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Focuser for Wide-Field DSLR Lens

Gina

By Gina
in DIY Astronomer

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davew

davew

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My skywatcher motor focusers both have a little lash,clearing the lash in/out of focus is a pain..

I'm looking for better motors, has anyone got any recommendations?

Unfortunately cheap gearboxes do have backlash. The way that works for me is this -

I replaced the 9v battery with 4 x rechargeable batteries. They give around 4.8 to 5v. I also set the control to the lowest setting. I then extended the wire by about 5 meters so I could use it at the laptop ( I'm a tight wad and didn't buy a PC interface ! ) One tap of the control buttons takes up any slack. If you come from one direction with such a slow movement you may not even need to reverse. I now know roughly how many taps gets me focus on each lens.

As you have two already give it a try. I can't recommend any other motors because I haven't tried them.

Dave.

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Gina

Gina

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It's been clear for an hour or so tonight but clouded over again now. In that clear time I was able to test me crude manual focussing thumbwheel on the moon. It's not good enough really. Not a lot of image movement with EOS Utilities Live View with full magnification but focussing was still too dificult. May be easier with a focus mask but I think it really needs a remote system so that I can adjust it from my warm room sat right in front of the monitor. Viewing through the door from the mount position is not satisfactory.

I'm hoping the belts and pulleys will arrive tomorrow. I've also ordered a bigger aluminium plate for camera mounting so that I can have room for focus motors.

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Gina

Gina

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The belts and pulleys have come this morning so will be working on the remote focussers shortly :) For a start I'm going to use the same system as I did for the scope focuser - at least get things working, in case we get some of those elusive clear nights :D Then I plan to make improvements like Arduino controlled stepper motors but firstly I need to know just how much movement is required and how precise this movement needs to be. Steppers um... move in steps and these steps need to be small enough when transmitted to the focussing sleeve of the lens.

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Gina

Gina

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Have mounting plate now and mounted cameras on it. Been setting up motor/gearboxes and testing with 13.8v from my bench PSU and focussing control box. Initial test with 4 reducing gear pairs was far too fast so went with the full set as suppiled of 4 sets giving 4096:1. This seems suitable and I think will give sufficiently fine control with the motor speed set near minimum. This photo shows the first motor/gearbox fitted in place and driving the focus sleeve of a 200mm f3.5 telephoto lens.

post-13131-0-82149800-1351870848_thumb.j

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Gina

Gina

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I mounted the WF rig on the dovetail tonight and connected up the focus control box in the warm room. The focussing action works well :) Next thing is to make a Bahtinov mask for the 200mm lenses to get accurate focus. The focussing rate is fine enough, when set to a slow rate, to permit good focussing.

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drhemlock2

drhemlock2

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Gina the mad scientist at work!! haha but i love your stuff and as i am garthering different items to try some your projects for myself this one appears to be working for you. i wonder? if your camera(s) with the installation of the motors w/belt would there not be slight movement because of the addtional torque(s) on the camera frontend. in other words would the lense move side to side without some kind of support or am i just not using my left brain the correct way?

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Gina

Gina

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Gina the mad scientist at work!! haha but i love your stuff and as i am garthering different items to try some your projects for myself this one appears to be working for you. i wonder? if your camera(s) with the installation of the motors w/belt would there not be slight movement because of the addtional torque(s) on the camera frontend. in other words would the lense move side to side without some kind of support or am i just not using my left brain the correct way?

They do move very slightly - there is a slight shift in the image when reversing the focussing drive. Not enough to be a problem though.
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Gina

Gina

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Could be reduced by putting a spacer between the two lenses (across a non rotating portion), so one counteracts the movements of the other

Yes, might help :) I was thinking of something along those lines. At least the change is not cumulative and the slight angular difference in direction is constant when the focus is obtained from the same direction. A small framing error can be taken out in DSS and between cameras in the alignment of the Ha and OIII.
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Gina

Gina

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Hi Gina, what do you focus on ? I try to use something bright enough for live view. Are you using longer exposures with a Bhatinov?

I focus on a bright star with the lens wide open and use live view with ISO 6400. The aperture of the lens is considerably more than the scope so the image is brighter.
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