I started on the mirror box yesterday, Sunday, February 19.
I’ve been concentrating on the mirror cell for weeks now, and have more or less completed work on the basic cell (which is virtually identical to the Kriege/Berry cell described in The Dobsonian Telescope). I still have details to work out about mounting the collimation motors.
I made the split bolts for the mirror sling on Saturday, finding, to my surprise, that 3/8ths stainless steel rod is quite easy to cut with a hacksaw. Initially, I used a jigsaw, but I had picked up several hacksaw blades and thought I might try one out, figuring it would do a better job for cutting the 2-inch splits. The hacksaw makes a cleaner, faster cut than the jig saw.
While I closely followed the Kriege tailgate plans, I knew from the start my mirror box would be nothing like the Kriege design because I wanted it to fit in the outside storage compartment of our RV (meaning it could be no more than 9-inches tall). In addition, I have never liked the idea of installing the truss pole mounting sockets on the outside of the mirror box. I understand the arguments for doing so, but think the disadvantages outweigh the benefits. The altitude bearing compromises, in particular, are annoying. To fit the mirror box into the RV compartment, I realized the altitude bearings would have to be removable. I found several impressive German ATM designs that incorporate this feature, both of which have inboard truss pole sockets. (See the web sites of Achim Strnad and Stathis Kafalis.) To accommodate inboard truss tube sockets, I made my tailgate wider than the standard Kriege design. Kriege allows for 1-1/8 inches on either side of the mirror. My tailgate is 26-3/4 inches wide, allowing 2-3/8 inches on either side of the mirror.
StarMaster telescopes are another source of design inspiration for my 22-inch Dobsonian. Based in central Kansas, StarMaster is a favorite among Astronomical Society of Kansas City members. In fact, every commercially produced large Dob owned by club members is a StarMaster instrument. They have a well-deserved reputation for excellence. Fortunately, I have a large collection of photographs of 20-inch StarMaster telescopes which I took years ago for a club newsletter article that I haven’t gotten around to writing yet. 
Despite the overall excellence of the StarMaster design, the fact that the tailgate (with mirror!) is transported in a separate storage box and has to be mounted in the scope each time it is used gives me the willies. I can only think of two words to describe it — IN-SANE. (Apologies to Rick Singmaster, but I had to get that out of my system.) 
One of the reasons Rick went with this design option is that the mirror remains in its protective case while most of the scope (truss tubes, etc.) is set up. The mirror is mounted after everything else is installed, which minimizes the risk of dropping something on it. Separating the tailgate from the mirror box also lightens things up by separating the two heaviest components. I must say that the tailgate mounting installation is well-conceived. I have not heard of any tailgate mishaps among ASKC members (who have been using these scopes for years). Still, my 22-inch mirror is going to stay in one box!
One aspect of the StarMaster design that I particularly like is the truss assemblies. ASKC member Scott Kranz is shown above demonstrating how easy the StarMaster truss assemblies are to handle. Instead of using separate poles that must each be installed in truss sockets, StarMaster scopes use two-pole assemblies permanently fixed in base and secondary fixtures that are screwed onto the mirror box and secondary cage during scope assembly. The advantages of this design from both construction and setup perspectives seem overwhelming to me. I had initially thought of constructing inboard pole sockets similar to those used by Stathis and Achim. After considering how to go about doing it, though, several issues caused me to reconsider.
First, I didn’t want anything projecting above the light baffle mounted on top of the mirror box (except the dust cover.) This was important to keep the box under 9-inches in height. If you study the images on Stathis and Achim’s websites, you’ll notice that the bicycle clamps used to lock the pole sockets are inside the mirror box, which would rule out the light baffle or require a slot through which the clamp would protrude. Not an option for me. I also don’t like the idea of manipulating clamps and other hardware near the exposed surface of the primary mirror. I considered several ways to use inbound pole sockets with clamps on the outside of the box. Rather than using bicycle clamps, I thought about using stainless steel hex-head socket screws. I’ve found large hex keys easy to use in the field and actually prefer them to finger knobs. The details of making the socket clamps externally accessible seem messy, though. You end up either with eight socket head screws (two per mirror box side located near the corners of the box), or four socket head screws at the angle of each corner.
I leafed through my Dobsonian idea file while pondering this and came across my StarMaster image cache. One look at the StarMaster trusses reminded me of the advantages they provide and I dropped the idea of using truss sockets altogether. Fortunately, I made the tailgate wide enough to permit the truss bases to be screwed to the top of the mirror box while the dust cover is still in place (pretty much a requirement from my point of view). Here are a couple more pictures showing Mike Myer installing StarMaster truss assemblies. Note that the tailgate and mirror are still safely in their storage box while Mike assembles the mount.
With the truss design determined, I was finally ready to start constructing the mirror box. I decided to use 3/4-inch Baltic birch plywood rather than 1/2-inch for the mirror box. It’s not really 3/4-inch, though, more like 16mm, a couple millimeters shy of 3/4-inch. Baltic birch is made to metric dimensions, but American lumber companies typically refer to it with English dimensions. Some time I might try asking for 16mm Baltic Birch to see whether they know what I’m talking about. It wouldn’t surprise me if they did: a lot of native cleverness often lurks behind the good ‘ol boy front these guys put up.
Anyway, I decided to use the thicker plywood (after hauling in a sheet of 12mm!) for several reasons. First, I wanted to inset aluminum angles to dress up and protect the box corners and felt that the thicker material would make a stronger corner joint (see details below).
Secondly, my tailgate will not be fastened to three sides of the mirror box. It will be fastened only to the altitude bearing sides. This is because I made the tailgate wider, but still wanted to have the top bumper positioned correctly in relation to the mirror without fussing around with some sort of extension for it. I suppose I could have accomplished this easily enough by running a section of square tubing centered on the center rung up to the top rung of the tailgate. Then I could have positioned the top bumper where ever I liked. I didn’t think of that earlier, though, and planned on mounting the tailgate with four 3/8-inch stove bolts on each side of the mirror box (to ensure rigidity). Counter sinking for the 3/8-inch bolts, which are flathead, would only have left about 1/8-inch between the bolt head and the inside of the mirror box. Pretty close.
Finally, I need to move the truss assembly bases I’m planning to use as far out from the center of the mirror box as possible to allow space for the dust cover. Making the sides thicker contributes there, too.
I’m shown above cutting a rebate on one of the mirror box sides, which I cut to 8.75 inches on the table saw before setting it up with the dado blade set. I cut the sides to length on the table saw as well, using a large cross-cut sled. I’ve found this setup to be more accurate than my 12-inch DeWalt sliding compound miter saw, which is good for framing and interior trim carpentry but not accurate enough for furniture making in my experience. 
I considered using a finger joint (also called a box joint) for the corners of the mirror box. It’s a strong joint and would be appropriate in this case, given the shallow design of my mirror box. But when I decided to use the aluminum angle corner treatment I gave up on the finger joint, which can be fussy. Instead, I used a rebate and dado joint which is much easier to construct than the box joint and quite strong given its simplicity. Most of the mirror box strength comes from the steel tailgate and from interior corner bracing anyway. In the picture below I’m cutting the corresponding dado for the second side of the corner. Two mirror box sides have rebated ends and two have dadoed ends.
Here is a picture of the mirror box before the aluminum angle corner treatment. I did a dry assembly with the dummy mirror for a reality check. I decided to mount the tailgate 1.5 inches in from the back of the box, allowing room for the collimation drive motors and for a removable bottom for the box (so I can seal it when the scope is not in use). I set the tailgate on scraps of 2×4 to raise it to the correct height. The box looks great so far. This shot doesn’t really give a sense of the box scale, which is pretty large. I know it will fit in the storage compartment, because I’ve done the measurements, but it seems pretty big for that space somehow. As soon as I get the box bolted together, with the carry handles on it I’m going to take it out to the RV for a test fit.
Here is a close-up of one of the mirror box corners with the aluminum angle in position. I cut the aluminum stock to 8.75 inches with the miter sled on my table saw. (You can actually see the sled in the background of the wider angle mirror box picture below.) Then I set a piece of it on top of each mirror box corner and traced the outline, extending the edges along the width of each box side with a machine square. I used these marks to set up rebated cuts on my table saw and made them to a depth of 1/8th inch, bringing the aluminum angle surface to the level of the box side. I trimmed the corner with a round-over bit in my router table. I plan to attach the angles with either #8 or #10 stainless machine screws, four per side. When I’m ready for final assembly, I’ll glue the whole thing up as well. The aluminum will be finished with fine silicon carbide paper to give it a brushed look and then several coats of clear lacquer to keep it bright. The purpose of the aluminum is to protect the corner and give it a finished look.
