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Misadventures In Telescope Building

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Misadventures In Telescope Building
by Daniel Gillis Click to Email Author

First, I guess that I need to state that I have no financial stake in any of the companies that produced products that I will mention in this article. Any mistakes and errors in this article are mine alone.

I confess. I have had a thing for buying telescopes. My father did cars--I have done telescopes. Over the course of several years I have purchased, sold, bought, and traded at least thirteen different telescopes. I have owned smaller high end refractors and an excellent five inch f15 achromatic refractor. I have owned quality reflectors like a ten inch Portaball and an eight inch Portaball. The list goes on. And yes, I lost money on almost every sale. But to keep things in perspective, I lost much more in my investments over the same time without the fun!

Money aside, these scopes have given me wonderful views of the heavens. I have enjoyed many a night viewing my favorite celestial friends. In addition, these scopes have allowed me to share some of these wonders with others through public observing events.

After a point, I decided that I wanted to settle on owning one "somewhat large" dobsonian and a smaller quick look scope. I am still undecided about the quick look scope. I would love to own a 5+" APO refractor; however, my finances say no for now.

As far as the dob, I did try several. I learned from them that aperture wins on deep sky--period. You see more of faint fuzzies with increasing aperture--at least I do with my aging baby boomer eyes. I also believe that you can, given the right conditions, see more planetary detail with increasing aperture. Of course, the right conditions can be frustrating to achieve. I have also learned that a scope's design can either promote or hinder those magical moments of excellent viewing.

Thus, I decided that I would build my own "somewhat large" dob using what I had learned. It also helped that I had run out of money to keep buying complete highend scopes. My plan was to combine many of the features I believe promote those magical moments into one scope. At least that was the plan.

My initial design constraints were

  1. a 10" to 14" primary mirror of solid reputation (probably used to save money),
  2. weight as low as possible--an aching back let alone sciatic pain definitely hinders those magic moments,
  3. an open primary mirror mounting system--from experience I believe cooling to be very important in higher power viewing (Alan Adler published an article in January 2002 discussing the effects of active cooling on high power images),
  4. quick setup and quicker takedown--the use of monopods as trusses could allow for a compact shape during transport (yes, I got this idea from one of those highend dobs I once owned),
  5. primary mirror collimation adjustments above the mirror rather than behind the mirror (I liked the convenience when collimating--a Portaball feature),
  6. a curved secondary mirror support rather than a four vane support--I prefer the views it gives of double stars as well as of the planets (I know some would disagree),
  7. the use of common construction materials--excluding the optics and monopods (i.e. stuff from common hardware stores),
  8. the use of my current limited tool set--an old drill, a somewhat new jigsaw, various wrenches, and screwdrivers,
  9. and the most important constraint, cost--keep from spending more than about $1200.

Due to cost and weight constraints, I would attempt to build my own mirror mounts (primary and secondary) and my own lightweight helical focuser.

How to start?

For the optics, I started reading reviews about various mirrors. I could not spring for a mirror used in Portaballs and the like (even if one was available), but I could settle for a mirror from a manufacturer with solid reviews. I also scanned the Web for listings of used primary mirrors matching my research.

For design ideas, I began by looking for articles on telescope building. After some reading, I focused in on a couple of articles. One by Barry Leger (December 2000) detailed the building of an 8 to 10 inch dob out of 3/4" plywood. The design featured large altitude bearings resting on an ultralow rocker box. The design also used a simplified upper "ring" to hold the secondary and focuser. I wondered if I could change his design to use four monopods rather than using eight fixed length trusses. To do so, I would need to mount the monopods differently--abandoning the "tilted" appearance of the scope. Could I also reduce weight and open the mirror box up more by making the altitude bearings more "hollowed out" (i.e. a frame). Another article by Alan Adler (August 2000) detailed what size secondary mirror I would need for a given primary. And another article detailed the use of springs to assist in balancing rather than using counterweights.

So many details to think about!

I started to draw pictures--lots of pictures from various angles. Doing that helped me decide on the general design of the scope. I was now ready for details. I felt it would be better to have the major components in hand (and within eyesight) before drawing plans with measurements. Time to spend money!

Foremost, I needed to decide on the primary mirror size which would drive all the other dimensions. The size was set when I purchased a used five year old 12.5" f5 primary mirror. The mirror was from a long established telescope company that is known for its high quality larger equatorial newtonians. The next important dimensions were the "footprints" of the monopod heads. These sizes were established when I purchased from an Internet vendor four black monopods. And finally, I purchased a new 2.14" secondary mirror from an American telescope company that builds moderately priced dobsonian telescopes and optics.

Taking measurements of these components, I drew up more detailed scaled drawings. Not being comfortable with this alone, I made actual paper cutouts of some of the major parts (altitude bearings, major crossmember connecting the bearings, mirror cell). My drawings called for two twenty inch diameter altitude bearings. Drawing smaller diameter shapes was not too difficult, but twenty inches? Being lazy, I searched the house for an object that closely matched my desired circular shape. The base of a floor fan matched very closely the altitude bearing size--thus my full size cutout had a pattern to trace.

Time to make some parts. I had earlier decided to use wood as the basic construction material rather than aluminum or plastic. I had toyed with the idea of using composites but good quality 3/4" and 1/2" oak plywood from Lowes was much cheaper without being too heavy. Besides, I had very limited cutting skills. I knew my cuts using a handheld jigsaw would work better with materials that were forgiving (i.e. correctable using sandpaper and woodputty). And I was proven right.

My curved cuts were not bad, just not true enough without a bit of eyeballing, sanding, woodputtying, sanding, and ... Eventually, the two bearings matched very closely in their curves and were basically true in shape. Of course, the Ebony Star formica I planned to use on the bearings would smooth out the smaller imperfections--I hoped. Next I cut out the major horizontal crossmember that would fit between the two bearings, support the mirror cell, and support the monopods. Funny thing was that it did not quite fit with the bearings as I had planned--a bit too big to fit in the groves I had cut for it in each of the bearings. So I re-cut (chopped off) some of the crossmember. Drat again, now it was too small and looser than I wanted! Hey, glue and woodputty could do wonders to hide these little things.

Speaking of glue? What to use? For better or for worse, I used what I was familiar with--Wood Goop. It forms a very strong wood bond and a tight but breakable bond between wood and bolts. It takes a while to reach maximum strength (48-72 hours). However, it is not benign (nasty warnings on its label) and even with the wearing of gloves I got more of it on me than I should have. I did have the sense to use it in a well ventilated location--outside. Since that time, I have tried other glues like a combination of a quick setting epoxy wood glue with a regular wood glue.

I next re-measured the distance between the bearings to determine the two upper crossmember lengths. I cut the pieces out--basically two long flat thin rods. At this point I should have been less worried about saving a few dollars and used two 3/4" thick oak dowels instead. They would have looked much better and been stronger--especially if there had been slots in the altitude bearings to fit into rather than just being glued in place.

If I had also been wiser, all the parts would have fit together tightly enough to glue without external bracing. I did it the hard way and had external bracing while the parts dried. Actually since the glue took time to dry, I had time to correct most of the placement of parts. However, since it took time to dry, parts had a chance to slowly shift out of alignment too. The biggest mistake was that one of the upper rods was not long enough (I swear I checked that at least twice), so I had to re-cut the piece on the fly. The other problem which I did not realize until it was too late was that the right bearing was ever-so-slightly splayed outward at the bottom as seen from the front (not quite perpendicular to the major crossmember).

Hopefully, the rocker skids would be able to handle this imperfection.

I next set about the task of mounting four wood inset threads into the major crossmember onto which the monopods would bolt. To guarantee no screwups, I bought a fifth inset to practice with on s****e 3/4" thick wood. I drilled the initial hole and "reamed" it out a little more with the same drill bit since I did not have the exact drill bit size required. I added some glue to the outer threads of the inset and proceeded to screw it in. It worked great! I proceeded to do the first inset in the scope; it worked like a charm. The next one seemed to work fine until I noticed that I had screwed it in crooked. Out it came (thank goodness), and I screwed it back in with much more care--yes the glue was a mess. The other two were also finicky screwing in.

The next major task involved the planning and building of the rocker box and base. Since I had the lower tube assembly (most of it) in front of me to experiment with, this phase went smoother. I cut several different shaped test skids and temporarily attached them (using carpet tape) to a large board. With this setup I tested various skid shapes and separations to see which seemed to work best with my bearings. The lower tube assembly at this point weighed in at about five pounds (excluding the monopods). I did try using teflon sheeting on the skids but found that with my slightly skewed bearings things tended to s****e too much. I switched to carpet gliders (Super Gliders I believe) which worked much better. I tried to save weight by building a square ground board with four weight saving "holes." Unfortunately, it allowed grass to enter the azimith bearing area and later the ground board sagged under the scope's full weight. I replaced the original ground board with a solid 20" diameter ground board. My earlier imperfection in the lower tube assembly seemed to be handled by the rocker box skids.

The primary mirror mount had to handle a 12.5" diameter mirror weighing about 20 pounds. Not light at all! The mount would be circular (15" in diameter 3/4" thick) with generous ventilation holes. To avoid gluing the mirror to the mount I would glue twelve felt pads to the mount providing a "cushion" for the mirror to rest on. To restrain the mirror I would use four retaining posts. Each post, made of balsawood, had a bolt running through it that allowed it to be bolted to the mount. One side of each post was shaped to be concave to fit the edge of the mirror. Each post was painted in a durable sealant (Bullseye 123 Primer Sealer) as was the rest of the wood in the scope. Additionally, each post was coated with Wood Goop for added strength. Finally felt and velcro were glued to the sides facing the mirror's edge. The mirror was fitted with matching velcro to mate with the velcro on the posts. As designed and built, the posts have held up well in keeping the mirror in place without intruding into the lightpath. However, this arrangement is not designed to restrain the mirror if the mirror and mount are ever turned upside down. I avoid turning it upside down.

For collimation I decided to use four adjustments rather than three. Why? I prefer four over three. The mirror mount would sit on four rather beefy springs with bolts running through the lower tube assembly crossmember, the spring, and the mount. Collimation would be done by tightening knobs screwed onto the bolts.

Cutting the upper tube member was easy. The dimensions of the upper tube member were determined by where the ends of the monopods were. I decided to go with 1/2" oak plywood, to make the outside shape a square, and to make the inside shape circular. How to mate the monopods to the wood? Boy, I looked all over the hardware store for some piece of hardware to do this. In the end, I went low tech--brute force. First, I removed each monopod's rubber foot. Then I drilled out a hole in the center of a wood dowl which was slightly wider than the monopod's inside diameter. I then whittled the dowl down to tightly fit inside the monopod, cut the dowl a predetermined length, inserted a bolt with some glue through the drilled hole in the dowl. Once the glue had dried, I gently tapped the dowl into the monopod. Finally, I drilled holes in the upper tube member to accept the bolts sticking out of each monopod. This arrangement allows for the disassembly of the upper tube assembly from the rest of the scope.

I had now come to another part of the project where I could save money--build my own secondary mount and support. I needed wood that was more "solid" than the balsawood I had used in the primary mirror posts, but not too hard to carve into a circular shape or to drill. At about this time, I happened to talk with a family relative who dabbles in wood carving. He had a s****e piece of basswood that he felt would fit the bill. Along with the basswood, I used three wood inset threads with matching bolts for collimation (yes, I now wish I had used four) and a threaded nylon dowl with nylon nut. I decided to be lazy and just glue the secondary mirror to the lower half of the mirror mount assembly.

How about the curved support vane?

I again looked and looked at hardware stores to find the perfect part--a thin metal strip with correct attributes. I even thought of using a metal ruler, but it just was not sturdy enough. Eventually, I compromised by favoring strength over some defraction noise. The hardware store had a stainless steel strip that was thin enough and sturdy enough. However, it was not long enough to span across the opening in the upper tube assembly. To bridge the span I added a flat metal bracket and an L bracket along with the shortest bolts I could buy to each end. The resulting support vane is very stable, but when using the full 12.5" aperture there is some added defraction noise (other than from the vane itself) from the short bolts and matching nuts.

A newbie like me had an initial problem wondering where to center the secondary. I took measurements, but you never know with my measuring. So I resorted to using a laser pointer mounted to the center of my primary mirror mount. I used a level to guarantee that the scope and primary mirror mount were level. As it turned out, my measurements very closely matched what the laser said.

One more major step was left before first light. The focuser. I had used some really nice helical focusers in the past--the Portaballs came to mind. I tried to look for articles on homemade focusers but could not find any. Later I learned that several books do discuss homemade helical focusers and there are older articles too. Lacking others' guidance, I experimented with various plumbing fittings. Eventually, I cobbled together a focuser that would work for testing purposes, but I quickly realized that it was not very good. In fact, it stunk. I eventually replaced it with a used 2" helical focuser from a highend dob.

First Light at Last!!

First the good news. The initial glimpses I caught of stars near the zenith looked good. No make that extremely good! At high power I saw tight little disks with one defraction ring. It did help that it was summer, so I did not have to deal with cooling issues. The scope also moved wonderfully in azimith using the solid groundboard.

Now the bad news. Nearer to the horizon was a different story. Stars were a mess--way out of collimation. Plus balance problems were raising their ugly head. I had a top heavy scope even with the heavy handed use of springs between the rocker box and altitude bearings.

Most important, I had a problem with flexture pure and simple. At first I thought that I could fix the problem by better securing the monopods near the base. I tried several rigged solutions but eventually glued "cradles" (brackets) to the sides of the altitude bearings that would give support to each monopod. To keep the monopods tightly pinned to these brackets, I bolted matching enclosing brackets onto the supporting brackets. Later, I replaced these bolted brackets with mini bungie chords that worked just as well at pinning the monopods against the brackets. With this fix in hand I tried again. The flexure was less noticable but still far from acceptable.

What to do?

I needed additional truss poles. No way was I going to ruin my original design by adding fixed length poles. Then it occurred to me that I needed a way to tension the monopods. Kind of like adding virtual trusses. I had some heavy duty fishing line from another ill fated project I had undertaken in the past. The line was originally designed for catching large ocean fish and could easily take lots of tension. Actually much more than I would need. I could use the line in conjunction with what I called "turn bolts" to provide tension between the top of the scope and the altitude bearings.

The trick worked! The amount of tube flexure was reduced to an acceptable amount judging by the images produced near the horizon. I now collimate the scope halfway between the zenith and the horizon to split the difference in the slight flexing that is still present.

I still had a problem with balance. I hated to do it, but I added a removable seven pound counterweight. Balance issues were thus addressed, but it was ugly!

About a month later during an annual astronomy club picnic I was showing off my new contraption to club members. One telescope builder and designer who builds highend dobsonians wondered why I had not mounted the primary mirror below the major crossmember rather than above it. By doing so the center of gravity would be lowered (remember that 20 pound mirror) and thus reduce the need for a counterweight. I went home the next day wondering why I was so dense.

I had to add additional clearance height to the skids, expand the existing center hole in the major crossmember, and remove most of the stuff related to the counterweight. It worked like a charm! The scope was much better balanced. Besides it looked better too.

You may be wondering about baffling. For the upper tube assembly I used black heavy felt sheeting to form a "tube." It works very well most of the time--moisture bothers it very little. High wind does bother it though, and gnats (those pesky little bugs that swarm at dusk) are attracted to it (they do not seem to set up permanent residence in it, thank goodness). I have also added some black cloth to the upper tube assembly for added light blocking. The focuser has a removable annular baffle cut from the same felt sheeting. For the lower tube assembly, I usually keep it open to promote cooling. But I did make a larger felt tube that can fit over the mirror and attach with velcro. It too is affected by the wind--during one blustery afternoon it tumbled quite a distance away before I noticed it had departed.

Results of my adventures in building

I now have a relatively lightweight 12.5" dob. The optical tube weighs in at about 35 pounds--15 pounds if the mirror assembly is removed. The rocker box weighs about 8 pounds.
The scope can be folded down to a size that fits in the backseat of my 4 door Civic sedan--it does take up most of the backseat. If I am forced to, the upper tube assembly and monopods can be removed from the lower assembly making the scope even more compact. Since I have marked the monopods (with my daughter's nail polish), I do not have to guess on the extension height when setting up. I just place the ota on the rocker box, attach the one balance spring, extend the monopods to their marked heights, tighten the two turn bolts on the tension strings, and tweak the primary collimation knobs (many times there is no need). Breaking down after viewing is fast too--nice on a cold night.

The cost? About $1300 including the used focuser.

Movement? In azimith very smooth at low and high power with no stickiness. In altitude smooth at low and high power, but with some slight stickiness at high power. In altitude not quite as smooth as the best dobsonian I have ever used (they move as a ballerina does!).

As for the views?

Wonderful. This is a keeper pure and simple. The mirror's figure is great.

I have never seen Saturn better than in this scope. Well, once I did see it a little bit better in a 10" Portaball I owned. However, that was at 2 am under very steady skies with ample time for mirror cooling. Mars at its closest was great. That's comparing it with views I got through other club members' scopes of all sizes and expenses. Views of doubles are great too--for example the triple star Iota Cassiopeiae is really neat.

When trying to view at higher power I use a portable floor fan. I position it so that it blows across the front of the mirror. I believe this helps higher power viewing. I see the difference with and without the airflow. Plus I do not live in a terribly cold climate either. Cold to me is 30 degrees with the wind blowing at 10 miles per hour.

Oh I forgot. Deep sky viewing is nice too. About what one would expect for a 12.5" reflector with standard coatings that are starting to show their age. The Ring Nebula is nice in this scope. The brighter globulars are gorgeous. I do need to get out more with it to dark sites.

Problems? Regrets?

Being a prototype, the scope bears some scars of my misadventures. Like those handcut rods rather than oak dowels. Like my wobbly cuts that pass for straight lines. Like that ugly flat black paint job throughout the whole scope.

On warm nights (above 85 degrees), balance problems still come up. The altitude movement gets "slippery" near the zenith and near the horizon. I have tried to add more spring tension but movement is adversely affected at other places. I have resorted to placing a piece of fuzzy velcro on one of the carpet glides to firm up movement on those warm nights. However, that fix tends to make altitude movement a bit sticky for my taste.

And the mirror does need a recoating in the not too distant future.

And being so lightweight it is not the best scope on windy nights.

Would I do it again?

Yes I have. Remember I can be somewhat dense. Six months after I built this scope, I constructed a single folded 5" f15 achromatic refractor on a dob mount. I have also built a smaller (6" to 8") version of the 12.5" dob. Both are other stories.

Thanks for reading my ramblings.

Wishing you clear steady skies,
Daniel Gillis


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