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DIY Equatorial Telescope Mount

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  • DIY Equatorial Telescope Mount

    As if I didn't have enough projects going on all at once I have started yet another, an equatorial mount for my telescope. This stems from trying to set up my 10" Meade LX200 for astrophotography. By the time I added the off axis guider, the filter wheel, focuser, and finally a camera on the scope I lost a lot of sky due to everything hanging off the back. Add a wedge to turn the alt/az form mount into an equatorial means I lost any chance of pointing north.

    IMG_2393 by macona, on Flickr

    The solution seemed to be to take the OTA (Optical Tube Assembly) off the forks and mount it to something like a German Equatorial Mount (GEM). A GEM allows the scope to swing around without clearance issues for the most part. But, boy are they expensive. A low end one if $1500+!

    So I figured it is time to build one myself. Probably 10 years ago I picked up an old PRI robot that had come out of intel or something, I think it had moved around wafer boat or something. Took most of it apart, gave the steppers away, and kept the useful stuff. Bunch of linear slides and stuff. The one gem was the big harmonic drive that the robot rotated on. The OD on the driven disc is 10" in diameter and it is supported by a nice, large ~8" bearing under it. All made out of aluminum so it is still reasonably light. This is going to be my Right Ascension (RA) drive.

    Harmonic drives are very cool, they use a flexible spline on an eccentric bearing that wobbles inside another spline with a couple more teeth than the flexible one. Since there is alway hard contact between the flex spline and the outer splice the drive is truly zero backlash. They are used in a lot of places like robots, cnc and even the moon rover. It used one harmonic gear in each wheel. Here is a video of the one I am using, you can see the flexing of the spline. This is 102:1 gear ratio.

    I have since taken it apart and cleaned it out, it runs much smoother now. The drive had a DC servo on it with a 500 count encoder. I tried setting it up with one of my little Elmo Harmonica servo drives like what I used with the laser cutter but it would just not tune. I dont thing the drive could supply enough current. Also after doing the calculations I found 500 lines is just not enough, that is less resolution than the mount I am already using. After messing around with various combinations of servos and gearboxes to drive it I have settled on a Mitsubishi MR-J2 200W brushless servo with a Bayside 10:1 gearbox. This drives the harmonic drive and gives me a resolution of 8,355,840 steps per rev. This puts me way above useful resolution.

    For the Declination (Dec) drive I am using another Mitsubishi 200W servo, this one is a MR-J2S series with a 131,072 count encoder. This will drive another Bayside gearbox, this one 3:1 and then that will drive a 80:1 harmonic drive which will have the saddle for the telescope on it. This gives me stupidly high resolution of 31,457,280 steps per rev. I may try direct driving the harmonic drive bypassing the 3:1 gearbox, I am pretty sure it will have enough torque. The OTA is adjusted on the dovetail mount to balance it.

    The Dec harmonic drive is pretty neat, it is basically a cross roller slewing bearing with a harmonic drive built in. I think these were pulled out of SCARA robots, I got one from S. Korea shipped for $180. More than I would have like to pay, but oh well. Considering I have had all the rest of the parts lying around I cant complain too much!

    Im now drawing up the design for the tilt mechanism for setting the angle of the RA drive and figuring out how I am building the motor mount for the Dec drive.

    This is the back of the main harmonic drive with a servo and the gearbox attached. There are commercial GEM setups available but they start around $20k!

    Harmonic drive with mitsubishi servo by macona, on Flickr
    Last edited by macona; 05-04-2014, 06:22 PM.

  • #2
    That video is incredible; they should be called hypnotic drives.


    • #3
      I ordered the smaller harmonic drive from Ebay monday night, $180 shipped. It showed up on my door yesterday morning... From South Korea. Wow...

      Small harmonic drive by macona, on Flickr

      The drive is pretty nice and should work great. It feels a bit lumpy turning by hand but connected to a motor is seems fine. I did decide to use the lower res servo on this axis. This axis just stays in one spot one you are locked on, the RA axis does all the tracking. If all else fails I can pick up another motor for the other J2S drive I have.

      For a control I am going to try using an arduino, this guy came up with a ASCOM compatible arduino based controller for mounts with step/dir drives. It can connect to a PC and be controlled from ASCOM or LX200 compatible software or through bluetooth with an Android app he wrote. He also wrote an ASCOM compatible driver for the Quantix camera I am using. That saves a lot of time...

      Finished up a bunch of the CAD last night and should be able to chew up some aluminum this weekend. Digging around in the shop I found a bunch of the 1/2" aluminum that the robot was made out of.


      • #4
        Was this used or new? Do you have a link to the ebay item or a similar item?


        • #5


          • #6
            Spent a couple hours making chips tonight. This the plate that the Dec motor and gearboxes mount to, it attaches to the big harmonic drive. I still need to counterbore for the screws.

            And some video of chips flying.

            The part is 8" wide and 14" long.

            Equatorial motor mount by macona, on Flickr


            • #7
              Hi Macona, a couple of weeks ago I bought a MR-2JS-20 servo amp and a motor. How do you program them without the software ?
              I think if people knew just how amazing they were and if they were easy to set up more people would be using them. Any tips on getting one going would be appreciated.
              My neighbours diary says I have boundary issues


              • #8
                Which version, A, B, or C?

                You can set them up almost completely from the drive, it is all in the manual.


                • #9
                  Ill read the manual a bit harder !!

                  My neighbours diary says I have boundary issues


                  • #10
                    Oh, the B's are almost useless, that is why they sell so cheap on ebay. They are intended to hook up to mitsubishi PLCs with their industrial network. There is one guy who used a FPGA to talk to the drives from LinuxCNC, he says he is going to write a program so you can use one of the Mesa 5i25 cards to communicate with them. Well see...


                    • #11
                      Made the motor and gearbox mount tonight. One little goof. I entered a .1 when it should have been a -.1 and started a bore shifted but caught it early.

                      Next up is building a losmandy style saddle and the frame for the RA drive. Starting to get a bit heavy too, up to about 50lbs now.

                      Harmonic drive equatorial mount, north side. by macona, on Flickr

                      Harmonic drive equatorial mount, north side. by macona, on Flickr


                      • #12
                        I weighed the three parts I machined out the day and they came in at 12lbs! Ouch! I modified the design to incorporate lightening cutouts. After about 5 hours of milling and one destroyed cutter plus one remade part I have them finished. I still need to bolt them together and ream out for the taper pins to lock everything together.

                        Dec motor and drive mounts by macona, on Flickr


                        • #13
                          Beautiful work! Can't wait to see the end.

                          We need more projects like this here. And you should consider writing it up for the magazine.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.


                          • #14
                            I started on the frame that holds the main drive last night. I had the original piece the big harmonic drive mounted to as well as some of the aluminum for the base. It is all 1/2" aluminum. Finished up that part tonight. The plates have bores for bearings and curved slots for the angle adjustment. It will got up to 60degrees of tilt. I am still working out the design of the base section, I have got a good idea what I want to do, just not sure.

                            I might send some of the parts out for bead blasting and anodizing.

                            Side pieces and top before machining:

                            Telescope mount parts by macona, on Flickr

                            Completed frame:

                            Telescope mount parts by macona, on Flickr

                            Drive mounted in frame:

                            Telescope mount parts by macona, on Flickr


                            • #15
                              You do some beautiful workmanship . I know it's cnc, but still it is very neat thanks for showing Pal . Alistair
                              Please excuse my typing as I have a form of parkinsons disease