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Clement Focuser


Horwig

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The focuser I made for my Newtonian some time ago is proving not to be as rigid as I'd like. There's drift of the order of 100 microns due to sag between the imaging train vertical and horizontal, which is not good at f3.5.

I've seen this focuser, and thought that it looked possible to concoct my own, but can't see any user reports on the web, so anybody out there own a genuine one, or attempted to make a similar beast?

http://www.clementfocuser.com/

 

Huw

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That looks interesting :)  I'm having problems with focussing on my All Sky Camera with keeping the lens axis perpendicular to the sensor.  That design is evidently intended to take care of this.  My present design overcomes this by having three motors driving screw adjusters.  Driving all three stepper motors provides focussing while keeping the alignment correct.  I can adjust the alignment by running the motors independently.  Have to say, I'm far from happy with this arrangement even though I'm pretty much built it.  It's just far too complicated and takes up too much room.

The problem is that with a lens of 1.4mm focal length and focal ratio of f1.8 the focus is very critical.  Unfortunately, the fish-eye lens I'm using has no focus control.  I've tried using the CS thread as a helical focuser but it's too coarse and not accurate enough.  Even M3 screws driven by steppers with internal 64:1 gearbox gives quite a coarse control.  So I'm wondering if a miniature version of the Clement Focuser might be a solution.

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32 minutes ago, Gina said:

So I'm wondering if a miniature version of the Clement Focuser might be a solution.

It does indeed look like a clever engineering solution, I think he has a patent on it, but I suppose that does not prohibit copying the basic design for own personal use, as long as it's not then marketed, might drop him a mail just in case

 

Huw

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I have always thought that flexure hinges are the way to go for stable designs so I look forward to seeing if you manage to make one. The hinge material may prove a challenge.

I don't think they sold many and when I was looking at then they were not available.

Regards Andrew

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From Googling other things I know that a lot of my designs are out in the wide wide world so I wouldn't want to tread on anyone's toes by publishing a miniature version here.  I agree I could ask but I think I would be very hard pressed to engineer a miniature version anyway.  And yes Andrew, the hinges would be difficult though I would probably have tried Nylon - 3D printed.  It was just a thought and I'm always open to suggestions :)

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Interesting thread :)  I can see this working well for a telescope focuser but my current requirement is for a focussing range of one or maybe two millimetres.  That would mean precision engineering for which I lack facilities.

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There's an absolute rule that anyone is free to copy a patent for personal research or research but you must not sell or market the product.

The commercial protection the patentee gets is given in exchange for making their design public and thereby catalysing experimentation and furtehr development.

To quote Wikipedia " Patent law is designed to encourage inventors to disclose their new technology to the world by offering the incentive of a limited-time monopoly on the technology. "

The patent is here: https://www.google.co.uk/patents/US6260428

To be valid it should contain sufficient information to permit you to construct your own. You would NOT be infringing the inventor's rights - they got their monopoly in exchange for making this information available to you!

The patent will expire on 9 September 2019, so if you are really concerned you only need to wait less than two years!

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51 minutes ago, Gina said:

Interesting thread :)  I can see this working well for a telescope focuser but my current requirement is for a focussing range of one or maybe two millimetres.  That would mean precision engineering for which I lack facilities.

If you have the ££££££ you could try one of these http://www.flicamera.com/focusers/index.html probably the best ?

Regards Andrew

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2 hours ago, andrew s said:

If you have the ££££££ you could try one of these http://www.flicamera.com/focusers/index.html probably the best ?

Regards Andrew

Neeeeaaaa, if you want very silly money, how about this:

https://www.teleskop-express.de/shop/product_info.php/info/p9100_MoonLite-NiteCrawler-3-5--Rotating-Worm-Drive-Focuser.html

 

Now where's my piggy bank?

 

Huw

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12 hours ago, Horwig said:

Neeeeaaaa, if you want very silly money, how about this:

https://www.teleskop-express.de/shop/product_info.php/info/p9100_MoonLite-NiteCrawler-3-5--Rotating-Worm-Drive-Focuser.html

 

Now where's my piggy bank?

 

Huw

Possibly the only piece of astronomical equipment named after an AC/DC song?

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15 hours ago, Gina said:

Interesting thread :)  I can see this working well for a telescope focuser but my current requirement is for a focussing range of one or maybe two millimetres.  That would mean precision engineering for which I lack facilities.

Works very smooth, just try it Gina...:wink:
Made a few small modifs.
There's also a dual speed knob involved.

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OK, can I float a question?

My present focuser uses an ordinary M3 x 0.5 threaded rod as a leadscrew, it works, but there's 3.3 Kg hanging off it, not sure if I should increase the size.

I've found plenty of M6 x 0.75 rods and nuts, have also found an M6 x 0.5 rod but no nuts, the hunt continues.

Can anybody tell me the difference between a real leadscrew and a normal threaded rod?

 

Huw

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17 minutes ago, Horwig said:

OK, can I float a question?

My present focuser uses an ordinary M3 x 0.5 threaded rod as a leadscrew, it works, but there's 3.3 Kg hanging off it, not sure if I should increase the size.

I've found plenty of M6 x 0.75 rods and nuts, have also found an M6 x 0.5 rod but no nuts, the hunt continues.

Can anybody tell me the difference between a real leadscrew and a normal threaded rod?

 

Huw

Phew, that's a can of worms.

Ordinary threads are perfectly good as leadscrews when loads are small (3.3kg is small).  For heavier duty buttress or acme form threads are used, and sometimes (especially on old machines) square threads.

The method of manufacture is what determines accuracy.

  • Cut threads are generally good accuracy.
  • Rolled threads are usually less accurate.
  • Some of the best threads are rolled and then ground to finished profile.

Ball screws which use recirculating ball bearings have low backlash and high accuracy, useless for a focuser unless held in position by a stepper or servo motor as they have very low friction.

For focuser the most important things are smooth action and minimum backlash. Accuracy is unimportant as you will use some sort of feedback loop to find the focus point (such as you looking through an EP or at a screen and twiddling or an electronic system). it is lack of backlash that gives repeatability, not thread accuracy. Smooth action allows precise small movements to be made.

For cheapness, choose stainless steel allthread with a pair of standard nuts with a suitably strong spring between them to take up backlash.

If you have more £££ buy a ground screw and a phosphor bronze-anti backlash nut.

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1 hour ago, Stub Mandrel said:

For cheapness, choose stainless steel allthread with a pair of standard nuts with a suitably strong spring between them to take up backlash.

If you have more £££ buy a ground screw and a phosphor bronze-anti backlash nut.

Thanks for the full answer Neil.

Further question, I take it the spring must exert a force greater than the max load on the focuser to take up all the backlash, ie. 3.3Kg

What about a pair of nuts that can be tightened against each other till slack is taken up, and then locked down, would that work?

Have found a good UK online source of leadscrews, (https://www.automotioncomponents.co.uk). They offer a good selection of anti backlash leadscrews, but the price is eyewatering, about £200+

Think I'll stick to just a threaded rod with DIY anti backlash

 

Huw

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The KStars/Ekos/INDI auto-focus routine allows for backlash - you just enter the relevant amount in the client control.  Works beautifully  :)   I use gears on my focusers and there is backlash but it can be completely compensated for.

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13 minutes ago, Gina said:

The KStars/Ekos/INDI auto-focus routine allows for backlash - you just enter the relevant amount in the client control.  Works beautifully  :)   I use gears on my focusers and there is backlash but it can be completely compensated for.

Thanks Gina, I'm  using a Lightphase focus controller from Cheadle Astro, (http://www.cheadleastro.co.uk/) that too allows for backlash, but if I can minimize it mechanically, I would be happier

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