Secondary Cage

The foam cage

Del has far more experience with building things than I do. In particular, he has a lot of experience building things that need to be strong and light. Still, the secondary cage he built for his 8" scope out of pink Styrofoam, epoxy and wood veneer completely shocked me. It is far stiffer than I ever would have guessed. It looks beautiful and is really light. So I was thrilled when he offered to build one for me.

Del cut the foam with a hot Nichrome wire. Apparently it is tricky to get the wire hot enough to cut the foam without making it so hot that the wire melts. He managed to produce 5 rings though that were 18.5" O.D. and 16.5" I.D. and 1.5" tall. He then epoxied the rings one on top of another to create the 7.5" tall cage. I was really lucky that the scope's design required the secondary cage to be an even multiple of 1.5" tall rings, otherwise I don't know how we could have a made a good flat cut to make it shorter.

Del spent a fair amount of time sanding the foam smooth. He also epoxied in some small foam bits and blocks to fill in any voids that resulted from flaws in the foam or any cutting mistakes made along the way.

The next step was to build in hard points. These fall into three categories. The first is the hard points for the focuser. Next are the connections for the truss tubes. Finally there are the hard points for where the spider attaches.

For the focuser there were actually 4 separate hard points, one for each of the focuser's mounting screws. They were made out of a stiff lightweight wood Del found. Rather than cut them directly into the rings, he made a small foam block to work with that is the size of the focuser's footprint. He then sanded the underside so it would match the curve of where it would fit into the rings. He left the top flat to match the flat base of the focuser. Bringing the focuser an extra 1/8" closer to the secondary mirror was a nice fringe benefit of this. The photo shows the rings glued together. At the bottom of the rings is the inset piece to be used for the focuser hard points.

Del had Dick mill the hard points for the truss connects were from aluminum and he did an amazing job. They more or less made to match one of the designs suggested by Kriege. Total weight for the four hard points with triangular wedges and screws was almost exactly a pound. We could have made these lighter, but time was running out and we figured we had saved enough weight that it wouldn't be too big a problem. The photo shows the hard points in place for the truss tubes and focuser.

The hard points for the spider were just 2" square blocks of 1/8" thick aluminum. They probably didn't need to be quite this big. I wanted to leave them a bit large though for two reasons. First, I was told that the spider would need a lot of tension on it because its veins are only 3/4" wide. Big hard points spread that force out over a larger area. The other reason is that if I ever had to replace the spider, I would have lots of room to drill new holes for the new spider mount points. Del also ran a strip of thin fiberglass cloth around the cage at this end to improve rigidity and strength.

Del learned a lot during the construction of the cage. The first thing he found out is that the wood needs to be kept isolated from moisture. This lesson was learned on a previous project where he epoxied the veneer over cardboard. Even though the veneer was varnished, moisture in the air crept in through the cardboard. Eventually the veneer rippled, buckled and peeled off.

The strength of the foam/veneer cage is dependent on the composite effect, which in turn depends on the veneer being attached securely to the foam. So the buckling wasn't just unsightly, it was a serious structural problem. This setback in the prototype turns out to have foreshadowed the demise of the foam cage concept.

Applying the veneer to the cage was difficult work as the birch veneer tended to crack and split. Cutting it to size was a problem only made worse by the demands of the epoxy. After applying most of the veneer to the interior of the Miatascope's cage, Del was pulled away from that project for a couple weeks. Before he came back to it, moisture in the air got to the unvarnished veneer, which then rippled and buckled.

At great effort and with only a few days left to go, Del removed all that difficult work and did it again. This time he did the work in a room with a dehumidifier and immediately applied varnish to the veneer after the epoxy had dried.

Here I am standing next to the scope at first light including the nearly finished foam secondary cage. In bringing the cage to this point, Del learned a few lessons. He found that there were voids where the veneer wasn't fully glued to the foam rings. He said that this was a big enough problem that for the interior of the cage it probably would have been better to just use fiberglass instead of wood veneer. Since the plan was to cover the interior of the cage with flocked paper anyway, there was no need to make it pretty. And of course, there were also the serious problems with using birch for the wood veneer.

Del finished the work in time for first light. The triumph of this accomplishment turned to tragedy though. The evening of first light the dew was among the heaviest I have ever seen. Everything brought outside came back in with water everywhere. After only a couple hours of exposure to this environment, the veneer on the foam cage was already coming off the foam in a very noticeable way.

We hoped that the problem was due to moisture leaking in through the uncovered foam ends of the cage. Those hopes were dashed though when we noticed that another cage Del made earlier for an 8" scope was having the same problem even though it was fully varnished. The foam secondary cage concept would have to be abandoned, and there was only a week to go before Astrofest.

The Kriege cage

We so little time remaining, I decided to go with a secondary cage built the way Kriege and Berry suggest. Thankfully it turned out that this cage design is very simple and easy to make. I was amazed when I weighted the end result and discovered that it was close to the same weight as the foam cage would have been.

There were some construction issues though that could have gone better. The first was the cutting of the rings for the cage. I failed to mark where to drill the indent holes for the aluminum tubes between the top and bottom rings. I couldn't figure out how to get four points 90 degrees apart. The absence of the material in the center of the rings complicated things a lot more than I ever would have guessed. I wanted to be sure that they really were exactly 90 degrees apart so I would only get 4 diffraction spikes off the spider instead of 8. To solve the problem I ended up buying more wood, marking where the holes would go and cutting two new rings.

Later I discovered that I should have trusted a suggestion of Del's. He recommended that I wrap a strip of paper around the perimeter of a ring, fold it in half twice and mark on the ring the location of the four folds. Turns out that is the technique Kriege recommends. And then I read H. R. Suiter's remarkable book, "Star Testing Astronomical Telescopes". In it he said that 8 weak diffraction spikes could in some ways be better than 4 strong ones. Precious time and beautiful wood wasted for naught.

The second thing to be pointed out in the secondary cage is the way the aluminum spacer tubes are connected to the rings. Kriege suggests using "threaded inserts". I didn't find a source for them until after the scope was done (the answer is to try a bicycle shop). My uncle Alex though had a suggestion that saved the project. He recommended using a dowel rod inserted in the aluminum tubes and attached to the rings. Grandpa, in true child of the great depression fashion, then took me to his basement where his large collection of old broom handles resulted in finding one that perfectly fit inside the aluminum tubes.

I cut 4 x 2 = 8 dowel rod bits about 2" long. I attached the rings to the dowel rods using glue and wood screws. The screws were mostly to hold the dowel rods in place while the glue dried. I then put the aluminum tubes over the dowel rods and drilled holes through the aluminum and dowel rods.

This technique helped me shrink the overall height of the cage. That is because there were no bolts for the threaded inserts to get in the way of the two sets of bolts that had to go through the tubes. The first set of bolts is on the top end of the cage to attach the spider. On the bottom end of the cage are the bolts that hold the latches to the tubes.

Which brings me to one of the unique features of my Miatascope that I am proudest of. The cage attaches to the truss tubes by using latches like one would find on a trunk. These latches attach to woodblocks at the top of the truss tubes. To keep the woodblocks in the right place on the rings, wood screws protruding down from the bottom of the bottom ring go into divots in the woodblocks. The screws and divots keep the rings from sliding off the woodblocks and the latches keep the screws in the divots.

Naturally, there were some concerns that this solution would not work. Turns out though that everything works great. There is no problem with the cage moving. It doesn't even wiggle a little. It attaches very quickly and easily. I like this solution better than any other solution I've seen.

Because the secondary cage is so short, I needed an extension to the cage to block stray light. I originally made two Formica light shields. They attached to the cage with Velcro and when rotated 180 degrees were reattached to outside of the cage for easy transport. Too bad these light shields were too small. So I tried a taller two piece shield that blocked the light, but was hard to transport.

So now I use a three piece version that extends 12" beyond the top of the cage. It is made of black plastic, which are again held on with Velcro. Each piece of plastic goes a third of the way around the cage. I was fortunate to be able to find suitable plastic at a small neighborhood shop that makes signs. The stray light problem I had is gone, and the pieces are short enough for me to carry them in my bag with the side bearings.

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Last updated 1/12/06