Introduction: Smaller Dobsonian Telescope

I was been missing making a telescope, but didn't need another one. So I persuaded a colleague to make one with me. The optics were cheap--a 6" F/5 mirror (with a 1.5" secondary) from a Cloudy Nights seller for about $60. In classic style, this was going to be mounted in a Sonotube, with most of the rest made of 1/2" Baltic Birch plywood.

Step 1: Altitude Bearings

I used a router to cut altitude bearings from 3/4" red oak. They are slightly more than semicircles. It's traditional for altitude bearings to be circular, but using semicircles (or slightly more), the radius could be made larger for semicircular bearings without wasting money on a wider board, thereby making for greater stability. Moreover, it's easier to cut less than full circles on my fixed-base router. And I cut lightening holes with a hole saw.

Step 2: Mirror Focal Length

I measured the focal length of the mirror at around 29". The time before that I measured the focal length I projected the moon onto a piece of wax paper, and then measured the distance. That wouldn't work this time, as we didn't have a moon in the evening sky. So instead I put a red LED light some distance (d1) away from the mirror surface, focused it on a piece of wax paper, measured the distance of the wax paper from mirror surface (d2) and used the formula 1/d1+1/d2=1/f, where f is the focal length. I did this twice and averaged.

The mirror did have a chip on the back side, and I was worried that this may have set up stresses and distorted things, so I tested it as described in this post , and it was fine. Hurrah. 

Step 3: The Tube

My frined got an 8" (approx.) diameter Sonotube tube, but it was only about 0.095" in thickness and very flimsy. Following Cloudy Nights advice, I did a couple of things:
1. Cut four plywood rings with a router for outside reinforcement. Currently, one is installed on an end, and I'll put another on the other end, and I don't think more is needed. A pair of plywood rings rigidify a tube by a lot.
2. I only needed a 29" tube (that's the focal length; one can't assume the tube length is the same as the focal length, but that's how the calculations worked out for me). Since the Sonotube was 48" long, I had 19" to spare. I cut that into three rings, two 7" wide and one 5" wide. I cut snips out of these so I could roll them into a smaller diamater (snip width: 2(pi)(thickness)) that fits inside the larger tube for reinforcement. I would eventually glued the 7" wide rings at the ends of the 29" tube, and the 5" ring in the middle, but first...
3. I impregnated all the cardboard--cardboard reinforcing rings and tube--on both sides, with a 1:1 mix of Titebond II and water. Titebond III would have been better as it's waterproof, but II is water resistant, and it's what I had at home. After impregnating, I inserted the rings inside the tube (the waterly Titebond was good enough for gluing them in place). When I impregnated the insides, I replaced some of the water with DecoArt black acrylic, to make for an initial coat of dark (smudgy blackish gray!) for the inside, which eventually will have to be made much blacker.

The result is a very solid and strong tube, though right now I only have one plywood ring in place. (One problem: the inner diameter of the rings is too small by about 1/16", and it took about 1.5 hours of sanding to get the one that I installed the right size.)

The photo shows the plywood reinforcing ring on one-side.  Later photos will show more.

Step 4: Primary Mirror Mount

The mirror came with a plastic primary cell. Unfortunately, that would have required quite a bit shimming to fit in the 8" inner diameter tube, as the cell was rather small in diameter. So I made one. As I often do, I laid it out in Inkscape. (The annotations are for the benefit of this description--they weren't there in my original file.) I am very bad at measuring (but getting some cheap digital calipers for $8 from a Hong Kong seller on ebay helped a lot) so my best way for precision is to print out a cutting template, attach it to the piece and drill and cut right through it, or at least make marks with a pencil or drill bit through it.

The plan was to use scrap material. The cell itself was to be a triangle with corners snipped off made of two layers of 1/2" Baltic Birch plywood, cut from the circular inner waste generated by routing the tube reinforcement rings. I cut the cell with a jig saw, not very precisely, and cut a ventilation hole with a hole saw. I then drilled and countersunk holes for collimation screws in only one of the two triangles. I JB Weld'ed 2" long machine screws into their countersunk holes, and then I glued the other piece of plywood on top of this one (with Titebond II), covering up the screw heads, and clamping hard. As a result, the heads of the screws are trapped within what is basically a solid piece of plywood (two pieces of plywood glued together are basically one thicker piece of plywood!)

There would be a rectangular baseplate ("tailgate plate" in the diagram below) made from a piece of scrap red oak that I had. The baseplate would be mounted in the tube, near its bottom (not quite at bottom, as the plywood reinforcement ring on the tube would I rounded the corners of this scrap piece with the router using approximately the right diameter to fit in the tube, bearing in mind I could always sand it smaller (as I indeed had to) but couldn't make it larger. (By the way, here is a hint for how to measure things off from the router. I cut things with a 1/4" up-cut spiral bit. For measuring, I replace the spiral bit with a small length of 1/4" stainless steel rod, and measure from that with the calipers.) Notice from the diagram that the rectangle would be somewhat off-center in the tube--this was on purpose, to fit better with the collimation screws. I drilled holes for the collimation screws (actually, I did that while drilling them in the triangular cell, because they needed to line up exactly).

I also made a bit of a countersink around the holes, so that the collimation springs that would go on the screws (I had some springs that someone once sent me) would be countersunk, which would make the assembly have a lower profile. I didn't have a paddle bit of the right size for these countersinks, and I didn't think I could control my router for such small work, so I did something wacky. I chucked my 1/4" spiral cut bit into my cheap Harbor Freight drill guide, locked its height, and used a drill to route out the countersinks. It wasn't very neat, but it was good enough, and the lower speed was less scary. May not have been good for the bit, I already somewhat damaged the bit in an earlier episode when it was accidentally cutting against cement.

I also cut three ventilation holes in the baseplate with a hole saw. The result coincidentally (honest!) looks like a certain popular rodent.  I emphasize that the placement was entirely functional. I made the inner hole as large as I could while keeping strength around the collimation screw holes, and the small holes are also located in such a way as to be fairly symmetric, and not too close to any of the alignment holes or the inner hole. Four wood screws hold the base plate to the tube.

The mirror is glued to the cell with three 3/4" blobs of silicone sealant, with the positions optimized with Plop. I used some removable 1/4" particle board spacers to make sure the blobs wouldn't flatten out while the sealant was setting. One wants the mirror to float on the blobs, and not have stresses put into it by the differences between thermal expansion of wood and glass.

Step 5: Drilling a Focuser Hole

I was really nervous about making the focuser hole--drilling in a cardboard tube can be messy. On my Coulter 8" when I replaced the focuser, I had to enlarge the focuser hole, and it was a quite big mess. I thought of trying to use a rotary tool, but I couldn't figure out which bit was appropriate, and had no scrap to try it out.

But I decided that the glue-impregnated cardboard just might take a hole saw, and risked drilling with a hole saw. Success! There was just one little bit of tearout on the inside, but I glued that down while impregnating the hole with water-diluted Titebond II.

Step 6: Focuser

I now prefer the much simpler design of this focuser .  But for this scope, I had made a more normal Crayford. The draw tube is aluminum with 1.5" outer and 1.25" inner diameter. Actually, my eyepieces and laser collimator didn't fit when I got it, so I had to sand the inside of the tube. The method was to take a screwdriver, wrap some sandpaper and foam around the handle, affix it at one end with duct tape, and spin the screwdriver with a drill. Took a while, but eventually I got the bore large enough.

The main part of the focuser is a particle board plate, with two 5/16" dowels along the sides to fit the curvature of the tube (see last photo), and three posts. The two of the posts on the right are, I think, oak, and the one on the left is cut from 1"x1" poplar square rod, and they all have little plywood thingies behind them to keep them from falling over. They are both glued and screwed to the plate. The two posts on the right get ball bearings (the cheap ones for skateboards that one can buy on ebay, with wood screws JB Weld'ed inside them), and the one on the left has a hole cut out in it for a holder for the focusing shaft. There are two screws on the holder for adjusting tension.

The non-fixed parts consist of a little H-shaped poplar thingy that fits in the slit in the left post. It has some V-shaped holes cut in it and lined with bondable PTFE. Between the H-shaped thingy and its post there is a folded rubber thing to add some spring, cut from a bicycle inner tube. The adjustment screws on the post bear against the rubber. The shaft is 1/4" stainless steel. The knobs are cut from pine boards. They're not as straight as they could be, but they're light. I cut the knobs by first drilling about 1/3" deep into a pine board with a 1.25" paddle bit. Then I cut the pieces out with a larger (1.5" or 1.75") hole saw. Result: cylinders with a recessed hole that lightens it and makes it look better. I then put a bolt through each, and spun it against sandpaper on a drill. The shaft then fits in the holes that the hole saw mandrel made. I put some PTFE rings on the shaft, and I filed the ends of the shaft to make it rougher and less round, and glued the knobs in placed with, of course, JB Weld. (I use JB Weld for most bonds other than wood-wood. I use Titebond II for most wood-wood joins. Except that the dowels at the bottom of the focuser, to make the focuser hug the tube, were attached with Gorilla Glue, because I wanted a glue that would (a) dry quickly and (b) make little gussets.)

Motion is delightfully smooth.

In the photos, the focuser tube isn't trimmed enough.  Eventually it'll be trimmed to the right length (after checking where the focal plane lies--I like the focuser when fully racked in to be about 0.25" below the focal plane) and a set screw will be added.

Step 7: Rough Secondary Cell

I used a router and a hand saw to cut a simple secondary cell out of 1" square poplar rod.

Step 8: Secondary Mount and Spider

I also had some 1/8" thick 6" long 1" wide steel strips. I bent one near an end--sandwiched it between two others, and hit it with a mallet, and then did the final fine adjustments by grabbing the end with an adjustable wrench and just bending by hand. It was hard finding the exact right place for it and the exactly right bend. 

I drilled a 1/4" hole in the secondary support for where the spider will attach. 

I glued the secondary mirror to its support the standard way, with silicone sealant and some toothpicks to ensure spacing, which toothpicks I removed after a couple of hours of drying. It's a nice-looking 1.5" minor axis mirror.

In the evening my friend came. We put in the primary. And then tried to mount the secondary. I was very nervous about dropping something on the primary. My initial solution was to tie a safety line to my secondary support strip, and tie the other end to the focuser so if I dropped it, it wouldn't hit the primary. I never dropped it, fortunately. Then I had an idea for a better safety measure--I gently put a scrunched up T-shirt on top of the primary. Good idea! I dropped fender washers two or three times on it in the course of the evening.

With my friend there, we measured off half of the diameter of the tube and cut a piece of wood to help center the strip. Moreover, we cut a small piece of hardwood to act as a small carpenter's square, for squaring the strip against the focuser tube (which I racked in all the way). Also, to get the vertical positioning right, I put a laser collimator in the focuser and used Inkscape to draw a little centering target which my friend cut out and we taped over the front of the secondary. (My laser collimator emits a cross, so I aligned it with one of the crosses on the target.)

Eventually, I gave up trying to get it completely perfect, and mounted it. Then I drilled a hole for the secondary mount on the support strip after aligning the target on the secondary with the laser (sorry, no offset). And mounted the secondary to it. (By the way, that big screw will have to be trimmed slightly. And I will glue its head down so as to make the collimation entirely tool-free.

The secondary support is mounted to the tube with a carriage bolt. One end of the carriage bolt is capped with a plastic wingnut, and the other goes into a plywood disk on the outside of the tube.

The long screw in some of the photos eventually got trimmed to a more decent size.

Step 9: Initial Testing of Optical Tube Assembly

I collimated with my laser (see the collimation instructions here), and it was time for first-light. The sky was clear. I plunked my 30mm Rini in the focuser for 28X with an approximately two degree field. And, hurrah, stars came into focus, while handholding the tube. With another friend's help (he saw us over the fence working away, and dropped in to kibbitz), and the scope leaning on the earlier mentioned T-shirt on the trunk of my car in the driveway, we saw a lovely but tiny sharp image of Saturn. We then looked at Mizar and Alcor, and split Mizar. Didn't look at any deep sky objects. At one point it looked like I saw a dark nebula, but it was just leaves on a tree.

The diffraction spike from the single vane support didn't bother me and more importantly didn't bother my friend--it's his scope, after all.

So, the tube is done is done. Now it will be time for the mount. We'll make a wide hexagonal plywood ring, about 7" wide, around the middle of the tube (the center of gravity with my 13mm Hyperion plus a Barlow is in the middle; it'll be even further front once my friend attaches a Telrad), attach the altitude bearings to that, and make a standard Dobsonian box.

Step 10: Octagonal Wrap-around Box

We cut up the 1/2" (actually a little less) Baltic Birch for the mount on a friend's table saw (thanks!).

There is a hexagonal box around the center of gravity of the telescope tube. That was hard to do. I thought I had the table saw set to very close to 30 degrees from vertical (with a precisely cut piece of cardboard) and I thought we were getting 0.01 inch precision on the length of each cut, but the box was a bit too big, and the angles didn't quite close up. To compensate for the size being too big we cut two sides a bit smaller, and we shimmed between the box and the tube where needed. Here is the hexagonal box drying, held together with a giant rubber band (red) and Duck tape (silver).

Step 11: Mount

The rocker box came out really nicely. The round parts were done with my fixed router. We did need to use a home-made drum sander (plug cut by holesaw on a bolt, spun by a drill, with sandpaper hotglued--unfortunately, eventually the sand paper came off, but it still did the job after a fix or two) to make things fit. I also glued 1/16" bondable PTFE pads to the sides under the rockers, and used the rockers themselves to press the pads down. The clearance is pretty small--we'll see if any problems develop as the wood ages.  I also added wooden strips, with some more PTFE on them, to keep the rockers from falling off

The rocker base is very simple: just a round piece of plywood, with three round legs (2.5" plugs from using a hole saw to cut the lightening holes in the rockers), and PTFE pads near the legs (for stability), and a vinyl record inserted for the PTFE to bear on. Bricks used for clamping both the pads and the legs as in the first photo.

Finally, today, with the kids' help I attached the rockers to the tube. I also added little wooden thingies, with PTFE lining inside the rocker holes on the box to keep the rockers from falling off.

And here is the finished scope. Well, finished, except for finishing the wood, which is for my friend, and painting things black. Interestingly, the tube and the primary cell are really stable. Even though we did all that stuff with the tube in the meanwhile, and the primary cell was removed for much of it, when I put the primary cell back in, although secondary collimation was off (I think that single-stalk secondary holder may need an upgrade eventually), primary collimation was almost exactly spot-on. I also put some silicone on the outside of the inner edge of the focuser tube, where the Crayford focuser's bearings go, so the tube can't be racked too far out. To prevent it from being racked too far in, the set-screw for eyepieces is located so it'll hit a wooden piece eventually. To remove the focuser, I guess one either has to twist the focuser tube past the rockers (not good for the aluminum of the tube) or remove the secondary and get it out from the inside with the set-screw removed.

Final hints for painting: Make sure you put some sort of a wood finish especially on the edges of the plywood, as that's where moisture gets absorbed the most.  Also, paint everything that is inside the tube, except the mirrors (which should be removed for painting), flat black--use as flat a black as you can to eliminate light reflections.  I like to use blackboard paint--it's nice and flat.  (I tend to prepare wood for it by "priming" it with a mix of Titebond II and water first.)  I don't know what my friend ended up using.  Also, don't put the mirrors back in until the paint has stopped off-gassing.

Celestron Space Challenge

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Celestron Space Challenge