The Equatorial Platform: Suddenly Simple?

I have been contemplating an astronomy project for a couple of years now. Well, maybe two projects. Okay, one of two projects: either a split-ring mount for a moderate-aperture telescope, or an equatorial platform for any scope.* While I had a good mental image of the split-ring design, I had no telescope that was a decent fit: the 8-inch f/3.65 was too small, and the 22-inch was way too big for the ring system. On the platform side of things, I had no mental image at all. That meant paying good money for a ready-made platform, or a lesser amount for plans and hints. I just couldn't decide which way to go - until it hit me.

*Detour: what are these things??
Simply stated, they are telescope mounts that greatly simplify tracking stars with less-sophisticated astronomy equipment. They allow for use of just one motor to keep a star fairly well centered in an eyepiece, keep stars in one place while taking photographs, and allow both hands to stay free for sketching, munching or scratching one's head in puzzlement. Even without a motor, it allows a simple movement to recapture a field that has drifted out of the eyepiece. RCA's Howard Banich has his 20-inch telescope perched on an equatorial platform, while the JMI telescopes demonstrate a high-tech version of the split-ring concept. The split-ring design can be set up for longer tracking, while platforms are limited to about an hour to avoid tipping the telescope over. The platform's advantage is that it is a platform, on which nearly anything can be set; its versatility compensates nicely for its time limit.

So what hit me? While wandering about the house, doing nothing in particular (something I'm very good at), I was visualizing both designs simultaneously. The platform somehow mimics rotation about a cone, or some such thing, while the split-ring is based on a rotating circle whose central axis points at the north star. Wait a moment.. if the upper 75 percent of that circle were cut off, and a slab of wood attached, wouldn't that be an equatorial platform? It would, if the other end of the platform had a post pointing at the north star AND passed through the central axis of the phantom circle.. Was it really that simple?

Less than one week later, my astro-platform [Phase One] is sitting in the driveway! This rough draft uses simple parts to prove the math, thereby ensuring that another project awaits me next winter! I have a photo-tripod sitting on the platform, holding a 2-foot piece of thin tubing aimed at the sun. Once aligned, I walked away an hour ago; in theory, moving the platform should bring the sun directly through the pipe again without further effort. Even without precise alignment, all appears to be working perfectly. It works for over an hour, so I may have to put some stops in place to reduce the potential of tipping over one of my scopes.

The work began when I cut a piece off each side of the circle (all right, circles don't have 'sides'.. I made one cut, then another parallel to it on the opposite side.. better?), one at 5¼ from the edge and the other at 4½, using my power saw at a 45-degree angle. Do the math and these numbers make some sense: the difference between the cuts matches the thickness of the plywood, which is how 45° angles work. I bound these two strips together, and they make up the lower arc of the phantom circle. (For the first time in my construction career, the two strips fit together perfectly, just like I calculated!) I then trimmed the big plywood back to symmetry so it became a capsule/barrel-shaped board that would become the top of the platform. An added benefit of this trimming is a narrow strip of beveled plywood, which can sit behind the arc for added strength.

With a circle of radius 17+29/32 and a 45-degree angle, the post that points to the north star lies 25.32 inches from the base of the circle (more trigonometry). This is another item that can be done many ways; for Phase One it's made of ¼" galvanized pipe. Why? Because the local hardware store had flanges, short pipes and [surprise!] a 45-degree angled fitting. The flange is mounted on the platform at the correct distance and screwed down its threads until it's aligned with everything else on the platform.
I'm not entirely convinced that anything more than a properly-spaced marble is needed for the pivot-point; since I had gone through the trouble to get the 45-degree setup, that's what I am using..

Something needs to be placed between the arc and platform base for the arc to roll smoothly. The simplest method is the traditional Teflon/Formica combination; if the system hopes to become motorized, a roller has less drag for the motor to work against, and a simple brake can adjust for varying loads with or without a motor. Roller choices have expanded, with roller-blades competing with skateboards for easily-purchased wheels. To my surprise I found that several shops that cater generally to young adults sell wheels; good luck deciding exactly which wheel has the performance characteristics needed for astronomical equipment. Once again, the convenience of 45-degree cuts comes in handy; mounting the rollers can be done in several ways. For Phase One, I went for the cheapest method: furniture glides at the local hardware store, tacked onto pieces of 2×4 that were trimmed at the familiar 45-degree angle. I used three of these, two to support the roller as it spins and one behind to keep it firmly against the other two. The two for the bottom of the arc are angled slightly for best contact.

The platform needs to be moderately adjustable for observing at different latitudes; I managed to keep that part simple. By using two fixed legs (huge ½×3-inch lag bolts dipped in paint work great!) and a long threaded bolt, the system rests on three points and can be tipped slightly north-south; all other adjustments will require shims [i.e. rocks, wood chips, or astronomy publications] By mounting a bubble-level and compass on the bottom board, one can quickly make a rough setup for 45° latitude (assuming you know your local offset from magnetic north!); after that, watching stars drift will work for finer tuning. Phase One construction is just a slab today, but the bolts will be added before it leaves the driveway..

I was amazed at how easy this project became.. after two years of thought that went nowhere. Once the mental image became clear, the rest fell into place. May all your projects go as smoothly as this!