Focuser for Wide-Field DSLR Lens

[Gina]

I've been looking at how to make a simple way of focussing wide-field lenses on a DSLR without shaking the image too much. Just turning the usual manual focus on a lens causes an image in live view to jump about on the screen even with a good solid mounting onto the dovetail of an NEQ6 Pro mount. For DSO imaging the focus is extremely critical and the ribbed focus sleeve is too coarse. The focus sleeve only needs to move by a few degrees to achieve focus so a very slow-motion drive is needed.

My design is based on a device available from Telescop Service with some changes. Here is a photo of my first crude prototype of a vernier manual focuser for camera lenses used for AP. The method is to have two rings clamped around the focussing sleeve and a fixed part of the camera. Then the angle between these is adjusted with a bolt and nut with a spring to maintain adjustment. This test model has an ordinary nut but this will be replaced with a knurled knob. The lens shown is a 200mm f3.5 telephoto with the old style Pentax/Praktica M42 screw mount and adapter for Canon EOS cameras.

[pete_l]

If you have the clearance, would a long stick hot-glued to the focuser work as a lever? You may have to have it centred on the focuser to keep it balanced.

[Ags]

in-gina-eous!

[Gina]

If you have the clearance, would a long stick hot-glued to the focuser work as a lever? You may have to have it centred on the focuser to keep it balanced.

This sort of thing do you mean?

Not fine enough and still produces vibration

[Gina]

in-gina-eous!

Thank you

[allcart]

Hi Gina, Looking at previous pics of your setup, you have some sort of motorised focuser on the ED80. Can you not upscale and adapt the same idea for the camera lens. You would obviously need two for you dual camera rig. It would not have to be motorised, it could just be a knob attached to a bar with a small pulley on it. The bar would be fixed to the camera mounting rig and a rubber band round the pulley and the lens. It should be able to produce fine adjustment without touching the lens.

[allcart]

I bet you could do this in minutes if you could find some old Meccano

[Gina]

I looked into this method but the movement of the focus sleeve is so critical that only a toothed belt would have low enough elasticity and yet enough friction on the focus sleeve. I tried a rubber band but it was too elastic. The focus is quite stiff on the lenses I have. Several of the selection of springs I've got weren't strong enough to turn the sleeve.

[Photosbykev]

If you use EF or EF-S lens on the body you can use APT to remotely focus the lens using a FWHM process. So far it has worked remarkably well for me and gives a good feel for the seeing conditions as well

[Gina]

If you use EF or EF-S lens on the body you can use APT to remotely focus the lens using a FWHM process. So far it has worked remarkably well for me and gives a good feel for the seeing conditions as well

Apart from one lens which I've just acquired, all my lenses are M42 thread with adapter so all but the new one are manual focus. I'm hoping to use remote focus with the new lens though

[davew]

Would this work for you ?

Works well for me but there's only one at a time round here

Dave

[Gina]

I might try that Two stepper motors, pulleys and belts would cover all lenses with a suitable adjustable bracket. It would be simpler than making up bands for every lens individually. I have two stepper motors with drivers ready for connecting to an Arduino. Just need pulleys and belts.

[davew]

Pulley and belt from Motionco.

Bracket from a garage door.

Steppers would be good.

For a big lens I added a ring.

Dave

[Merlin66]

IMHO the stepper motor/ belt drive is the way to go....

It's been very successful over here in Oz - Bert got some great results with his big Canon lens.

Remember the CFZ (critical focus) for a f4 lens is only 35 micron (half the width of a human hair!), and it gets even tighter as the f ratio goes down.....

[Gina]

Thanks for your replies Yes, I think you're right about stepper motor and belt drive - I think I'll try that. I think any hands-on system is likely to cause movement and any other motor control would be more complicated.

[Gina]

I'm just now wondering how big a pulley to go for - I'm thinking of something like 15-20mm by 6mm belt width (1/4") and 2mm pitch. Motionco seem to have a good selection.

[Photosbykev]

Could you mount the stepper motor and a rubber faced roller on a sprung loaded plate that contacts the lens focus ring? Doing this would give you an adjustable friction drive onto a number of lens. If the focus rings are at differing distances then a slide plate running parallel with the lens axis would give that freedom of movement as well.

[davew]

That may well work ok but I thought it added a layer of complexity I didn't want. Or to put it another way, I couldn't work out a simple spring method and non skid wheel. A wheel faced with fine emery did sort of work though. My motor sits on a sliding plate and the motor turns right round so I can use it on a focus ring either right in against the camera or at the end of a long lens. It also slides for and aft plus in and out.

Gina. I can't do it now but I'll have a look and see what pulley I used.

As a foot note, it worked on a 200mm f2 lens so is quite accurate.

Dave.

[allcart]

Pulley and belt from Motionco.

Bracket from a garage door.

Steppers would be good.

For a big lens I added a ring.

Dave

Thats what I imagined Dave, but you already did a fantastic job of it.

[Gina]

Could you mount the stepper motor and a rubber faced roller on a sprung loaded plate that contacts the lens focus ring? Doing this would give you an adjustable friction drive onto a number of lens. If the focus rings are at differing distances then a slide plate running parallel with the lens axis would give that freedom of movement as well.

I'm with Dave on that - too complicarted. I looked at that method for my filter wheel but settled on belt drive. Also, the focussing on lenses is quite stiff and it would want quite a lot of force appllied to the idler wheel or motor.

[Gina]

Looks like I shall have plenty of time to sort this out - just heard the weather forecast for the next few days! It was cloudy last night and more rain is forecast for later tonight preceded by cloud, of course. Then it goes downhill again with cloud and some rain for the next few days I was hoping the high that was forecast was going to yield a few clear nights but it wasn't to be I'm beginning to suffer "cold turkey" from lack of imaging time but I guess fiddling with the gear helps

[davew]

Hi Gina,

It is a 24 tooth Ali pulley and 9mm HTD timing belt. The belt length in my case is 324mm.

If anyone does want a go with a Skywatcher motor then it's a 6mm bore. I also found 9 volts too much so use 5 volts.

Dave.

[Gina]

Thanks Dave

[Gina]

Ordered a couple of belts and pulleys from Motionco

[Gina]

Think I'll set up with a couple of the motor/gearboxes like I used on the scope (from Maplin). These have multi-ratio gearboxes - ratio can be changed by changing the plastic grear. I may change to stepper/Arduino when I've done some experimenting and seen how I get on. The focussing will need very minute steps and I don't know if the steppers I've got give fine enough steps.