Thin electric focuser/helicoid?

[Lou Jost]

Somewhere I had once seen a discussion of a very thin electrically driven focusing helicoid. It would have been designed mainly for astrophotography. Can anyone here remember it? Thanks in advance.

[dickb]

Somewhere I had once seen a discussion of a very thin electrically driven focusing helicoid. It would have been designed mainly for astrophotography. Can anyone here remember it? Thanks in advance.

I'm not sure whether this is what you were looking for:

https://www.astromechanics.org/ascom.html

You mentioned it yourself in this post:

http://www.photomacrography.net/forum/v ... 70#p258670

[Lou Jost]

Dick, thanks, but no, that is not what I am looking for. I hope for a thin helicoid that moves under power, though at this point I would be happy with a manual rack-and-pinion thin helicoid. The thinner the better (a few mm would be great).

[CharlesT]

Is this the thing? :
https://kenrockwell.com/tech/megadap/mtz11.htm
'6.5mm of self-motorised extension'

[Lou Jost]

Thanks, that would almost work (though it would need a different camera mount). But I don't really want autofocus, I just want precise hands-off manual focus.

[ray_parkhurst]

I'm very interested in such a contraption, though have never heard of one. It could make an excellent Z-stage as long as it does not rotate during actuation.

[Lou Jost]

Ray, I have a pretty strong memory of seeing a post somewhere (not necessarily on this forum) about such a device. Still, I could be mistaken.

[ray_parkhurst]

Ray, I have a pretty strong memory of seeing a post somewhere (not necessarily on this forum) about such a device. Still, I could be mistaken.

The Megadap that CharlesT posted gives me hope.

[joeD]

maybe this one?
https://www.heliconsoft.com/helicon-fb-tube-purchase/

[ray_parkhurst]

I think that one from Helicon is similar to the one posted by dickb and that neither makes a physical change to the length of the tube, instead acting to control the focus motors within the AF lens to move a prescribed amount.

[Lou Jost]

I think that one from Helicon is similar to the one posted by dickb and that neither makes a physical change to the length of the tube, instead acting to control the focus motors within the AF lens to move a prescribed amount.

Yes, none of those are helicoids. Those are fancy extension tubes. I'm hoping to find something that physically extends.

[dickb]

Yes, none of those are helicoids. Those are fancy extension tubes. I'm hoping to find something that physically extends.

Well, something like this will extend, but comes with a hefty price tag
https://eu.primalucelab.com/esatto-35-l ... cuser.html

[Lou Jost]

YES!!!! That's it!!! That's the very thing I remember. Thank you.

Very expensive, ugggh. Also too thick for many of my applications...But an excellent device.

Ray: step size of 0.01 microns! Could that be true?

[ray_parkhurst]

YES!!!! That's it!!! That's the very thing I remember. Thank you.

Very expensive, ugggh. Also too thick for many of my applications...But an excellent device.

Ray: step size of 0.01 microns! Could that be true?

I was asking myself the same question...

The helicoid itself is a large diameter, hollow screw. The drive mechanism is probably a rack and pinion arrangement around one end, orthogonal to the optical axis. Let's assume the following:

Helicoid pitch: 2mm
Rack pitch: 2mm
Rack diameter: 102mm (consistent with 90mm clear aperture)
Rack teeth: 320
Pinion teeth: 20

This gives 16 motor turns per rack turn. With a 400-step motor, this is 6400 motor steps (full steps) per rack turn. The result is a full step movement of 312nm. With quarter stepping, then ~80nm could be achieved. That's far from 10nm, but perhaps with finer pitches and more microstepping?

[soldevilla]

Helical focusers always have a bit of image shift (I've been using and making telescopes for 40 years, I know a bit about this). There is no great solution for very low profile focusers, and that is the holy grail for users of Newton-type telescopes. The most affordable solution is to use a low-profile Crayford focuser for newtons, but that's still far from your requirements. There is a design with two discs joined by 3 hinges at 120° that is possibly very close to the size and precision you are looking for, but I have not tried it, I only have readings about that focuser. I will try to find information