blast it and let me piggyback on some anodizing he was doing. He did the spider lugs too. What a difference! It's just a bit flatter than the Krylon UFB on the ring. Bead blasting rocks!
The cell is a pretty standard PLOP designed 6 point type. I made the center triangular part out of 3 pieces of 1/2" x 1" bar stock with lap joints held together by the collimation bolts. I came up with this design because I could technically make it with a router and a drill press, which is what I have, and I'm pretty sure I couldn't get the holes for the shoulder bolts that hold the rocker bars perfectly square to the face of an equilateral triangle. I had another lucky break when my neighbor offered to let me mill the parts in his machine shop. I wasn't really excited about removing all that metal with a router!
Only one hole in each bar is threaded, with the big screw cranked down pretty tight it feels very firm. The through hole size (.250) matched the screw (.249) so there's no wiggle room. I think I was lucky to get screws that were so close to spec, I dunno. I put some Loctite there though, just for insurance.
I thought it was a fun design since each end of each bar is identical (except for the hole sizes). Albert's castings are probably a faster way to go, but I just couldn't see myself trying to square up and drill a triangular part.
The completed cell weighs 19 ounces. I went through several sets of stainless springs to find the right ones. The ones that work have a load rating of 19 lbs. each, a bit less than twice the weight of the mirror.
I glued the mirror to the cell with silicone RTV. Twice. It just didn't feel right somehow, so I pulled it off and cleaned it all up and did it again without the penny spacers.
Several things happened with the pennies: first just after I glued it, I got an email from Albert in which he said he didn't think spacers were necessary. I trust Albert. Then I got to thinking about the difference in adhesion between something under some pressure vs. goo just filling a gap. Totally conjecture on my part, but I suspect you get more stick with some pressure. Third, when I did the pennies I didn't get a nice, complete wetting of the pads, squeeze out was uneven - you can see in the picture. Fourth, I discovered from another mistake elsewhere that the silicone doesn't stick as well as might be hoped to smooth stainless steel. So I pulled it off and did it again without the pennies after abrading the pads with some rough sandpaper. Not all entirely logical, but it felt more 'right' the second time.
My spider arrived, and I RTV glued the secondary on with the specified offset and installed it. At this point it's really a telescope! I started doing some light baffle experiments - tipped the scope over far enough to get a glimpse of Jupiter in it (not collimated or anything yet, just leaned the OTA over and waved it around). Dang it was HUGE and BRIGHT!! Exciting! I've only had an ancient 4" refractor and an 80mm up until now. This thing eats light! Clouds appeared within 5 minutes. It figures.
I weighed the OTA with all the gizmos on it - finder, dummy counterweight, even an eyepiece - 26.5 pounds. That was nice. Now that I had whole OTA assembled I could also find the exact balance point and locate the altitude bearings around the center of gravity, and figure the final height dimension of the rocker box.
The 16" diameter bearings are 3/4" ply with 1/4" glued on for a lip.
I used a web clamp to glue on the Ebony Star laminate, Gorilla Glue again. The paint can is to hold it down. The orange web clamp (just a nylon strap really) is tightened with a screwdriver. If you've ever wondered how to glue one of those spindly legged Victorian chairs together, this is the tool.
The thing I made the mistake in above is one of my torsion-box rocker box sides. It's a hollow 1/2" ply frame with 1/8" skins glued on to it. Very stiff and saves a lot of weight (but kind of silly on a part that is only 15x12 - it could be a great weight saver on a bigger Dob). A proper torsion box has more, thinner interior sticks or even foam or paper (like Albert's S&T scope) and is properly made with a vacuum press. Like they make hollow core doors, very light and stiff. Here's the inside parts:
Oops. Clamped everything except the workpiece and it moved while I was routing it. I turned it into a "detail." I liked the detail enough to repeat it on the front of the box. The rest of the box parts are 1/2" ply, only the sides are torsion boxes. The joints are done with biscuit splines (the mirror box uses these too).
For the ground board I got to handle a mistake using the industry accepted standard method.
The ground board has rubber feet on dowels too, this time 3/4". The Teflon pads are located directly above the feet for best weight transfer to the ground.
Here's the finished rocker box. The laminate is glued on with 3M Spray 90 contact adhesive. Great stuff. The 1/4-20 center bearing bolt is fixed to the rocker box with a tee nut, goes through a bronze flange bearing in the ground board, and is held with an aero nut (a nut with a nylon lock washer built in).
The counterweight is made from Schedule 40 brass pipe found at onlinemetals.com - it weighs 3 lbs./ft. so a 3" length gave me exactly the 12 oz. I needed. I tapped it for some nylon set screws, put a little 1/16" cork inside with double sided carpet tape and gave it a satin finish with a mill file. Sydney the cat supervised. It brings balance to the force.
For cooling while setting up - blows out. Both fans have "M" type coaxial connectors for DC power. Just batteries for now, perhaps a variable voltage circuit later.
For cooling while observing - blows in. The white cord is polyester wrapped elastic to help damp vibrations (fabric store, 60 cents).
All of a sudden - it was done! Spending evenings, weekends and quite a few lunch hours (I work at home) it took 4 weeks to plan and 7 weeks to build. The final weight just barely under 40 lbs., and the final cost just about $1,380.00
The business end...
Packed up for travel, it's about 16x16x24...
First, my famous last words plan to deal with the focuser tilt issue. I figured that with a 2" to 1.25" adapter I could put the two set screws opposite and thus cancel out some of the tilt. Prior to the Shingletown trip, I had tried this with only marginal success, but once I got away from the city and the high speed, no time to lose sensation that goes with it, I was able to spend a little time carefully aligning things. Not too bad at all. Not perfect, and I'm considering bribing my neighbor for some more machine shop time to add a compression ring to the HC-2, but in the end I was able to get a very satisfying collimation.
We had some fairly warm 90+ degree days. The exhaust fan cooling worked admirably taking perhaps 20 minutes to go from boil to simmer. Once observing the rear fan was sufficient to maintain equilibrium and mitigate boundary layer effects for all but the most excruciating planetary views (which the seeing didn't support anyhow). Especially happy that the fan at full throttle didn't vibrate the view at all. So I think I'll just stop there and not experiment further with more elaborate side exhaust ideas.
The 'wishful thinking' counterweight and 'it'll never fly' 2" focuser turned out to be perfect. If you recall, the idea was to be able to accommodate a heavy 2" eyepiece even though I don't own one, and even though it forced design and balance issues that others warned me wouldn't be all that useful in the long run. Well, someone did loan me a 35 Panoptic, and with the counterweight sized for just this occasion, it took all of 10 seconds to slide the weight, change the eyepiece and get focus all without losing the object, even going from 40x to 200x. Here the extra effort to place and size the diagonal for 70% illumination at the edge of a 35mm field stop really paid off too, just incredible views of M11, M101, all the Sag goodies. I hate to contemplate it, but I may have to buy one of these enormous door stop eyepieces. Please, nobody loan me a 31 Nagler, just keep telling me it won't balance. Another observer showed me how to find geostationary satellites, and I had a little wide angle tour of the clark belt as the one I was watching moved across the milky way, right through M11.
I've had some grand views of Jupiter, and my first look ever at Mars, but I'm still waiting for that night of perfect seeing to find out what it can really do on planets. I'm sure I won't be disappointed.
As a first time ATMer, I think the main thing I learned was to decide on goals first. Without that excellent bit of advice, I am sure I would never have wound up with a telescope like this. I know the biggest limitation I have now is my own observing skill. Hopefully that will improve, it will certainly be fun!
Oh, it's called Little Dipper because I thought it was a sort of bad pun on Spooner. Sorry Mike!
Cheers, Mark
Cost breakdown. Most figures include shipping and tax.
OPTICS
-Primary mirror $ 550.00
-Secondary mirror $ 86.04
-Secondary holder and spider $ 85.00
-Focuser $ 83.35
-RACI finder $ 70.36
-Cell material $ 6.68
-Cell screws, springs, weld nuts $ 32.61
-Focuser shelf springs and aluminum $ 8.82
-Silicone for mirror $ 3.98
-Stellarvue Red Dot Finder $ 20.00
-Misc stainless screws and washers $ 12.62
STRUCTURE
-1.5x.049 6061-T6 Tubing (3 x 60") $ 39.90
-Light baffle HDPE plastic $ 4.95
-Finnish birch ply * $ 97.00
-Tube & bearing tee nuts $ 7.33
-Bearing & tube end cap screws $ 11.63
-Threaded tube inserts $ 7.00
-Ebony Star & Teflon * $ 52.21
-Center bearing flange bearing $ .99
-Formica * $ 15.00
-Rubber feet $ 8.00
-Fans $ 9.18
MISC
-Gorilla Glue, Epoxy $ 25.57
-Krylon Ultra Flat Black spray paint $ 6.50
-Varathane Exterior Water Based finish $ 30.00
-Misc. shipping & tax $ 33.07
-Forstner bits $ 50.00
-Anodizing $ 25.00
-Total $1382.79
* these items I have since found at much lower cost.
Special thanks to Albert, Mike Spooner, George Feliz, Richard Crisp, TAC, and Brian Spitz!
:bow: to Doug, Tom, Ron, Mark, Rod and all the CN ATM forum folks!
Is it dark yet?