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ATM: 8" f/6 Rotating Dob
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My 8” F6 Rotating Tube
All telescopes have a story behind them; all ATM built scopes seem to have more than one. This is mine. I started in this hobby in 2002 when my wife took an astronomy course at the local community college and then decided to join our local club. For Mother’s day that year I gave her a pair of Oberwerk 15x70 binoculars. One night later, I was hooked. I started my hardware collection with a little 4.5” F8 reflector but as everyone knows, aperture fever soon took over. An upgrade to an 8” scope seemed like the right next step.
I began the project by doing the usual research, pricing components and weighing my options. One night as I’m trying to decide between the E-bay 8” F4 and the Orion 8” F6 my wife, looks at me and says “Spend the money to get something you will be happy with. If you put all this effort into building a telescope and don’t get something you can be proud of you will just be disappointed.”
She’s the best.
The next day I called Mark Harry and ordered an 8” F6. I’ve never looked back.
So now that I have a premium mirror on order I knew I would need a premium mount to do the mirror justice. I also knew that with the Missouri weather I would end up spending more time looking at the telescope than looking through it. After many hours of web searches and some hints from the folks over at Astromart I found Black Knight Telescopes, run by Paul Stock. If you hang around on enough mailing lists for long enough, just about every telescope maker out there will surface after a while and one day I saw a post from Paul describing a couple of his main secrets. He described how he makes his tubes look so good and how he makes them rotate in the cradle (more on that later). I took that chance to contact him and ask a few questions. It was at about that time I realized what a unique world I was about to enter. TMs, professional or amateur, are always more than ready to share their ideas with anyone who asks. Paul was very helpful and offered me many of his trade secrets while asking nothing in return except that I give credit where credit was due. Thanks Paul.
As a builder I don’t generate many original ideas myself, but I can copy other ideas very well. What follows is a description of what I consider to be a copy of a Paul Stock Black Knight Telescope. The key features of this telescope mount are its height and the tube finish and ability to rotate. The wood finish makes everything look nice and really tops things off but it isn’t what makes this scope a dream to use. The Mark Harry primary goes without mention as a fabulous mirror. I get comments on its performance almost every time I take it out. I am still amazed at what is visible in an 8” mirror.
So what makes the tube look the way it does? Simple, post form Willsonart. Willsonart makes the ATM staple Ebony Star. They also manufacture most of their other patterns and colors in what’s called post form thickness. Post form is flexible enough to wrap around a post (or tube) without cracking or breaking. The trick is to cap both ends with a stiff ring in order to prevent splitting. In my case I used a plywood circle notched with a router to fit over the tube ends. The laminate is held to the tube with a row of pop rivets along the seam. Because my scope is designed to be used either right-handed or left-handed I put the seam opposite the focuser. Next time I would put the rivets under the tube based on the users preferred eyepiece location. The following picture shows both the rivets and the top end ring.
The inside of the tube is flocked with black velvet. The fabric is held in place with Elmer’s Spray Glue. This glue works better than the 3-M products for this application as it doesn’t dry rigid, soak through and it can be worked for several minutes before it sets.
Possibly the best feature of the telescope mount is the rotating tube. Being able to adjust the eyepiece position by simply turning the tube to the appropriate location makes observing a dream. There is no need for an adjustable chair (I use a shortened bar stool) and I can sit for the whole observing session. The original idea came from Phyllis Lange in an Astronomy Magazine article a few years ago and was perfected by Paul Stock on his fabulous Black Knight telescopes. The mechanics are simple, if not a bit heavy. The tube rests inside a box that has holes cut in it larger than the tube diameter. Furniture pads placed on the inside of these cutouts allow the Willsonart to slide will very little effort. The altitude bearings are attached to the outside of the box. It makes for some nice square surfaces to attach everything. The handle attached to the top makes it easy to carry.
The tube is held in place with a plywood clamp ring, backed with Ebony Star that rests against three furniture sliders. Inside the ring is a strip of leather that allows me to put quite a lot of even pressure on the tube with the ring. I recently added an adjustable latch that makes setting the proper tension easy. This approach also allows for simple, in-the-field balance adjustments. Simply loosen the latch, slide the tube in or out and reset the latch.
Spider and Secondary Holder
Once again I need to thank Paul for providing the spider design. The plans had been on his website before he shut it down. The basic design is simple. Connect two fender washers with a central bolt about an inch apart. Use a dowel rod with a hole drilled in it for a spacer. Hold them together with three screws and have three more pass through them to adjust the secondary holder. The secondary holder is a piece of PVC cut at a 45-degree angle with another fender washer glued to the square end. That washer allows the central bold to connect the whole thing together. The key to the adjustment system is a rubber washer that goes inside the secondary holder and is kept in compression by the central stalk. It provides enough give to allow for easy adjustment but keeps everything stiff enough that you don’t have to collimate the scope every time you use it.
The spider veins are made from metal banding strap from your favorite home store that are bent at one end for the connecting bolts and attached to thumb screws at the other end to provide a mounting mechanism. My spider is attached with brass furniture mounts, but a simple º-20 nut would work just fine. The two pictures below show the spider and secondary holder just before installation and then after installation in the well-flocked tube.
Normally the focuser doesn’t get much mention but due to the special mounting for mine I thought I spend some time on it. The Kineoptics HC-2 is a wonderful focuser. Machined to extremely tight tolerances it works flawlessly. However, the HC-2 is best mounted on a flat focuser board. There are several methods for mounting the HC-2 to a round tube but the standard ones all involve drilling three precise mounting holes through the edge of the easily frayed Sonotube. During my tests every hole frayed to be unusable. I had been drilling the focuser hole first, followed by the mounting holes. I might have had more success drilling the holes in the opposite order, but I am happy with my solution.
The solution I used was to make a mounting block from plywood that would mount to the inside of the tube through a 3” hole. The block would then have a flat face toward the focuser that could have all of the mounting holes drilled on the drill press. The most difficult part was cutting the correct curve in the mounting block. The process involved setting the depth on my router to the correct curve radius and running the block through several times for each depth. Once everything was drilled and tested I glued the block to the inside of the tube and mounted the focuser to it. I’ve had no difficulties at all with the focuser mount in almost three years of use. Both pictures show the outside face of the mounting block. The inside of the block is covered with the tube flocking so it doesn’t show in any of the pictures.
The base seen in these pictures is actually the second base built for this telescope. The first was built from home-store quality birch plywood. After all of the “beauty” cutouts were made it was quite flexible and had a noticeable backlash while moving the scope. After several attempts to stiffen the base with aluminum channels and angles I decided that a rebuild was in order. This time a sheet of æ” Baltic birch was used. At one time I thought that the wood choice was overrated, but after using the higher-grade plywood I am convinced that you get what you pay for. Baltic birch is a joy to work with and it’s quality helped speed the rebuild time for the base. This base would end up being slightly wider than the old one because I laminated two pieces of plywood together to get 1 º” thick rocker box sides. The reduction in the flexure and the elimination of the backlash, even at high power, is remarkable.
The finish consists of a coat of white stain and then six (yes six) coats of gloss water-based polyurethane. I liked the ease of use of the water-based poly but since I used a painting pad for the application the finish has a few bumpy spots. I will probably go with standard polyurethane for the next scope.
The mirror cell is the standard University Optics 8” mirror cell. It works with no problems whatsoever.
Optics and Performance
A telescope is never really finished and this one could use a touchup or two to the finish, some of the baffling could be glued down better, the azimuth movement could be better and I want an equatorial platform under it so I can take full advantage of the Mark Harry mirror on Mars this fall. However, this scope brings a smile to may face every time I use it. A good telescope is a joy to use, and a good one you build yourself is even better. I can easily split the double-double at about 100x and my recent tour of the Hershel objects in Virgo confirmed that this scope is one worth keeping for years to come.
I am extremely happy with the results of this project. I’m a much better wood worker now than I was when I started and I understand a lot more about telescope making. The scope looks great and its performance is outstanding.
I would like to dedicate this article to my two youngest children, Elizabeth and John, who remind me every day that there is always something new to see and learn. Thanks kids.