new base

After testing the previous mount, I came to a couple of conclusions. 1) The split trunion design that served as the foundation of my "ultra-compact scope" was going to require too much maintenance to keep it from feeling very rickety. The hinges were a little loose to start with and having any sort of gap in the altitude bearing surface is bound to cause the scope to jump around as you're aiming it. That could easily lead to it being essentially unusable in the field. 2) As the date of arrival of the (expensive, fragile, etc... ) primary mirror arrived, I began to reconsider the wisdom of not having it in a very secure box at all times. These conclusions made me think that the ultra-compact base was really not practical, so I basically remade the whole thing in the "classical dobsonian" style. Here's a picture:



You can see the same mirror cell and truss mounts within two boxes. The inner box is called the "mirror" box, the outer one is called the "base". The inner box moves relative to the outer box. Instead of the split trunions I was using earlier, I decided to just fork over the cash and get a pair of cast aluminum C-arms from Obsession Telescopes, and use them instead. They will rest on bearings attached to the base. I will probably just use the track rollers I have and see if I can avoid teflon pads altogether.

The aluminum C-arms are used in the classical style Obsession Scopes made by Dave Kriege, one of the authors of the guide book I'm using. These parts would be very hard to make by one's self, bending aluminum rectangular stock was the only way I could think of to come up with something similar, but that seemed like quite a headache and prone to irregularity. So I just forked over the cash, and I'm glad that I did! I haven't mounted the arms yet. To do so, I still need to drill holes through the 1.25" of solid aluminum, and am not looking forward to that process (especially since my drill is a wireless 18 volt Hitachi that I have a feeling will need to be recharged multiple times).

The mirror box is a very simple 2' x 2' x 1' box made of imported birch 13-plywood. Most importantly, it passes the "trunk test", which is that it fits laying flat into the trunk of a '97 Nissan Altima! The base is 30" x 18" x 12" and just serves to support the trunions. I'll also use the same lazy-susan style azimuth mount that I had earlier. I'll make some variety of lid for the whole thing too, but this is what it will look like when in use. I'm feeling better about this new base already!

primary mirror fabrication underway

Well, it's been a while since I've last posted, but in the interrim, fabrication of the key part of the scope has been undertaken! I commissioned Portland's Steve Swayze to make the primary mirror and he's well underway. His mirrors have an excellent reputation in online forums and people I've talked to. His prices are very competitive relative to other mirror makers. I won't say exactly what I paid but the index number of the check that I wrote him for a 50% deposit was the same as the amount of the check. Steve has been great about keeping me in the loop about progress on the mirror. It will be 16" diameter, f/4.5, 1.6" thick, and made from pyrex that was slabbed from a previous thicker mirror. Here are some pictures of the work in progress, copyright Steve.



These are the two slabbed pieces of pyrex. I don't know the provenance of the original glass, but it looks like it must have been from a 2.5-3" thick mirror or so. Slabbing it to 1.6" reduces the weight (it will be 26 lbs total), lets it cool quicker, and is generally not felt to cause the mirror to bend to any noticeable degree given the right flotation system supporting it.



This is the glass on the grinder. The grinding tool itself seems deceptively simple to me, it's just made of store-bought square tiles arranged in a circular pattern underneath the plywood disc. The gray material is "pitch", made of various different grades of abrasives dissolved in water. Using several grinding patterns and finer and finer pitch material, the glass is first ground to a spherical shape, then refined to a parabolic shape. There are a lot more details of this process that I haven't learned, so it's great that there are mirror makers like Steve who have!



The rough surface of the mirror status post some degree of grinding. At this point the shape still needs to be confirmed to be parabolic, and of course it needs to be coated with a reflective material.

Okay, I'll post more on the primary mirror as it progresses!

base improvements

After last week's dry run with the whole frame assembled, I decided that the altitude bearing wasn't light enough. I decided to buy a couple of track rollers from McMaster, along with brass threaded inserts, and installed them in the central beam of the base. I'm glad I did, because this modification makes it almost as easy to adjust the altitude as the azimuth. I can kind of adjust how much weight sits on the teflon pads by shimming them with washers, although it's not especially convenient to do this.



I also built the tripod-ish base that the azimuth bearing attaches to. Nothing too fancy there. The whole height of the base is a little on the tall side, but still not enough to require anything more than a step stool to use when viewing.

nearly-complete frame

Well, despite the lack of posts from the last month, I've continued to tinker away on the scope and now have it in a form that pretty strongly resembles the final product.



For starters I painted everything. Rather, I used a black gloss stain-plus-polyurethane product from Minwax called Polywax to varnish all of the wood components. This is nice because it preserves a little of the grain, which is attractive. Although technically an indoor stain, the polyurethane should keep everything waterproof to the degree that I will need it to be.



I had to bisect the trunions, in order to allow the scope to collapse down to a portable size. I had to buy two 8" strap hinges made by Stanley to allow the trunions to bend in the middle and fold back on themselves. I also bought two somewhat heavy-duty draw-latches from McMaster-Carr to attach to the inside to secure the trunions in their extended position while viewing.



The bisected trunion design actually presents some significant design dillemmas. First, the strap hinges are not load rated, and there is what looks like a plastic washer in the hinge which prevents metal-metal contact. This plastic washer could be the site of failure, depending on just how much compressive force it can take. For now, though, it seems to be able to hold the full weight of the optical tube as I've been testing it. I had no way of knowing this at the outset, though, so this caused me much consternation when I was trying to choose what type of hinge to buy. Regardless, I am happy with these. I would still rather have the torque applied across the 16" span of the hinge, even with its plastic washer, than have it exclusively borne by the wooden trunions.

The second dillemma is that the extended portions of the trunions flex a small amount, despite being secured with the draw-latches. I still have to make a cross bar to hold the trunions the appropriate distance apart. While not difficult, this is still another piece of the scope that I'll have to set up prior to viewing. Also, it was surprisingly hard to find the tube-connecting nuts that I'm using for this bar. These nuts look like the plastic things on the end of ski poles, but they are made of metal and have a nut in the middle. You jam them in the end of a tube and they stay there and let you thread a bolt in parallel to the tube. Very useful, but not a single hardware store clerk in all of Seattle had ever heard of them.

All in all, the trunion issues makes me reconsider the ultra-compact design. Oh well, I can always build a classical mirror box in the future!

Next, I made the altitude bearings. I bought a 1/4" thick 6" x 6" teflon pad from onlinemetals.com. I sliced this into four 3" x 3/4" sized pads, and secured them to the base with flat head screws. The pads are about 20" apart on the base, which makes for a nice wide margin of error. If the center of gravity moves outside the pads when rotating the scope, it will tip over. So it's nice to have a relatively large margin of error, even though I think my estimate of the CG location is actually pretty accurate.



I then bought a couple of cheap vinyl wall-base panels, which I sliced into 3/4" x 20*pi/2" strips. I epoxied these strips to the trunions, using a Loc-tite gel-epoxy product that seemed to work well. I don't know the precise coefficient of friction between vinyl and teflon, and I think melamine-teflon may have it beat, since that is what is recommended in K&B. However, I couldn't find any melamine laminate, and I don't think it could be that dramatically different anyway.

That said, in my current design, the friction of the altitude bearing is significantly greater than that of the azimuth bearing, which, again, is a ball-bearing containing lazy-susan from McMaster. There are reasons why this may be nice. Tipping over is a significant risk, and is more likely if altitude bearings let the tube descend too quickly. Plus, at high latitudes such as Seattle (47 deg north), the azimuth bearing is more closely aligned with the Earth's axis than the altitude bearings are. When tracking objects as the Earth rotates, it's therefore more important to have an easy azimuth bearing. Again, that only applies at high latitudes. The opposite would be true at the equator, and as for San Diego (32 deg north), one would probably want both bearings to be easy! For that reason, I will probably purchase some track rollers from McMaster to use for the altitude bearings.



I finally drilled the truss tubes to secure the tube adapters in place. I will have to do this again after shortening the tubes when I know the exact focal length of the mirror. The whole optical tube is very rigid now.



That's most of the frame of the telescope. I still have have to make a tripod for the azimuth bearing to sit on, as well as the trunion strut, but these are pretty simple to do. I'll post about the optics next, but suffice it to say for now that the primary mirror has been commissioned!

trunions & base

I made some progress on the trunions and base this week (ie, the altitude and azimuth bearings). I'm still waiting for a couple of components from McMaster-Carr to get here before they will be complete.

The trunions are two large arcs of 13-ply wood. They are positioned so that their center is 14 inches above the mirror surface, which is approximately where the center of gravity of the optical tube will be. This makes the mount points of the mirror cell 17 inches from the center, and so I made the inner and outer radii of the trunions 14 and 20 inches respectively. Hopefully this thickness will be sufficient to support the 60 lb optical tube - if not I may need to add some aluminum struts. I will eventually need to put a hinge and latch in the middle of each trunion, since there's no way this would fit into my car at the present time. This issue makes me debate the merits of an "ultra-compact" design without a dedicated mirror box, vs the more traditional Dobsonian design, but I still think I'm happier this way.

The base is a a relatively simple palette-like structure, using the complementary pieces of plywood left over from making the trunions, held together with framing lumber 2x4's. It will eventually sit on a lazy-Susan turntable from McM-C, en route currently. The footprint of the base is 24 x 28 inches.

At some point, I need to buy the teflon pads and plastic molding that makes the altitude bearing smooth to maneuver.

truss tube & focuser board

Several components for the truss tube and focuser have arrived. Here they are assembled:



I connected the truss tube to the mirror cell with three blocks of wood of dimension 1.5 x 2.5 x 3.5", cut right from a two-by-four (a misnomer, two-by-four's are actually 1.5 x 3.5"), and bolted to the mirror cell at a radius of 9" very close to 120 degrees apart. The inner diameter of the blocks is 8.25" which allows them to serve as bumpers for the mirror if it slides around accidentally. The blank in the photos is a 16.5" diameter piece of plywood left over from making the secondary ring, and fits nicely between the blocks. I still need to stabilize the blocks on the mirror cell, since now they are prone to rotating a little around the central bolt. I may do this by routing a pattern into the bottom so they will fit snugly on the metal frame.



I mounted three ball-and-socket joints from MoonLite Telescope Accessories (MTA) to the wooden blocks, and another three to the secondary cage ring. These joints greatly simplify the design of the whole telescope, since they make the complex angles of the truss tube possible, are are easy to dis/assemble. The tubes themselves are aluminum, 1.25" in outer diameter, 0.05 thick, and are also from MTA. "Do you know what the @#$% you can do with an aluminum tube?"



The distance from the mirror center to the ball-and-socket joint seats is approximately 10.4", and they are about 7.58 degrees apart at that distance. This makes the truss intrusion 8.55", just larger than the aperture of the secondary cage. By truss intrusion, I mean the distance of the truss tube to mirror center at its closest in projection. Picture a hexagon inscribed in a circle - the midpoints of the sides of the hexagon are closer to the center of the circle than the vertices of the hexagon are.

Right now the tubes are all 6' in length, but I will shorten them by about 1' after I order the mirror. I am planning to buy a 16" f/4.5 mirror but the exact focal length has to be measured after fabrication, so I won't chop the tubes until then. I also need to drill the tube caps to seat them a little more securely.

I also made a focuser board, dimension 6 x 8", with a 2 & 3/8" diameter hole centered at 5 inches from the end. The board made is 13-ply "import birch" 3/4" plywood from Dunn Lumber, same as the secondary ring. It will eventually hold the MTA focuser I bought, but I haven't attached it yet. I'm waiting until the spider and mirror holder arrive before I do any more with the focuser board. I've ordered these from astrosystems.biz and they should be here in a couple of weeks.



The fasteners for these components are (mostly) all from McMaster-Carr, which I again will recommend for their extensive but easily navigable catalog.

The assembled truss tube and focuser board have a very light but rigid feel. I am eager to see how things improve after the two improvements to the wooden blocks and tube caps that I mentioned. After I install the spider/secondary mirror holder, I will probably be able to get a good estimate of the weight of the entire secondary cage, which I need to calculate the diameter of the trunions that I will build next!

secondary cage

At peril to both my relationship and, no doubt, my lease, I have completed the secondary cage ring, shown below, in my wood-shop/living room. I'm planning on just a single ring, hopefully that will suffice for rigidity and focuser board stability.