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As I discussed in the first of my three blog articles about my new Orion 120 mm f/5 refractor (found here), I purchased it originally as a tube assembly only, hoping to use the telescope primarily as an alt-az instrument, at low power, to augment my 10 and 11 inch telescopes and give alternative views to many of the objects seen during my high altitude deep sky observing sessions. It is not practical to try to use my large aperture Schmidt Cassegrain telescope at 3-4 power per inch of aperture; but the 4.7 inch short focal length refractor is ideal for such views, providing high contrast sightings of large diameter objects.
But, the refractor was sold either as a tube assembly, or with an equatorial mount. My experiment in modifying an existing Orion alt-az mount was unsuccessful for a tube assembly as heavy as the 120 mm (which is more than three times the weight of my Orion ST-80, used during the test setup shown below, my first try at fabricating something like this more elegant version.)
Discovering that I did not have a satisfactory alt-az mount that would allow the new larger refractor to reach the zenith, I decided instead to convert the purchase of the Orion tube assembly into the complete instrument with equatorial mount (as an Orion AstroView 120ST EQ.) Furthermore, my research indicated that few such alt-az mounts (which could point at the zenith) were available; the ones that I could manage to get by mail-order were incredibly expensive (and marginal with a massive and heavy telescope tube and 2" star diagonal with finders; most of them are optimal at best with an 80 mm scope.) It occurred to me that a pipe mount arrangement could suffice, especially since I had the very heavy duty tripod that came with my Celestron NexStar C-11. I could either use the refractor by itself, on this tripod, or in conjunction with my 10" Dob. If I wanted to use it with the Schmidt-Cassegrain, I would eventually have to acquire another tripod (such as perhaps this one, a slightly lighter duty model for the Celestron NexStar 8i, at a very affordable price.) In fact, I already had one of those tripods: it came with my Orion f8 refractor using the Sky Pro equatorial head:
This tripod was perfectly stable with a 120 mm scope, and would do nicely to serve as a platform for the alt-azimuth pipe mount I was conceiving with my 'mind's eye'. However, it was in permanent use in my "home" telescope, setup in the garage for quick and easy planetary observing. I could buy another tripod; but first, I'd try my idea out and perfect it with the C-11 tripod to test my concept. I've had lots of experience building and finessing pipe mounts, dating back to my earliest years as a budding observer and ATM in the 1970s.
My friend Rich Page built a pipe mount for my first large reflector, a 10 inch Newtonian, which I later duplicated for 8 inch and 4 inch scopes, as well as a sort of Frankensteinian binocular pipe mount system for my 11x80s. It's easy to do even if you have no fancy metal working equipment (other than a drill): just go to the hardware store's plumbing supply department and let your imagination run wild. Here's a nice looking German equatorial version built by Michigan amateur astronomer Dennis Allen. A more massive one by "Joeastro" looks very much like the original Rich Page mount for my old 10" f/5.6 scope. I soon found myself at the local Orchard Supply Hardware on Branham Lane in San Jose, trying out pipe nipples, flanges, and adaptors. The threads of such parts are cut very crudely and no two of them seem to fit exactly the same way. So it is always best to TEST EXTENSIVELY with the very parts you intend to buy. (This means also that you must fend off 'helpful' salesmen, who have no idea what you're doing -- and no comprehension if you TRY to explain!) If you can't fit the parts together before you select them for purchase, you'll probably need gritty rubbing compound (3M makes a recommended version) to work the threads so that they mesh smoothly. Luckily after a half hour of testing, I found pieces that fit together nicely. The major parts necessary for my alt-azimuth pipe head were these: • Two 4-inch "closet flanges" (or "field flanges") (Note that plumbing part alleged diameters are a matter of convention and are NOT exactly what you will determine with a caliper; the 'claimed sizes' might even vary from country to country. That's why it's advisable to try to test and put together the parts at your local hardware store or plumbing parts dealer. If my numbers given differ from what "fits" for you, then by all means adjust the sizes as necessary or practical.) Be certain when you buy the flanges to test them for flatness. Many of them have curved or irregular "bottoms" and may even have the threaded part cut at an angle that is off 90 degrees. Put a pipe nipple into the flange to test that. Then, set the flange down on the floor and make sure the bottom is perfectly flat. The Celestron C-11 tripod was already drilled and tapped with six holes for 5/16" coarse threads. These holes unfortunately did not line up with the flange but were arrayed slightly outside its periphery. Rather than modify the top of the tripod, I elected to use those existing tapped holes for bolts and large washers that could hold down the flange tightly onto the tripod top. I could have mounted the flange merely with hex-head bolts, but these would require a wrench to tighten them. Since I am always worried about losing tools at a remote observing site, I chose instead to use 2" long 5/16-coarse thread bolts, nuts, and fender washers, with a wing nut to tighten them down onto the flange. It takes only a moment to install them, and requires no tool. (It was necessary to file down the edge of the flange a bit at the position of each bolt-nut-washer so that the nut could fit, and serve as a "stop" to center the flange on the tripod top.)
The top vertical pipe shown bolted to the Celestron tripod top:
The neoprene washer -- which serves to create friction for the altitude bearing, keeping the scope pointed steadily when one focuses or moves its altitude to a new position -- did not quite fit the 1.75" OD of the pipes so I carefully trimmed its inside diameter with a pair of shears. Then the washer could fit between the altitude bearing (the end of a field flange) and the locking nut. An alternative to the washer, if you can't find one, is a 1.75" ID o-ring, which might be found at a nearby auto parts store if you're lucky (I tried both, and preferred the flat washer.)
I already had an 8" dovetail mounting plate, purchased earlier from Orion, to attach accessories to the Sky Pro mount, and had not used it for anything significant; so I decided to put telescope mounting rings on it for the 120 mm scope tube assembly (116 mm OD, Orion part no. 07372.) The holes in the field flange did not line up with the existing drilled and tapped holes on the dovetail bracket but it was not difficult to drill and tap new ones. The drawing above is not entirely accurate, as the dovetail plate is in reality much larger; and to get the scope to balance, I mounted it off-center: see the photograph several paragraphs below for a better illustration. My mounting balance was measured with my heavy right-angle finder, plus the reflex sight, and two extra tube assembly rings, in place, plus the 2" diagonal; your weight balance point may differ, so determine it experimentally before drilling the holes. WARNING! I have purchased several sets of these cheap but effective Chinese-made telescope tube assembly rings from Orion for various instruments, and had no difficulty with them whatsoever. So, I failed to examine carefully the ones I bought for this project. But I discovered -- AFTER the finish on the telescope tube was badly messed up! -- that one of the rings had a blob of goo on the isolating felt, which smeared all over my scope. It took me a good part of an hour to clean it off, and left the scope's paint permanently damaged and scuffed. I tried then to remove the now flattened gunk from the ring, but it pulled off the rest of the felt pad; the entire ring had to be returned to Orion the next day for an exchange. BE SURE to examine your rings' felt surfaces very carefully before installing the scope tube assembly! The altitude bearing only has to turn 90 degrees to move the scope from horizon to zenith, so the flange is not likely to be unscrewed from the vertical pipe shaft (especially with the lock washer tightened down slightly.) But the azimuth bearing allows the whole assembly to rotate all around the scope base; one can lose track of how much the azimuth bearing flange has been unscrewed, resulting in a disastrous accident, the flange coming entirely off the vertical pipe shaft, dumping the valuable scope, diagonal, finders, and eyepiece on the ground (and perhaps injuring the observer)! So it was absolutely essential to include a SAFETY STOP on the azimuth bearing. I conceptualized and sketched one, as shown below, using a long bolt attached to the top of the tee, inserted into the vertical shaft. At the end of the bolt a pair of nuts holds several fender washers in place in a small stack. Paddle bolts are fitted into matching drilled, tapped holes, and inserted and screwed down until the washers cannot move above a certain point, noticeably tightening the azimuth bearing and keeping it from unscrewing. During operation of the mount, you notice that the az-bearing suddenly becomes very stiff, and then realize that you have rotated it too far. (You may also use the paddle bolts to 'lock' the azimuth bearing though they won't hold it very tightly; but they can add a bit of friction if desired.)
The safety stop system required the following parts: • One 7" long 3/8ths threaded bolt Measure the length from the top of the bolt attached to the tee, to its end; position the flange on the vertical pipe; and screw it down far enough for safety, to the point where the threads start locking; back off slightly. Then rotate backwards a bit more than one full revolution. Using a ruler, mark the position on the outside of the vertical pipe that signifies the end of the bolt, and where you think the fender washers will have to be placed so that they are just BELOW the two paddle bolts. Take off the tee, and attach the nuts, lockwashers, and fender washers; position them correctly and tighten; replace; and adjust as necessary. When partially disassembled, the pipe mount system looks like this:
On the Celestron mount, with the counterweights from the Orion EQ-3 mount attached to the long horizontal 1/2" shaft, and the altitude bearing/tube rings on the opposite 2" shaft, the entire system looks like this:
You will note that I have attached an old handle, for convenient scope pointing, that I took off a small camera table tripod I found in my junk parts box. If you don't have something convenient, you may fabricate one from a small alumimum rod and other necessary parts; or just move the scope by grasping the tube assembly. The entire telescope, with the finders, looks like this (I'm not going to bother to paint the pipe mount, though some craftspersons might indeed desire to make it look as elegant as possible by doing so):
As you can see, the 2" star diagonal clears the top and legs of the tripod, which even the new Orion "Versa Go" system does not quite allow with this tube assembly (I tested it recently at the local store, using an Orion 80 mm refractor and 2" diagonal: no, sorry. No pointing RIGHT at the zenith!) Here is a picture of Regina trying out the system to see if it was comfortable for her:
She found that with either our 'drummer's stool' or the expensive Orion Quick-Adjust deluxe chair (No. 05939: at the moment, they have the wrong picture in their catalogue, but both are shown here), positioning of the scope in all directions was a breeze, and she did not have to crick her neck to see objects at the zenith. Depending on whether you will look mostly straight UP, or lower down, you may have to adjust the tripod leg extension to a compromise distance; so far I've found that for my own height, they need to be out about 6-8 inches (and I do have to stand up to see into the eyepiece when the scope is pointed right at the horizon, though most of the time I can sit conveniently at a chair height that only has to be adjusted a couple of inches for perfect comfort in order to sweep my scope all the way up to the zenith.) I have now tried this system at my mountain top observing site on several occasions, and find that it works very well at "non planetary" powers of magnification. One adjusts the locknut on the azimuth bearing until there is a little pressure on the washer so that the movement in altitude is slightly stiff. This makes tracking easy, and also allows one to adjust focus without moving the scope off the correct position. I have had no particular difficulties observing up to about 120x to 150x with this system, though I prefer to use an alt-az mount at lower powers. In order to test my refractor's planetary performance, on 14 October 2007 I used it at 240x (with a 2.5 mm focal length eyepiece) to get the same magnification as my nearby 10" Dobsonian: viewing Jupiter with the alt-az refractor pipe mount wasn't too much harder than using the same power on the Dob, and the shaking during focusing was somewhat less jumpy. And, viewing objects at the zenith is MUCH easier than in the Dobsonian telescope. It takes me about five or six minutes to set this thing up, compared to the much longer time required for assembly, polar alignment, and balancing of the Orion EQ-3 mount that I got with the 120 mm telescope. And, the alt-az mount is much more stable, considering the mass of the wonderful tripod for the Celestron 11-inch scope! Observing objects at the zenith is supremely easy with this system, and fulfills my objective. When my budget permits, I will acquire a second Celestron tripod (as cited above) so that I can use the system in conjunction with my C-11. UPDATES, January 2008:Rainy weather: the perfect time for improving or modifying your telescopes! As I discussed in one of my earliest "Jottings" articles, the facetious essay "The Mount from Hell", I have a near-useless alt-azimuth mount that cost me $150, and won't point up to the zenith. Why not use the tripod from this mount for my new alt-az pipe mount? As I described in that article, I have modified the tripod to beef it up and increase the mass: the center stabilizer tray was changed, and the aluminum legs were filled with sand. While it would not be as heavy and stable as the massive tripod for my C-11, perhaps it would do for the much lighter 120 mm f/5 Orion refractor. And so I contemplated how to mount the vertical center pipe.
I found that I could bolt a heavy plywood board to the top of the tripod, with the alt-az head removed. Then I drilled four holes to fit the field flange, and countersunk them slightly with a paddle bit, installing 5/15-18 screws (2-1/2" long) into the holes (they could certainly be shorter, but those were the only ones I had on hand.) I bolted the board down to the center of the tripod head, and then attached the pipe mount flange, using matching nuts and lock washers. Because I hate to have to get out tools in the middle of the night, I attached a cheap wrench to the top of the board with a bolt and wing nut, so that it is always available in case the mount needs to be dismantled or further tightened. I made up a sliding counterweight for the 120 mm telescope, using some heavy steel washers weighing a total of one pound, attached by a 1/4" threaded bolt to an Orion telescope tube mounting ring that fits the tube assembly (I already had a set on hand from an earlier project.) I can slide the counterweight back and forth to compensate for heavy 2" oculars or light 1.25" ones. Finally, I modified my cheap Chinese "clone" or knock-off of the Telrad™ finder, which I had purchased a few years ago from Apogee Optics (a cheap and rather unsatisfactory $15 gadget that is no longer available.) I added an on-off toggle switch so that the unit could be set at a carefully adjusted brightness level by means of the potentiometer, and then powered by a separate switch (I also put a similar modification in the Telrad™ unit on my 10" Dobsonian.) I bolted the red dot finder to another matching Orion telescope tube ring with an angle bracket. I can rotate it for convenient positioning and minimal parallax.
To complete the improvements, I purchased a new Orion 9x50 upright, correct image finderscope during their winter 2008 sale, at a huge discount. This finder is identical to the one I use on my 10" Dobsonian, so I don't have to mentally adjust to an entirely different kind of instrument.
On the way home from the hardware store, I stopped by Big Lots and discovered that in the tool department they were selling a marvelous adjustable height padded seat -- called "Big Red" -- on a cart with four wheels, for only $25. Since my venerable Orion drum-stool was more than 20 years old and nearly worn out, this was an ideal replacement, and covered the same range of height adjustment, perfect for the alt-az refractor (except when it is pointed nearly at the horizon; then I either have to stand up, or rotate the diagonal to place the eyepiece at a lower angle.) I can't wait for the end of the current storm front and the next new Moon! After I took the photos, above, I decided that I wanted to improve the stability to prevent any tendency of the mount to tip over (especially in my windy observing environment.) So I removed the center tray and increased the spacing of the legs by drilling new tray mounting holes that were 1.5" further apart: now with the legs extended about 8 inches, there is a 36" separation at the ends: the same as my C-11 tripod. The wide overhang enables me to point the scope straight up at the zenith, with the eyepiece at a very convenient height. Update, Thursday 29 January 2008:
A careful test of the pipe mount system, switching between the Celestron C-11 tripod and the Orion tripod, revealed that the latter had a low frequency resonance of a few cycles per second, making focusing slightly frustrating at 240x. I traced this problem mostly to a flexure in the top piece that holds the tripod legs together, which is the bottom base for the detachable equatorial (or alt-az) mounting heads that match the tripod. To reduce this flexure (though I could not completely eliminate it) I purchased a 3-foot section of 3.8-16 threaded stock at Orchard Supply hardware, and ran it through the threaded center of the tripod top, and through a hole I drilled in the central mounting tray. Underneath the tray I placed a heavy board with a matching hole in the center, and tightened down the vertical rod with a wing nut (much in the fashion of the vertical support stabilizer in the C-11 tripod), which pulls the top and tray together, tightening the system significantly. The photo above also shows the tripod leg tightening rods, which are removable for collapsing the tripod for transport or storage. These "reversible" modifications reduced the resonance so that I was able to focus the 120 mm scope at 240x in order to study Mars on the night of 29 January, from my driveway, during a momentary clearing in the bad weather we've been having. Mars in fact looked better than I had seen it previously with this scope, showing the polar cap and distinct dark smudges. At lower magnifications, I found the focusing a bit easier than without the central supporting rod; but the massive, rigid top of the C-11 tripod is still a more stable mounting system. At least now I can use BOTH systems together at the same observing session. Warning! While I was testing my refractor setup, a most unpleasant thing happened, which reminded me that I had not yet made the necessary modification to the metal foot of the Orion finderscope. Even though the refractor's finder base setscrew was tightened down as hard as it would go, the finder came loose when I pointed the telescope straight up, and clattered to the concrete driveway three feet below, breaking off one of the plastic adjustment thumbscrews. I urge all of you who have an Orion finder to make the necessary safety modification: take out the setscrew on the telescope finder base; insert the finder; mark the position of the hole for the setscrew; and then use a drill to dig a small indentation into the soft aluminum of the finderscope mounting foot, as shown below in the picture I just took: |
