Modular Altitude Azimuth Telescope Mount

Hi,

I am working on making a large diameter friction based Alt-Az mount, for my amateur astronomy hobby. My intent is to make each rotary axis modular so I can play around and change the configuration by swapping brackets etc.





So far I have only designed the rotary axis. It is 5" in diameter, made of two plates, one with bearings, the other with a threaded hole to receive a shaft. One plate has a groove cut in it to receive a strip of Rulon J bearing material(from my work). The two plates are assembled and a tension adjusting nut is used to control the friction. Each plate has a standardized 6 hole pattern that I will use to attach modular components and brackets.





I am using fusion 360 to design the parts. For the cast parts I 3D print the patterns. I have recently started to use UV curing resin to coat the parts and smooth out the layer lines. Unlike epoxy, I can take my sweet time and carefully coat every nook and cranny, and when I'm ready, I pop it in a UV light box and it cures rock hard in seconds. Although, it leaves a greasy surface agent that can be wiped of with alcohol. Using this rapid high build finishing technique, it moves the bottleneck from finishing time to the printing time, so for small parts it's now possible to design, print, finish, and cast in the same day.




Here's the mold for one of the disks







Here is my set up, dragged out of my apartment, on the left, my mold making/ sand ramming station, the prepared mold in the middle, and the furnace on the right.





Here is the raw casting as it is broken free from the sand mold. I don't have pictures of the pouring.





I only need two disks to make one axis, but wanted a spare for when I screw up the machining.





I used my lathe to bring the parts to final dimension (almost)





Here is a 80% machined casting





I screwed up the bearing bores on two parts(so much for spares), so I am going to try again this weekend. The best part about casting is you can just melt your mistakes.



I don't think I actually need the bearings, a Teflon bushing plus a thrust bearing would probably be simpler and more temperature stable (no grease) but I have the bearings and I want to learn how to use my new lathe. I'm particularly challenged by boring at the moment.



I can't wait to get the first axis assembled to play with it and to feel the action. The closest thing I could find to this design is the "DiscMount", which is highly regarded for it smooth action, so I hope mine is half as good.



-Corey

 

The first one I screwed up by not measuring and overshooting the eyeballed rough "undersized" dimension. The second one I was more cautious and roughed the bore under measurement and reading dials. On my last pass(2 or 3 thou) I screwed up some where between my measurement, arithmetic, dial turning, or un-accounted tool spring back. Knowing me, I was probably so focused on not screwing it up that I forgot to divide by 2. ha ha.

 

Nice work and project Corey, I have the need for a camera mount along the same lines for milky way long exposures...

Is it Ultradome resin you use?

 

Hey, Cory, nice project. I was wondering how you got the riser in the hub perpendicular to your part for subsequent chucking in your lathe.

 

DavidF, Thanks for the tips, I tried again this past weekend and nailed the two press fit bearing bores.

Negativ3, This is what i used. SolarEZ "thick hard" or "thin hard" should be equivalent. That Ultra dome looks good too, it might be a little thick, but it should fill better, and since it's UV curing you can leave it to self level as long as you want. For a curing "oven" I bought a 12V UV LED strip and lined a box with it. Be careful with your eyes, you can not sense the damage done by intense UV light.


Al Puddle, The nub on the top of the pattern is sized to fit the ID of a 1" PVC Pipe. I can twist and remove the PVC pipe before removing the pattern, This leaves a good cylindrical boss to grip. perpendicularity depends on how straight the pipe sat on the pattern (not usually great, I had to file the nub to adjust some). It would help if my PVC pipe was faced on the lathe.

I have since changed the way I grip the part on the first set up(compared to the picture above). I now use the large OD jaws, and cut the nub so it is short enough not to bottom out on the chuck, but long enough to be griped by the inner most jaws. I then rest the 3 of the 6 pads on the outer jaws and tighten. This is way better, and more repeatable. good enough to chamfer where the turned 5" OD meets the raw cast surface.

I'll try to get a picture next time.

 

I cut up and melted the bad parts. I always like cut away views, it's neat to see a CAD Section view come to life.







Over the weekend I made another set of castings, and machined them. This time i did not mess up the bearing bores. In addition to the the cast plates, a stainless steel shaft and a brass tension knob where made.









I am really happy with the results. Every thing came out true, parallel & perpendicular enough for the action to be very even throughout the rotation. I wish I used a finer thread pitch on the tension knob for more friction adjustment resolution, but I think a spring washer should do the same, and it would be more compliant with temperature swings.

As far as the action, it's a little hard to tell with out a mounted telescope tube for leverage, but it seems smooth when the friction knob is just right, and the Rulon J surface is clean.

It's a little hard to get the tension just right, but a spring washer will help here.

If the sliding surface gets contaminated with dirt or debris stiction becomes noticeable, But if its clean I cant notice any, at least by hand with no leverage, we'll see when I mount an telescope on it.

Now I just need to make another Axis, a 90° Bracket, tripod adapter, and a "Vixen" dove tail clamp.

for a tripod I am planing on getting a affordable surveyors tripod, they are pretty sturdy and competitively priced compared to asto-gear.

I am going to try to design and print the 90° bracket this week so I can attempt to cast it along with a second set of axis plates this weekend.

It Verks!! Yahoo!

 

Nice work. Looks great.

 

I ordered and received the heavier duty CST/Berger 60-ALQRI20-O Tripod for $60. This thing feels very sturdy, but as expected the legs have no hinge stop so I'll have to add some chains or something.



For mounting the telescope mount head to the tripod, I will use the same friction plate casting but with a 5/8-11 thread in the center and 1/4-20 counterbored clearance holes on the 6 bolt pattern. I am also thinking about painting the un-machined cast surfaces orange to match the tripod.


I modeled the 90° bracket and some parts for a fine motion control concept. The bracket is fairly simple but very easy to mold and cast, and requires no post milling; I don't have a mill so I would have to hand file anything I designed to be milled. The only downside is that it puts the altitude axis at a large offset from the azimuth axis. I am concerned that the scope will be too far out when the dovetail clamp and scope rings stack up. We will see, I can always make a different bracket later thanks the the modular design.







The fine motion control concept uses a capstan line drive/reduction. A 100lb test braided fishing line will wrap twice around the 5" diameter of the friction disk and the then twice around a small 1/8th spindle in a Teflon bushing with a 2" knob on the other end.

So 1° of scope travel would be about 40° on the knob which should be about 0.7" travel on the radius of the knob. This doesn't seem like much reduction, but to get the same 1° scope rotation with the same 0.7" tangent displacement directly driven on the axis would required a 40" lever arm, which is not happening. With this capstan drive, I am concerned about maintaining adequate tension, and how well the knot will traverse the system. Also the spindle friction will be multiplied 40x on to the main axis when direct driving the scope, maybe miniature ball bearings are in order, we will see.





I was able to print the patterns last week and cast the parts over the weekend.


The bracket molded very well. The big angled sides make it release for the sand easily .





I made a pair smaller flasks, so now I can make two molds with the same amount of sand (1 Bucket) that one mold used to take.





This is a huge help to productivity. Now I can crank out more parts per melt.





I didn't get to machine the second axis this weekend, so that will have to wait until next weekend.
I need to order some miniature ball bearings, a Tap, and a counter bore cutter anyway.



Almost there.

 

With the telescope COG being off the center-line of the tripod, does the tripod have a hook at the apex of the three legs on the underside? I ask because you may want to add enough mass at the center to counter any tipping moment you may experience. I used to carry an empty bag with string which I could fill with rocks wherever I was, to provide extra stability.

Depends on the mass of the telescope of course.

 

I don't actually have a telescope for this mount yet. For testing I'm planing on using my 80mm spotting scope. My end goal it to dual mount two telescopes over-under style, one 5-6" high power narrow field catadioptric SCT or MCT for looking at bright small stuff like planets and moon details, and one 3-4" low power wide field achro refactor for looking at expansive star fields and big dim things like Andromeda. These two scopes would be nice portable complements to my main scope, a big 10" dobsonian which is a great deep space scope(faint and fuzzy), but a little inconvenient to drag outside for a 30 minute look at the moon, although the views would be stunning. All together the two scopes would probably weigh about 16-20lbs and be pretty short and stumpy(4-8" in diameter and 12-22" long).

I am trying to get these scopes off the used market at a good price. I have my big scope, so I can wait and watch.


The tripod does have a D ring hinged on the center bolt that could be used for ballast. I'll keep it in mind during testing.

 

That's some really nice casting work. What I wanna know is what is going on in your neighbors backyard that is so interesting???? lol

 

Very nice work. Please keep us updated hopefully to include some astrophotography.
Robert

 

or is that asstrophotography?

 

Over the weekend, I was able to machine the altitude axis parts.



I am really happy with how it is coming together. The new axis is a little rough compared to the old one, but I think the parts just need to run in. The older azimuth axis is shiny and burnished where the Teflon ring bears, the new one isn't quite there yet, but getting better and better.

I made the tripod adapter plate too. The connection to the tripod is very rigid and fast to assemble/breakdown.




I need to start working on a temporary bracket to mount my 60mm spotting scope to test the movement while I wait for the right deal on a used 4-6" catadiopric telescope .

Getting really close to having something that works!!! can't wait

Oh yeah, Al Puddle, here is the picture of how I held the casting reasonably true in the lathe, I tried describing it in post #8.




-Corey

 

That is really beautiful! Looks like your castings machine better than mine.
Robert

 

I noticed you switch jaws from the last picture. This looks like a good setup to use for the facing and outer diameter turning. If the hub wasn't quite perpendicular a shim or two will fix the problem good enough. Thanks for the picture.

-Rod