Building a Bigger Telescope

The first telescope I built was a 6-inch f/8 sonotube Dobsonian. When I started building it, I had never even seen a Dobsonian telescope in real-life, only in pictures on the web. It turned out well, and gave me wonderful views. I took what I learned from building that scope, and put together a TravelScope that I could take with me on airlines. Once again, I learned a lot more about building and using telescopes, and the second scope was much better than the first!
Still, I was yearning for more aperture -- a bigger scope that would reveal detail in things that were only "faint fuzzies" in my 6-inch instruments, and let me see dimmer objects completely out of reach of the smaller scopes. I took what I learned from building the first two telescopes, spent over 8 months grinding a mirror, planning the design, and putting it together...Papa Joe is the result.

(A quick aside -- my grandfather, who all us grandkids called "Papa Joe," passed away in 1999 while I was building this telescope. When I was a young child, Papa Joe gave me my first look ever through a telescope...a 6-inch Criterion. I hope that somehow he's able to see this big beast that is his namesake, and smile when he sees how much joy it gives me. This scope is dedicated to Walter Joseph Nunley, 1911-1999)

The design of Papa Joe is mostly right out of the book, The Dobsonian Telescope by Richard Berry and Dave Kreig. These are the guys that make the commercial Obsession telescopes (the book can be purchased here). I'll go over the various parts of the telescope, going into the greatest detail when I've departed from the designs in that fine book.

Specifications:
12.5-inch f/6 primary mirror (ground and polished at home), 2.25-inch thick Pyrex
2.14-inch secondary mirror
Spider and secondary holder from ProtoStar
2-inch Crayford focuser from JMI
All ApplePly(tm) construction, 1/2-inch and 1-inch
Home-designed-and-built mirror cell made of 2x1 Oak
18-inch Altitude bearings, 1.5-inches thick (made from two 3/4-inch plywood rings bonded together)
1-inch diameter aluminum truss tubes

The Mirror Cell


Krieg and Berry use a steel mirror cell in their telescopes, and show how to make one in their book. Their scopes also use a sling to hold the mirror in place in the cell. This design is almost a necessity for a REALLY big scope (more than 16-inches), but for a 12.5-inch scope I prefer to firmly attach the mirror to the cell -- with silicone.
My mirror cell uses some of the Krieg & Berry design, replacing the steel struts along the bottom with good, solid oak 1x2's. This is cheaper than steel, almost as strong, and doesn't require any welding! On top of the cell is a 1/2-inch plywood disk slightly larger than the mirror itself -- on this disk I located points for a nine-point flotation system for the mirror, which is where the dabs of silicone will go when the mirror is put in place (you can see the outlines for the points in the photo above).
Collimation bolts go into countersunk holes in this disk, through slightly-too-large holes in the oak frame, and are adjusted by wing nuts on the back of the cell. Heavy-duty springs sit between the disk and the frame, keeping tension on the collimation bolts and holding the mirror in place once it's collimated.
The silicone holds the mirror VERY strongly, and supports it in a 9-point flotation arrangement. It all works great.
Since these pictures were taken, I've begun to add a small 12V DC fan to the back of the mirror cell to cool the mirror off more rapidly, even though the ventilation holes and open back allow it to cool down in less than an hour without the fan. I'm just impatient!

The Secondary Cage and Attachments


Again, the main part of the secondary cage is right out of Krieg & Berry. The main differences are the attachments to the secondary cage for the truss tubes. For the idea I used, I owe a debt of gratitude to Rashad Al-Mansour, a very creative ATM that I've had the pleasure to observe with a number of times.
The truss tubes attach to small wooden pieces that consist of a 2x2-inch piece of 1/4-inch thick oak, by 1/4-inch bolts that go through holes drilled in the top of the truss tubes. The oak piece attaches to the secondary cage with a snap-latch, purchased at a local hardware store. A small block on the top of the bottom ring of the secondary cage holds the catch for the latch. A 1/2x1/2-inch piece of hardwood (birch), 2-inches wide, is mounted inside the oak piece to ride on the bottom of the secondary cage for extra stability. This provides a very quick, easy, no-tools method of attaching the secondary cage to the truss tubes. Just insert the tubes into the mirror box (the tube tops stay attached to the oak piece when the scope is disassembled, and the two tubes fold together), set the secondary cage on top, and snap the latches shut. It's a great way to do it. Thanks, Rashad! See the smaller picture for a close-up detail of the arrangement.
The secondary cage is lined with 1/8-inch thick plastic sheeting that I purchased at the local TAP Plastics. It was $4 for a 4-foot square sheet. It's not "real" Kydex as Krieg & Berry recommend, but it has a nice flat-black side (the inside of the cage) and a shinier side that goes out. It holds its shape well, was easy to roll into place, and doesn't get white spots on it when it gets nicked like Kydex does. Highly recommended!

The Mirror Box and Altitude Bearings


The mirror box is all 1/2-inch ApplePly(tm) plywood, with inside dimensions of 14x14x14-inches. It's painted flat-black on the inside to cut down on internal reflections.
The truss tubes attach to the mirror box with split blocks, right out of Krieg & Berry's book. The blocks are made by laminating two 1x4-inch strips of hardwood maple together, then cutting with a table saw to make 2x2-inch blocks. Holes are drilled for the truss poles, a cutout is made at the back of the block, then the blocks are split lengthwise to make a moveable piece that, when tightened with the wing nuts, holds the truss tubes very securely. See Krieg & Berry's book for more details on making these split blocks -- they work great!
I deviated once again from the Obsession design with my altitude bearings. The traditional method, if you're not using cast-aluminum bearings, is to use full half-circles as the altitude bearings. This leaves the problem of mounting two of the split blocks on the outside of the mirror box, which go right where the altitude bearings are! Krieg & Berry solve this problem by cutting a huge chunk out of the bearing, and having to reinforce it from behind...I took a different approach.
My bearings are half-circles, but instead of leaving all of that extra wood around, I made them only 2-inches from the outside diameter to the inside. This meant that I could mount the altitude bearings on the mirror box and have them go right around the split blocks, with only a small notch cut out of them for clearance. I worried a little about the bearings not being sturdy enough, but a test proved they had plenty of strength. They're made of 2 pieces of 3/4-inch plywood laminated together, to form a 1 1/2-inch wide bearing surface. An 18-inch diameter circle was then cut out of this thick laminate using MANY passes of a router on a homemade circle jig (cutting about 1/2-inch at a time). Then the inside circle was cut out of the 18-inch circle, leaving a 2-inch ring. Finally, the ring was cut in half on the table saw, leaving two half-rings. The bearings were then notched for the split blocks, sanded and finished, and had Ebony Star glued to the outside surfaces. These bearings are extremely strong, do not flex at all, and look good as well!

Final Details


The Rocker Box sides and front are made from pieces of 1/2-inch ApplePly laminated together into 1-inch thick pieces for extra strength. The bottom of the rocker is covered with a sheet of Ebony Star laminate, and it rides on 1x1-inch virgin teflon squares mounted on top of the ground board (a 1-inch thick, 18-inch diameter pine circle). The altitude bearings also ride on teflon pieces mounted in the arcs for the bearings cut into the top of the rocker box.
All the wood surfaces were finished with 2 coats of Spar Urethane (for UV and water protection), then 2 coats of Polycrylic finish to give them a very nice, glossy shine. The truss tubes were covered with pipe insulation picked up at Home Depot for about $1 each. Finally, a cover with a handle for easy removal covers up the mirror box when the scope is not being used.
Papa Joe had his "first light" in January, 2000 -- and performed marvelously. A slight bit of top-heaviness (when using heavier eyepieces) still needs to be dealt with with some counterweight in the back end, but the fan I'm installing will probably take care of this. I'm also awaiting the arrival of digital setting circles that I've ordered, to allow me to more easily find those now-visible faint targets!
The scope has wonderful, smooth motions -- that "buttery" feel that owners of good Dobsonians always talk about with a gleam in their eyes. The effort to move the scope is also well-balanced between altitude and azimuth, making it easy to track objects and not worry about which direction you're pushing or pulling!
Compared to my 6-inch scopes, the view is amazing. Much more detail is visible in everything I look at, and objects I never saw before pop right into view (I saw NGC 2024, the beautiful nebula around Zeta Orionis, for the first time visually one night -- in the 12.5-inch scope it was obvious, but I had never seen it in a 6-incher!).
This was certainly a worthwhile project, and I'm very pleased with the results. I'm looking forward to many nights out observing with the big guy...

The alert reader will notice the same car parked behind Papa Joe and my very first 6-inch scope on my home page. My scopes have grown larger, but my car hasn't...however, I'm barely able to carry the 12.5-inch and my 8-inch SCT scope, along with my accessories, in my little Nissan. Time to start looking at an SUV...:)