Project:
Nikon D800 for astrophotography ?

Contents:

1. Nikon D800 overview
2. Adapter Nikon to Pentax 645
3. Lid, Dove tail, Tube ring
4. Focuser mechanism, driver
5. Off-axis guide camera
6. Computer module and DC/DC converter
7. DC/DC general triple converters
8. Main bracket
9. Wiring & USB-HUB
10. Test with Windows and APT
11. First Light
12. Balance system
13. Nikon vs Canon, Pentax FA vs Pentax
14. Tilt adjuster
15. KStars/INDI setup for Nikon
16. To be continued

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

4: Focuser mechanism, driver

A motor focuser is a must. I have already built one based on the open source project MyFocuser Pro II. One demand is to have it to handle the backlash better than my earlier timing belt construction.

Focus arm:

Last time I built a motor focus I used a timing belt to connect it to the lens' focus barrel. It worked but introduced a bit too high friction and backlash. Maybe I can use a simpler construction with just a tangent arm or a worm gear. Here is the tangent arm design. The square box is an illustration of a NEMA17 stepper motor, I hope to use a smaller one.

The focus arm. With a stroke of 40 mm I can focus a bit in/outside the infinity, this extra space is needed to get the auto focus to work. It will be direct driven by a stepper motor with 200 steps per revolution. That gives 8000 steps from one side of the focus to the other side. It gives 5 my per step in full step mode, but normally used in half step mode and then 2.5 my per step. This construction will give less backlash than the older timing belt construction I have on the older setup.

Stepper motor pivot:

The stepper motor must have the ability to tilt when it follows the angle of the screw. This is how it's solved, a bracket divided in two parts to attach to the stepper motor.

The pivot brackets mounted on the main bracket. They are designed for the NEMA 17 standard of stepper motors.

The motor must connect to the M3 threaded screw that is mounted on the tangent arm. It's designed like this, a 10 mm long thread lock to the screw.

The Axle coupling in place, now just missing the stepper motor.

With the stepper motor installed. Looks like it will work but need a more flexible cable, this one is too stiff. At the same time I also changed the PolyFuse from 2.5 Amp to 1.6 Amp which is more reasonable.

Limit switch:

Somewhere I have to place a limit switch, otherwise it could smash into the mechanical stop. I will place it on the upper hing.

A block on the screw hit this switch when it's in its start position.

HPSW, Home Position Switch mounted on the tangent arm. I'm not sure if I really need this switch, but now the mechanics is at least there. Have to make some sort of "hat" on the screw that act on the switch, the green tip. No wiring found where to connect this switch, but in the setup for the software it says it should be connected to D12 on the Arduino. A 100k to 470k ohm pull up resistor is needed.

Focus motor driver:

I plan for a stepper motor to the focuser. To that motor I need a driver which is the interface between the focuser and computer. I use the same open source driver as I used earlier: MyFocuserPro2. Everything has to be built in an enclosure. My idea how it should look like. I have changed the switches to ones that have built in LEDs, make it less complicated. The push buttons are two transparent bars that push on the switches soldered on the circuit board.

Focus driver circuit board:

It use the software for the INDI driver standard that's used in Linux systems. The CPU board take its power from the USB cable and there is no need have that extra 5 Volt built in, only the power for the stepper motor. In my earlier focuser I had some problem to feed the stepper motor direct from 12 Volt. In this layout I have added a DC/DC converter where I can lower the voltage, but better if I can make it work without this. Later I decided to have this extra DC/DC converter in a separate box together with other general voltage converters. It can also be solved with another driver circuit maybe or a higher impedance stepper motor.

Enclosure:

The circuit board need some enclosure, I use the same design I did to the earlier one but decreased the thickness of the walls from 3 mm to 2 mm.

Push bars:

It is the first time I 3D print with a transparent material. These bars will push on the switch on the circuit board.

The lid with bars inserted.

The lid installed on the focus driver. When the focuser moves the push buttons will shine beautifully in red and blue light.

An early auto focus test:

Even if the auto focus parameters are not optimized yet I can see a huge difference. The low backlash in the mechanics give a much more even focus curve. It will result in better precision of the focus.