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How-to Make an Alt-az Pipe Mount
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How to make an Alt-Az Pipe Mount
By: Ging-Li Wang
Why make an alt-az pipe mount? 1) It is cheap. I made my mount for a 6" refractor for around $70. Commercial mounts rated for this load are $300 and up. Since the economy will stink for the foreseeable future, this is a nice option. 2) It's easy. I used a hand drill and handheld saw. Most DIY projects take a little tweaking, but my pipe mount worked the first time out. 3) Making stuff is fun. It's very rewarding when it comes together, and you'll know how to fix it because you made it! 4) It works. More on this later.
A little background: I live in Oakland, CA and have only been in astronomy for 2 years. I own 3 refactors: a Vixen ED100sf, Orion ST80, and a Celestron CR150HD. Mounts I own are a Vixen Porta Mount (altaz) and a Vixen Super Polaris (German Equatorial). When I bought the CR150HD, I wanted an inexpensive mount for it, so I looked into pipe mounts, which been around for a long time. Google "telescope pipe mount" for good advice and experience. I especially like Jeff Duntemann's Junkbox Telescope Gallery for its journey though time. These were the sources I consulted when I made my mount.
How it works: By "mount," I am only referring to the mechanical head that provides smooth motions for aiming your telescope. The tripod/pier will be something you make or already have. I attached my pipe mount to the wooden tripod of my Vixen SP. You can make altazimuth or equatorial versions. I chose the altazimuth because it has simple left/right up/down motions, like a Dobsonian reflector. It will not track the motion of the stars like an equatorial, but this is a simple undriven mount. The beauty of the pipe mount is its use of off the shelf hardware store parts. The pipe threads themselves are the bearings. You will need to lap the bearing threads for a smooth motion but it's easy.
FIgure 1. The parts: I chose 2" galvanized pipe fittings. They were the largest available at my hardware store, and they seemed the right size for my long 17lb. scope (not a scientific process). Based on the size of your scope, you can use larger or smaller size pipe. I've read galvanized steel is dangerous to ingest/inhale because of zinc content, so take precautions if you are planning to sand or file. 2" flange A and B: these are standard 2" flanges. Flange A has two additional holes drilled for mounting the scope (I didn't use the original holes because they are countersunk). 2" nipple A: nipples come in different lengths. The length of this one is important. Too short and your scope will hit the tripod at zenith; too long and you may tip over, not to mention needing lots of counterweight (CW). 2" Tee: this is a standard part that allows me to attach a CW opposite the scope. If your scope is light, you may not need the CW and use a 90 degree elbow to simplify things. 2" to 1/2" adapter: I felt most lucky to find this part because it has a 2" male and 1/2" female thread. It does the job of several reducers. 1/2" nipple: This is the CW bar. They come in different lengths; mine is 1 foot long. CW serves two purposes: it centers weight over the tripod legs and it helps the azimuth axis move smoothly. I use standard freeweights (with 1" hole) and secure them with a spring clamp. The weights do not need to fit snugly over the bar. 2" nipple B: it is 1 foot long because I wanted to get my long refractor as high as possible to help viewing at zenith. Feel free to use a shorter part if you have short scopes. Magnet: The magnet is convenient for holding the wing nuts that secure the telescope. It also holds an allen wrench for tightening the tripod legs.
The string: The string deserves its own paragraph. On the altitude (up/down) axis, the telescope is restricted to a 90 degree arc from horizon to zenith. There is no danger of the flange unscrewing and dropping the telescope. However, it is possible to lose track of the azimuth (left/right) axis as we pan around during the course of an observing session, resulting in disaster. Others have devised contraptions for preventing this mishap, and I have called on the simple string. It has a loop that goes around the CW bar, and the other end is looped around one of the tripod legs with a nylon clip. As you turn your scope, the string will wrap around the mount and stop you from unthreading the azimuth axis. I sized the length of the string to give me 3/4 turn in each direction, which is plenty of freedom.
Lapping: The two threads that are lapped are labeled in figure 1. All other threads are tightened and should not turn. I'd leave the CW bar removable for ease of packing and travel. Pipe threads are conical, not cylindrical (like a machine screw), so they will not fully engage when new. You will need to lap them for more engagement and smooth them out. To lap the threads, you can use valve lapping compound from the auto store. I just mixed some grit from 150 grit sandpaper (peel it off, smush it, remove paper fibers) with some lubricating oil. Apply the lapping compound to the threads and repeat cycles of tightening and loosening until you engage most (or all) of the female threads on the flange and tee. The motion of the threads should feel pretty smooth after lapping. After you're done, clean out the lapping compound as humanly possible, don't worry if a little grit is left. Lubricate the thread as needed.
Figure 2. Attaching the telescope: I considered attaching a dovetail clamp to the flange, so that I could hold other scopes, but I decided against it. It would add weight, cost and complexity. You can certainly make one or add a commercial model if you please. My scope rings are countersunk for 1/4-20 screws on the inside, so the thread protrude out. I drilled a couple of holes in my flange, making them big enough to easily line up the screws. A couple of wing nuts hold the scope plenty securely. I've found it easiest to mount the scope in the vertical position, so I don't worry about balancing the tube immediately.
Figure 3. Connecting mount and tripod: This was the hardest part of the project. I had to devise someway to couple the mount to my Vixen SP tripod, which has a single hole and a protruding spike for polar alignment. I made the adapter out of plywood using a handheld jigsaw (coping saw will also do), drill and files. The two layers are epoxied together. As you can see, there was a bit of filing and trial and error. My Vixen SP tripod is a bit light for this scope, but it was on hand. It is easy enough to make a pier out of a 4x4 or stouter pipe. There are also plenty of DIY tripod designs on the internet.
Under the skies: How well does it work? Good to very good. The motions are similar to my old Orion XT4.5 dob. My criteria for gear is simple. If I can observe for hours without fussing over my equipment, then the equipment is good. I've had this rig out 5 times, the last being a 3 hour session hunting down open clusters and galaxies in winter constellations. I was able to star hop with ease by first lining up the tube to a bright star and finding the object through my RACI. The azimuth axis is very smooth. The altitude axis is a bit stickier, because the scope is cantilevered on the threads (another reason to keep the scope close to center). I thought about attaching the CW rod to the scope flange, but it would complicate things. I can also try using finer grit to lap the threads for a smoother motion. As is, the motions are smooth enough to center and track an object at 200x. I'd like altitude axis to be as nice as the azimuth, but it is certainly functional. Compared to my Porta mount, the motions are similar except, of course, the lack of slow motion controls. The bottom line is I can observe for hours without wishing I had a better mount.
About friction: With the scope properly balanced, I can switch eyepieces without losing my aim. My lightest EP is a cheap plossl, my heaviest would be a barlowed TMB planetary. I can interchange them without rebalancing the scope. Mac McAvoy's portable mount webpage describes a system for adjusting friction on his pipe mount. I was have not found this to be necessary in my setup. If I ever needed to add friction, I'd try adding teflon tape to the threads, or something along those lines.
There you have it: a cheap, easy to make, relatively light, compact and functional altaz mount. It's probably not as nice as a Unistar or Giro mount, but it WORKS. Here's to all of us who enjoy our hobby on a small budget. Clear skies!