The following is an edited down version of an article I wrote for "Amateur Astronomy" that was published in their 4th edition. AA is a fun magazine (not just because they publish articles from me!) but it is written "for" amateur astronomers "BY" amateur astronomers. Click HERE to vist the "Amateur Astronomy" web site

One Step Closer.....

by Chuck Shaw

Anyone that considers themselves an Amateur Telescope Maker (an "ATM’er in the lingo) either has found, or will find the one basic truth to telescope making. That truth is: There is no such thing as the "ultimate telescope". I think there are several reasons that assure this to be a fundamental law of ATM’ing. The first is related to the saying that beauty is in the eye of the beholder. Specifically, what seems to be approaching the ultimate telescope for one person isn’t even close for someone else. For instance, I love Newtonians. However, there is another whole group of folks that prefer trading the ability to see dim little fuzzies for a higher tech approach that requires having a hole in your mirror! (Can you imagine, having to drill a HOLE in your mirror? It won’t even help drain off all the dew that always forms so easily on the front of those things!) To each his own I guess......

Another reason there will never be an "ultimate telescope" is ATM’ers are such a clever group of folks (yes, even those with holes in their mirrors!) Just as soon as you complete the latest version of your effort at combining all the latest ideas ever thought up you will see someone else that has incorporated at least one thing that is either new to you, or does the same thing but in a somehow "better" way! Going to a big Star Party like TSP can be a BIG blow to your ego when you check out all the other ATM entries!!

The healthy part of all this is there are very few, if any, ATM’ers that "compete" with anyone except themselves when trying to achieve their version of the ultimate telescope. That’s good. Its good because we can openly share ideas and learn from each other on our own path towards what we envision as our ultimate scope that satisfies our own needs. That’s the reason I am writing this article, and why I enjoy reading every article I can find on what everyone else has done. So, what have I brought to this pot luck dinner of telescope making ideas? Hopefully, I have brought two things. The first is to add my vote on confirming the basic approach to figuring out what you want to do. The second is how I applied the approach to my telescopes.

The approach is simple. The first thing you need to do is figure out WHAT you want to do with your telescope. Then, let your requirements drive the design. Don’t let yourself fall into the trap of becoming enamored with a particular design detail (unless its part of your requirements!) and then let that drive everything else, including having to change your requirements! There will be enough compromises made when bringing the design to life (materials availability, cost, schedules, etc). Take the time to really think about what you want this creation to do for you! Do you want high power moon and planetary views? How about 15th magnitude galaxies instead? Oh, you want BOTH?(bring money!) How about photography? Long exposures require accurate tracking and the ability to accurately guide. Oh, you want it simple, easy to assemble in the field, and able to collapse into a briefcase ..... Hmmmmmmm, maybe this approach isn’t so simple after all! The way to get through this hurdle is to talk to folks, use other scopes, and even just jump in and get an inexpensive scope and start using it. You will eventually figure out what you like and don't like, and what peaks your interest. As with most things, compromise is a way of life. You will soon learn what designs support what requirements. That’s when the basic design for YOUR version of the ultimate telescope starts to gel!

For instance, I wanted deep sky views, but I also wanted to be able to use a reasonable amount of magnification. That suggested something slower than f4.5, but faster than f10. I eventually settled on f6 when looking at available optics. Aperture and cost seem to go hand in hand. The only way to maximize aperture while still staying within my budget was to go to a Newtonian system. So now I had settled on an f6 Newtonian. I wanted to do photography, but that was a long way off in the future. Better in the near term was to be able to track the sky good enough for a high power field of view to stay centered long enough for me to sketch it. However, after using dobsonians, I soon learned how easy it was to nudge a properly made dob along, even at high power. So, the tracking requirement could be put on hold for the time being. That freed up money to go towards increasing the aperture a bit more at the start. Sonotube made a great tube and I was in business!! I spent several years with my 10" in that configuration and enjoyed learning the sky with my "window to the stars!"

As I used my scope, I learned more about what I considered important to improving my enjoyment. Eventually, the scope was to undergo the first of several rebuilding efforts that incorporated my constantly growing list of "really neat ideas". One of the first things I discovered is the need for portability. A 10" f6 is not small. It is considerably bigger than a 10" f4.5 (or seems like it anyway!) I also discovered my need to be able to rotate the eyepiece to more comfortable positions to better share the views with folks smaller than me and to make it more comfortable for me when looking at low elevations. These two requirements have been fundamentals for every version I have made since then! The result was the upper 1/2 of the tube was a serrurier truss and the lower 1/2 was a cage with a rotating tube inside. This was a major advancement towards the ultimate scope and was a GREAT improvement from the all sonotube configuration.

After a while, as more and more truss tube scopes were made, I noticed that they all had sacrificed the ability to support a rotating eyepiece capability for longer tubes and subsequently smaller end pieces (i.e. the primary box and secondary cage). So, I had to at least try it. The problem was my mirror did not weigh enough to counterbalance an f/6 tube and the rather heavy finder I had made (and was emotionally attached to), plus a telrad (the single greatest accessory I have ever purchased!!). This required counterweights. Ugh.... I ended up with a very compact scope when disassembled, but I also had 35 lbs of counter weights!! I also discovered how much I missed my rotating eyepiece! It was during this time I acquired a 14.5" f/5 Newtonian. It is the size of a 60 gallon hot water heater (and seems to weigh about the same a full hot water heater!) But the increased aperture was intoxicating and the figure on the mirror is wonderful! It was also the exact opposite of the tradeoff I had just made. It totally sacrificed portability but the whole huge Sonotube rotated in a cage in its dob mount, so I had my rotating eyepiece back! I found I could put up with the loss in portability for the increased aperture and rotating eyepiece! The addition of two detachable wheelbarrow handles with wheels helped relieve the portability crisis quite a bit. Many times since then I have considered moving up to something larger than my 14.5, but portability efforts seem to increase much faster than the limiting magnitude drops. Again, this is a personal trade, but I have found a 14.5" f/5 to be an excellent compromise between portability and aperture, and the f/5 to be a good trade between RFT/portability and high power/long tubes.

As I added more "good ideas" to the list to my ever growing list, I decided I would take my venerable 10" and build a prototype that I could test my designs on. If successful, I would scale them up for my 14.5". A major consideration was to eliminate counterweights! I also had discovered that I did not like the altitude bearings way down at the end of the tube near the primary. That causes the eyepiece to swing in a larger arc. That means you have to move off of your stool quicker when either tracking or looking at something different, but more importantly, the larger arc makes it harder to hide in a shadow to evade all the street and security lights (which you have to do if you are going to use your scope in the suburbs). These two things actually work to help each other, since the lack of counterweights requires the altitude bearings to be moved up to nearer the midpoint of the tube. As a side benefit, the tube is slightly less affected by wind. I also absolutely had to have the eyepiece rotate. Rather than have the entire tube rotate as with the 14.5", I had seen several scopes that had only the secondary cage rotate. The ability to maintain collimation was a concern, but accurate craftsmanship and a clever design would go a long way towards overcoming that (I hoped).

By this time I had designed and built several equatorial tracking platforms using a cylindrical bearing geometry. The tracking accuracy was quite surprising, considering the most precision tool required to fabricate the platforms was an electric drill! The addition of a tracking capability meant the old stirrings about photography had started again. That meant the design had to support a heavy secondary end, and the secondary mirror could not be the really tiny thing that you could get away with for strictly visual work. It also meant the focal plane had to either be far enough outside the secondary cage to allow a camera to reach focus, or the focuser had to be removable to allow an ultra low camera focuser to be attached in its place. To keep the secondary as small as possible, the changeable focuser was the best answer. In addition, the ability to remove the focuser helped later when I decided that all the pieces would need to "nest" within each other to make it even more portable.

The mirror cell was another element that could be optimized. I wanted to have the mirror to be able to be "sealed" inside the cell, since my scopes live in a dusty garage most of their lives. In addition, I wanted a 9 point flotation system (mandatory on the larger 14.5" mirror) and an orthogonal collimation adjustment configuration to make collimation super easy and quick. Orthogonal, in this context, means that the three collimation adjustment screws are not 120 degrees apart. Instead, they are on three of the four corners of a square. That allows one screw to make pure "pitch" adjustments, and another to make pure "yaw" adjustments. The third one acts as the pivot. All three are push/pull arrangements, with the push adjusters acting as "locks". The pull adjusters have a strong spring compressed between the cell and adjuster that steadies the mirror during collimation. The push screws then lock it in place. The mirror would be centered in the cell by 8 "cams". The cams were to be wooden dowels with a hole drilled off center so that when rotated they could adjust to hold the mirror in the center of the cell. A thin piece of cardboard inserted between the cams and mirror temporarily during adjustment would assure that the mirror would not be pinched too tightly. Larger fender washers on top of 4 of the cams prevent the mirror from falling out when transported.

The truss assembly was a problem. Even though the earlier serrurier truss version worked great, I did not like having a bunch of loose parts to keep up with when being assembled. And getting all 8 tubes to stay still long enough to attach the secondary cage was sometimes a small but annoying pain. And then there was the time I showed up with only 7 tubes after a long drive out to dark skies.......(at least I remembered the eyepieces that time!). A design breakthrough came one day when my wife bought a couple of unique folding chairs. They collapsed into a small cylinder yet were able to re-expand back into a chair instantly. This configuration of attaching the 8 tubes together meant I could have a center ring to hold the altitude bearings, and have TWO sets of truss tubes without a big pile of loose parts and virtually no increase in assembly time! A little thinking on how to attach the systems together with a clamping system that also had no loose parts made the fundamental requirements of no counterweights, very portable, and altitude bearings near mid tube very compatible and realistic. I think this configuration for truss tubes may open up all sorts of possibilities for design improvements for open truss scopes since short truss assemblies are now quite manageable without lots of loose pieces!

I wanted the entire scope to be able to "nest" together to minimize the space it took to stow it in the car. In order to do this, the rocker box was another area that had to be improved. One of the problems that appeared with the altitude bearings nearer mid tube was the rocker box became taller. All my rocker box designs disassembled by using hangar bolts and wing nuts. However, I was left with two relatively heavy sides, and a front board in addition to the ground board and base. By changing the sides to a "picture frame" design, using square tubing for the vertical pieces, and wood for the horizontal pieces, the two sides could lay on top of each other when disassembled and surround the mirror cell box. All of this could sit on top of the ground board. With removable feet on the ground board the whole assembly collapsed to about 7 inches high. To stiffen the sides, two flat aluminum straps clamp in place as cross braces on each side. The front board is also replaced with two flat cross braces and one piece that goes straight across. All pieces are the same length so they are all interchangeable.

The design for the bearing for the rotating secondary cage had to insure the axis of rotation was aimed exactly at the center of the primary mirror, the two bearing halves were exactly concentric, and there could be no play either longitudinally or radially. Accurately fabricating the secondary cage rotating bearings turned out to be much easier than I thought. I routed two circles 13" in diameter and 3/16" deep in two 14" diameter circular pieces of 1/2 inch birch plywood. I then coated the wood with West System Epoxy, and re-routed the circles. I repeated this about two more times. Then I cut the centers out of the circular pieces with the router to make two rings of plywood, with 14" OD’s, and 12" ID’s, with a 3/16" groove in one face. The faces with the grooves were solid epoxy, hard and smooth. I attached 9 small Teflon blocks into one ring groove and trimmed them to fit with very little clearance. The blocks keep the rings concentric, and the two epoxy faces provide two flat faces that prevent any detectable tilt as they rotate against each other when the rings rotate. The secondary cage is attached to the upper ring and the attach points for the truss clamps attach to the lower ring. U-clamps with nylon screws hold the two rings against each other and provide a friction adjustment. A Novak spider and secondary holder complete the secondary cage assembly.

The open truss assembly requires either a very long secondary cage, or a light baffle opposite to the focuser to keep stray light from entering the focuser. I opted for the light baffle, made of a lightweight sheet of plastic that was a "for sale" sign sold in a hardware store. It attaches with snaps to the inside of the rotating ring. A sock is really required to really improve contrast. It attaches to the front of the primary mirror cell box where the front plastic dust cover attaches, and to the lower fixed ring for the secondary cage. It is inside the truss tubes to prevent reflections from the tubles, and also leave the tubes accessable for use as handholds. The sock also helps keep the primary from dewing up since it is otherwise quite exposed (it also keeps June bugs from doing their little "buzzing on their head" trick on the mirror too!) In the absence of June bugs and dew, but in the presence of wind, I do not use the sock and the slight loss in contrast is worth the increase in stability. In a really dark location, I cannot tell any difference in contrast with or without the sock.

To insure the axis of rotation of the secondary cage bearings is pointed at the center of the primary, I either use a collimation eyepiece or a laser collimator I made (the subject of a future article perhaps), to collimate the system. Then I rotate the cage 180 degrees and notice where the laser dot moved. I losen the clamps holding the secondary cage assembly to the upper tube assembly and take out half of the observed movement, and then readjust the secondary. I repeat the process and the error in that plane is virtually gone. Then I rotate the cage only 90 degrees and adjust the tilt in that plane, followed by another 180 degree adjustment as before. This all sounds complicated, but I assure you it takes much longer to tell about it than to do it!

As shown in the photo at the top of the article, the two boxes that the telescope stores in also double as an observing station with the addition of four screw-on legs. One box serves as a table, the other as a seat. The configuration in the photo has my 8mm camcorder attached to a bracket that allows it to be used afocally with a 32mm 2" erfle. With the scope sitting on my equatorial platform I can sit at my "desk" and study the moon in the small TV with books and charts in front of me. I also take time lapse TV shots of Galilean moon events such as shadow transits and eclipse disappearances and reappearance’s. Altogether, pretty lazy and GREAT at star parties!

With the Primary Mirror cell already only a small box, the natural addition to the system was an even more spartan configuration that would all fit into a cloth tote bag, or into a small suitcase for checking aboard an airline. Two altitude bearings that use the same clamps as the truss assemblies are used. A single square aluminum tube that comes apart in the middle holds a lightweight Novak helical focuser. The same secondary mirror removed from the other secondary cage with a wing nut quickly mounts in the lightweight secondary cage. An ultra lightweight Rigel finder mounts out of the optical path onto one of the aluminum spider vanes , and a light weight plastic sheet that snaps on to the secondary mount provides a light baffle for the focuser. The ground board and base are 1/2 inch plywood and due to the extreme lightweight of the scope are more than adequate. The square aluminum strut is a bit springy. However, a light touch with your fingertip instantly damps the vibration. When sweeping an area, the springiness takes a bit of getting used to since the initial motion of the scope makes the image go opposite to the actual push till the initial stiction of the bearings is overcome, then it rapidly catches up. After a few minutes you tend to forget it even does it. Perfect collimation is hard to maintain, but the extreme ease of transport more that offsets that problem (I wonder how many perfectly collimated scopes end up being left at home because they are too much trouble to take places when slightly more crude, but more easily transported scopes would be constant travelling companions?

The success of the prototype effort with my 10" f/6 convinced me that this design could be easily scaled up for my 14.5" f/5, and in fact I have finished that project and it works GREAT! Set up time is similar to the "normal" serrurier truss designs. That means the end of lugging around my big water heater! Of course, by the time I finished these projects, the list of new ideas has grown even longer!!! But in the meantime, my 10" is one step closer to my version of the "ultimate telescope"!