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Frustrating focuser problem


Astrofriend

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Hi,
I have a 3" focuser on my APO 5" telescope. High quality and works very fine, except it could slip sometimes in the friction coupling. Even if I set the force higher with the adjust screw it slip sometimes. When the telescope get wet from moisture the water act as an oilfilm and lubricate the axis and steel plate friction surfaces.

Now I must do something about this problem, took the focuser apart yesterday and looked after what was wrong with it. I have as usual documented what I do, for myselfe and all other that maybe have the same problem:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/project-focus-friction.html

I will update the page when I have done the first test after my fix.

Any smart ideas how to increase the friction?

/Lars

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  • 2 months later...

Hi,

It's not so easy to change the axis surface. It needs to be of low friction surface because the press friction bearing acts on it at the same place as it press on the steelplate.

I have thought about to instal two roller bearing on each side, but it's complicated.

Lars

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Hi,

Now I maybe have found a simple and quick fix of my slipping focuser.

I have done a simple test if I can increase the friction with just sandpaper glued to the steelplate. It looks to work very well, maybe it will not last forever but then I just glue a new sandpaper on the steelplate. The sandpaper is a replacement for the diamond shape surface.

I hope I can do outdoors test in the coming nights.

Here is a bit more text and photos what I have done:

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page05-project-focus-friction.html

/Lars

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The cheapest, easiest instant fix is to use a couple of rubber bands, one end fitted to the eyepiece locking screw and the other to a fixed part of the focuser. using the correct tension will keep the focusing action firm yet balanced.   :icon_biggrin:

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1 hour ago, saac said:

This is an interesting thread. I've often wondered what are the perceived pros and con of the different type of focusers. Does anybody know?

Jim

I have been amazed that in recent years I have seen lots of people saying they want to upgrade their focuser by moving from a R&P a crayford. I don’t see the sense in that. Yes there are differences in manufacturing tolerances  between focusers but with other things being equal I would have a R&P focuser any day. If they are adjusted correctly they will not slip.

The Baader steeltrack may be exception amongst crayfords, but as I have never used one I can not say for certain.

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Sorry, I was not clear enough. I use this telescope only for astrophotographing. It already have motor focus. Camera and pheriphals weight about 1.5 kg.

 

But same problem, focuser slip. Especially when doing automatic focus, then the motor move back and fort. If it slips then it will newer find the sweet spot focus.

 

But I think I have solved the problem now. Just need a clear sky to do a test. Please give me that before the summer sets in!

 

Lars

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I've often wondered if substituting the working surface with diamond nail files or diamond paper would help.

The price of diamond "abrasive materials" has fallen remarkably in the last couple of decades.

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  • 2 weeks later...

Hi,
Today I have solved a lot of problems.

The problem with the wrong parking position of the focuser must have been caused by the fact that the focuser motor has too low torque now when I have increased the force on the focus axis. Not the temperature compensation which I first thought.

Here you can see how I have temporarily solved it with help of a spring:

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page06-project-focus-friction.html

Maybe I will have a better solution to it later, but first I must test it out.

/Lars

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  • 10 months later...

Now I solved one more problem, this time the high friction in the fine-tune focuser. I found two things that cause that.

First:
The friction in the fine adjuster gearbox was very high, caused of aging grease.

Second:
One of the screws that press the bearings against the stell plate was to short.

Here you can follow the details if you find it interesting:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page07-project-focus-friction.html

I think that the problem with aging grease can be a very common problem, my EQ6 mount is very stiff too. Maybe time to take it apart and regrease it.

/Lars

Edited by Astrofriend
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  • 2 years later...

In these rainy days I thought a lot about my focuser. The original friction focuser which is very good when only attaching low weight oculars but doesn't work very good with heavy loads as a camera. It will never work as good as I wanted it to do whatever I try to do with it. Now I try to rebuild it with a rack & pinion construction, and more, I try to 3D-print the parts.

Have a look on my first prototype gears:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page07-project-focus-friction.html

This is very exiting if I can use the 3D-Printer to make this rack & pinion mechanics.

The 3D-Printer is my best friend !

/Lars

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Today I did some more work on my implemention of a rack & pionen gear into the friction type focuser. I took apart the focuser and did some measurement how much space I have and what parts I can reuse. It's very limited space and will be difficult, but not impossible.

Here is what I did:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page08-project-focus-friction.html

/Lars

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  • 2 weeks later...

I continue with my attempt to build a 3D-printed 3" focuser. I have now made the CADs for the rack & pinion gears, started with the worm gear, make the focus shaft holder and the focus chassi. Lot of details left to solve.

Here is my document, see page 7 to 9:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page07-project-focus-friction.html

Maybe it will not work with enough precision, but it's exiting to do at least a prototype. I can always hand over the CADs to someone who has a metal 3D-printer.

/Lars

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  • 2 weeks later...
  • 7 months later...

I realized that I put too high demand on the 3D-printed plastic parts. Now I have changed the construction and use three push pull screws to move the focuser draw tube. At the same time I now don't need the extra gearbox. The screws itself is the drawbox. If I direct connect it to a 200 step stepper motor each step move the focuser 3.5 my (M4 screw). With the timing belt gears I can extend or reduce it. Less backlash with this construction.

Here is my drawings over the design:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page12-project-focus-friction.html

Lot of small details still to solve, but much easier now compare to the earlier design.

/Lars

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In case it does not work, due to the heavy weight, you also could try a collapsible focuser.
There's zero play and can hold a massive weight without sagging down.
The only disadvantage is it's rather massiveness. But it does the job perfectly.

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My friend had a focuser of that type, very interesting. But I think I have a lot of extra torque to handle the weight.

 

Have a look at the bottom, the last version and I think it's ready for the 3D-printer:

 

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page12-project-focus-friction.html

 

/Lars

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Today the roller bearings I orderad arrived. The are of a thin design, 6x12x4 mm. The diameter can't exceed 12 mm because of the narrow space inside the focuser.

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page13-project-focus-friction.html

It's relly exiting when you can start assembling the parts, so far I'm really satisfaid over how good this part of the focuser work. Now I can proceed with the push pull rods, I have changed it from M4 to M3 to have a better resolution, about 2.5 my per step (without micro stepping).

/Lars

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  • 2 weeks later...

Now I have 3D-printed more devices to my motor focus. I can start to assembling the different parts and see if they fit together. Looks very promissing to me.

Here are the last details:

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page14-project-focus-friction.html

I must decide if I will use half step or full step mode drivers. Both have pros and cons.

/Lars

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