Question for those using SLR lenses and autofocus for widefield

Using some spare parts I’m cobbling together a motor and belt system for autofocusing with old Pentax SLR lenses. I plan to use a robofocus motor and timing belt to focus the lenses (55mm and 135mm). For anyone using a similar system, how far does the belt need to move physically around the lens for autofocus? Does it need to make a complete revolution or does it just move a little bit back and forth? What is your step size and how many data points do you use?

And if anyone has some pics of their setup I would be glad to see it.

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There was a similar thread at Cloudy Nights recently:


Joel, it shouldn’t need to move very much. A few revolutions around infinity should give you enough to deal with your focus.

Neat idea.

Once again my email reply didn’t come through on this…
Chris, that’s what I thought as well. I was hoping the belt wouldn’t have to move very much. I have the motor, camara and lens kit all mounted up, I just need to get the motor cog and belt. I’m looking forward to autofocus for my wide field imaging rig! Once you get used to autofocus, it feels like a huge pain in the rear to have to manually focus a lens when I’m out at a dark sky site away from home.

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It’s really cool. You’re on CN, you should go to the Observatory forum… some guy just built a similar setup and posted it in the observatory (it’s going in a small robotic dome…). He might be able to give you some real insight into how it all works. I think he used two robofocus.

Wow that guy on CN is serious about widefield imaging. He has three Robofocus motors on three cameras/lenses. Too cool.

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Yea, his setup is pretty legit :smile:

Here’s a setup that QSI sells…might give you some ideas although I doubt it will directly work with your equipment:

I’ve been contemplating something like this for a while. Although I don’t think my dome opening is wide enough to actually accommodate it so it would likely be a “star party” only thing.


That QSI system is essentially what I’m trying to do…just much cheaper and with parts I have lying around. :slight_smile: I have the mount all put together, I’m just waiting on the Robofocus motor cog to drive the belt. I’ll post a pic when I get it all together.

So here’s what I’ve come up with. I had a spare Robofocus motor, Losmandy dovetail adapter, and a few brackets I made up for this. I just needed to buy the motor cog and belt on ebay. The dovetail adapter obviously slides front to back to accommodate various size lenses. Then the bracket that the motor is attached to can slide left to right to facilitate tightening the belt.

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That is awesome. Where did you get the brackets?

I just made them. The silver aluminum camera bracket is made from 3/8"x1.5" bar stock that I got from a local metal shop. Aluminum is soft enough that I can cut it with a metal blade on a standard mitre saw and can drill holes with a regular cordless drill or drill press. The robofocus motor bracket consists of a spare aluminum bracket that came with the robofocus motor, and the black “L” metal piece that sits on top of the Losmandy adapter I just got at the local Home depot in the lumber section. It’s not hard at all and just takes a little time. It doesn’t look all that professional but it gets the job done.

I also made a small guidescope out of a spare 8x50 finderscope that bolts down beside the camera on that black small dovetail base. So I have everything in a small, somewhat lightweight package. This is part of my travel astrophotogarphy setup.

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Looks good! Looking forward to seeing some images out of it!


Joel, it’s beautiful. Nice job. The brackets are always what I’ve had issues with.