My astronomy project:
3D CAD design of a 3" Push Pull focuser

Content:

Implement of external push pull screws:

1. New design with push pull screws
2. Stabilization of the focuser draw tube
3. Assembling the Push Pull system, 1
4. Assembling the Push Pull system, 2
5. New simplified design
6. To be continued

Related projects:

• Repair and adjusting focus friction
• Tangent arm focuser
• Rack and Pinion focuser
• Push and Pull focuser
• Focus and camera rotator
• Focus and temperature drift

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

5: New simplified design

I have now used my push pull focuser for three years with great results. But it can always be more developed. As it's now it's a relative complicated construction with the timing belt, roller bearings and other stuff. If I have two stepper motors direct connected to the threaded push pull screws it will be less complicated through a mechanical view. But not so easy to sync the two stepper motors to each other. How about to have only one stepper motor and only one push pull screw ? I'm really not sure if it will work for heavy loads, not 5 kg as my first design, maybe 2 kg.

It's not so difficult to built and I do a prototype here to do a test with.

Single push pull screw:

The stepper motor bracket will be clamped around the field flatter. Then it's placed closer to the center of the tube and the side forces will be less. More back weight on the telescope means that I can push it forward to get it in balance. Then less risk for the camera to hit the mount's legs. No back lash through the timing belt because the stepper motor is now direct connected to the push pull screw, less friction too. I can then use a low ampere stepper motor.

The left part isn't finished yet, maybe it need to be adjustable in height and the two free screw holes I have on the optical tube is a bit difficult to reach. Or shall I drill new holes in the expensive APO refractor ?

The problem, can the focus drawer tube handle the side forces when the push pull system isn't symmetric ?

If it work I have a much more compact system and even less back lash.

After I had it 3D-printed it look like this. This stepper motor is a bit oversized.

The shiny black part in the middle is the field flatter, the plan is to attach the stepper motor around this barrel. Look how good the adapter between the 3" field flatter and the Nikon camera look. 3D-printed in matte black PLA with 3D-printed M80x1 mm threads, tilt, off-axis and 2" filter holder built in.

The new stepper motor to the right, look how much closer it sits to the telescope tube's center. The two white brackets to the left is of no need anymore, just need a small bracket for the M3 nut to the push pull screw. Can the stepper motor absorb the axial forces that arise or do I have to design some kind of axial bearing for it ? What is certain is that the stepper motor must have some support at the rear end.

After some thoughts I decided that the sing axis design will not work, can't handle the force with a heavier camera. And the stepper motor take too much space too. But if I go back to the original design with two push pull screws and use two stepper motors I dived the load on two motors and then they can be smaller. With no timing belt the friction is much smaller. With a smaller stepper motor it will look like this, this is the motor I use today but can only handle 2 kg camera load when pointing to Zenith. I can connect them in parallel or in serial to the driver. After a while they will be out of sync, but easy to restore the sync. Later I add something that handle this sync, mechanical or electronic.

Strengthened up the design to handle the load better and mirror the bracket. The two bracket behind is the nut, the M3 threaded holes where the push pull screws attach. With a load of 5 kg camera equipment each bracket must handle 2.5 kg. As it's now I think it will flex, but easy to add another bracket that join them together.