Sunday, February 12, 2006: I realized this project would be a lot of work when I started it. I visualized certain construction steps, but in my mind’s eye the whole thing ran in a sort of fast forward motion from the beginning to the finished scope. A lot of details were left out. I think this is common. You don’t realize how much time specific details will require.
This image shows the mirror cell partially completed on my workbench amidst a chaos of tools and parts. I wanted to make progress on the mirror cell today, but I agreed to help clear up some clutter in our attic first. It didn’t take that long. Elly also wanted to get the kitchen floor scrubbed, which I hadn’t realized was on the agenda. We have a large kitchen, and I could not see leaving Elly to the task on her own, so I helped with that too. I started on the mirror cell around 11:30 AM.
The Kriege/Berry book lays out a specific plan for building a large dob, but you still have to figure out lots of stuff for yourself (even if you decide to follow their plan to the letter, which I am not doing). The mirror cell, in particular, is a complex component. Building it requires obtaining lots of hardware like all metal locknuts, machine screws of various sizes, washers and lock washers, etc. I spent a day and a half visiting Strasser Hardware, Home Depot, Office Depot, an Ace hardware store, and Metals by the Foot, collecting these parts and considering different options for the flotation pads that mount on the mirror cell triangles which support the primary mirror.
I found the EZ Glide pads shown in the Kriege book (on page 128) at the Ace hardware store. But I suspect the manufacturer has changed the design of their pads since the book was published 14 years ago (probably to lower production costs thanks to Walmart). Anyway, they weren’t extra-dense as described in the book. In fact, they are thinner and squishier than many similar biege felt pads. None of the pads I found were thick enough to provide what I thought was a comfortable buffer space between the back of the mirror and the panhead screw attaching the triangles to the collimation bars.
I experimented with screw-on rubber bumper feet, but at 3/8 inch these are thicker than necessary. Since I want to keep the mirror box height to a minimum, I need some way to trim the bumper feet down a bit. I tried cutting them with a razor blade but couldn’t get a smooth enough surface. So I drilled a recess in a piece of MDF scrap with a forstner bit, sized to leave an appropriate slice of the bumper exposed for cutting. This helped a little, but finished result wasn’t level. After going to so much trouble to make an accurate cell, I wasn’t about to mess the thing up with cockeyed flotation pads. I also tried sanding the pads on my stationary sander. This gave a smoother surface, but it still wasn’t level.
I abandoned the idea of using rubber bumper feet, and rooted around in a box of odds and ends I thought might contain some floor protectors I got after refinishing our hardwood floors a few years ago. I found a partial card of Feltac Faultless bumpers. These were a little wider than I wanted (1 & 1/8 inch in diameter), but they were both thicker and denser than any other pad I’ve found and they came with a convenient plastic mounting cup with a recessed hole sized for a number six machine screw. I thought they probably came from Strasser so I went back for another look. I didn’t find any with the other floor protectors, but in a moment of inspiration I looked in the Closeout aisle and found five or size cards of the Feltac protectors (8 pads per card). I bought three cards, enough for my mirror cell, and got them at 50% off (about $2 per card). I found a website for the Canadian company that makes these, Madico, and they still list the product in their catalog.
Having worked all this out on Friday and Saturday, I planned to get the flotation pads installed today. Doing so required drilling out three mounting holes at the apices of each of the six mirror cell triangles. Fortunately, I centered punched the apices for each triangle when I had the paper templates attached (just in case I decided to use machine screws to mount the pads). After drilling out the triangles, I mounted the flotation pad holders using 1/2-inch #6-32 stainless steel flathead machine screws with split ring lockwashers and hex nuts. I also torqued all metal locknuts onto the collimation bolts, mounted the collimation bars, and fixed them in place with type-C insert lock nuts (which are thinner than standard), making sure to leave enough play so the bars can angle up and down. The triangles are attached to the bars using #8-32 1 & 1/4-inch machine screws with hex nuts and split ring lock washers, again leaving sufficient play so they can self-adjust to the back of the mirror.
After finishing that, I put my 22-inch mirror dummy on the cell and cut three lengths of 3/8-24 stainless steel threaded rod for the mirror bumpers (which keep the mirror from bouncing out of its cell during transport). I determined the size for these, 6 & 5/8-inches, by raising the mirror to its maximum collimation height and threading a long rod into the cell with the retaining clip assembled on it. I adjusted the height of the clip to give sufficent spacing above the surface of the mirror. The bumpers and retaining clips aren’t actually supposed to touch the mirror. After cutting the rods to length, I threaded on all the necessary parts (picked up while searching for the flotation pads) and mounted them on the cell. (Note: the mirror bumpers are not shown in the image below — I am thinking of making them from wood dowels coated with rubberized paint.) I took a brief lunchbreak with Elly at 1:00 PM and finished around 4:00.
