Frustrating focuser problem

[Astrofriend]

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

[glend]

The TS130 APO, has a few focuser options, and they are all very strong, with the 2.5" and 3" Rack and Pinion also used in other TS APOs. I have rhe 2.5" R&P on two of my APOs, and have never has a problem with them slipping when the correct TS motor focus adaptor is used.. Did you contact Teleskop-Express before you disassembled the focuser? They have an excellent service department. Your focus motor bar is a non-standard part for that focuser and it appears your using the wrong holes to secure the bar to the focuser body, this could be a problem. Was there a reason you could not use the standard TS motor focuser mounting bar?

[Astrofriend]

Hi Glen,
I think my focuser was more meant to visual observations from the earlier owner of the telescope, that's why it don't have rack and pinion. I normally do most of my repair myselfe, in this case it looks that it's not a repair that's needed, more a modification.

I built my own motor focuser system from USB-Focus, that let med get it more compact with less protruding details. I can't see why my mounting holes should cause problem, was it something special you had in mind?

I have got answer from TS Germany, they have the press bearing and steel plate as spare parts. But I think even if I buy that parts new I will get in the same slip problems.

Now I'm eager to test if the fix I did make it work, just waiting for a clear sky. Outdoor tests is different from indoors test.

By the way, I constructed a Depth of Focus web calculator:
http://www.astrofriend.eu/astronomy/astronomy-calculations/depth-of-focus/depth-of-focus.html

Good to find what distances needed when design the gearbox. I have 1.7 micrometer per step for a f/7 instrument. I could have much bigger steps, but the focuser draw tube is only 100 mm long and the digital range 64000 steps, or maybe half if doing half steps.

Thanks for the comment!

/Lars

[ab1963]

My advice as far as focusers go is just get a Feathertouch,I have played around with stock focusers in the past and it's not worth the heart ache ,It is the only part of the optical train that moves it has to be the best money can buy and a Feathertouch is it they just do what you want them to do ,Hold weight and snap in and out of focus then hold that focus as long as you want ,The best.....period .

[Astrofriend]

Hi Andrew,
Yes, Feather Touch focuser are like a Rolls Royce. I have looked at them earlier and belive it must be perfect in precision and it has rack and pinion too.

I think this model best fit my needs:

http://starlightinstruments.com/store/index.php?route=product/product&product_id=403

But one problem, the threads doesn't fit my telescope or my field flattener.

M120 inlet compare to my M92 and M86 outlet compare to my M80. If I connect them the thread adapters will take some space, maybe it still could work.

For the moment I think I try to get the most out of what I have.

Thank you to remind me about Feather Touch as an option!

/Lars

[ab1963]

If you speak to Wayne at starlight instruments he can make any adapter you need to suit,Let him know what you want and they do the rest ,I know they're not cheap and my focuer is worth as much as my scope just about but it carries a Baader 2" diagonal ,4x powermate and Ethos effortlessly and performs like it's carrying a 1.25 diagonal and an ortho,They just take away any hassles

[AstroApprentice]

Similarly Precise Parts can make any adapter you need. Beautifully machined, but not cheap. Like with FTF, quality costs more

[Astrofriend]

The problem with the low friction has increased.

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page04-project-focus-friction.html

I see that the backside of the steelplate has much higher friction. I just have to flip it upside down, but then I have to drill new holes. Before I do that I shall order a new steelplate. This will be a summer work, for the moment there are so many other things that take my time.

Other then that I'm very satisfied with the focuser, so I don't want to changed if I can repair it or get it more functional then it even was when new.

Lars

[Rkonrad]

I had lot of scopes with theses toothless focuser's some work well ,WO, Moonlite etc but they have to be machined 100% there are a few sites on how to improve it on the lesser ones , but you need the equipment to do it

[Astrofriend]

Hi Richard,
I hope I can fixed that friction problem, it's not so easy to change to another focuser and that will be expensive to.

Do yuo have any link to the pages you have seen that have a fix for it?

Lars

[Astrofriend]

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:

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

/Lars

[Saturnine]

Hi Lars
I've made a few crayford focusers over the years and to achieve a reasonable friction for the drive I've glued a strip of the finest grain wet & dry ( 1600 g ) to the flat on the drawtube. Have worked almost trouble free for me , giving a positive drive.

[LewisM]

The way I always fixed Slip-o-matics (Crayfords) is by removing the pinion shaft, removing the knobs and then shrinking a shrink tube onto the shaft. This puts tension on the drawtube and increases friction. 100% better and cheap as chips, no glue needed, no abrasion of components as you get with sandpaper.

$1 per foot...http://www.readytoflyquads.com/3-0mm-heat-shrink-tube-black

[Astrofriend]

Short report:
It works perfect, no slip, more like a rack and pinion. Maybe it will destroy the axis in the future because of wear from the sand paper. But until that happen I will got lot of photos with perfect focus :-)

/Lars

[Saturnine]

For the amount of drawtube travel and the speed of rotation of the shaft there should not be any wear to speak of as they are moving together, unless there is a great deal of slippage, which there should not be. Should last a lifetime with normal care, maybe replacing the fine sandpaper once or twice.
Like Lewis' method of using heatshrink fitted over the shaft, why didn't I think of that, much better than metal on metal contact.

[Astrofriend]

Hi Jeff,
We will se in the future what happens.

But the sand paper for sure will make the surface of the axis more rough after some years of use. That could cause problem because in that area of the axis there is a plastic piece that by force press the axis against the steelplate, a force that you can adjust by a screw.

See int the middle of this page:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page03-project-focus-friction.html

By the same reason I think there will be a problem to put a schrink plastic tube on the axis.

If I can replace this palstic piece with two rollerbearings on each sida of the axis center point it will be ok, but there is no space for that.

What we say, croos the fingers and hope it works :-)

/Lars

[Astrofriend]

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.

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

[Astrofriend]

I spent a day to investigate why I had that high friction in my 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 common problem, my EQ6 mount is very stiff too. Maybe time to take it apart and regrease it.

/Lars

[Astrofriend]

One more night with a clear sky but moonshine.

I took out my equipment to the balcony to do a test.

The focuser worked very well up to the highest altitude I can point my telescope, about 55 degrees. More than that and the neighbour's balcony above block the free view to my telescope.

This is what I got:
http://www.astrofriend.eu/astronomy/astronomy-photo/globular-clusters/m13/m13-globular-cluster.html

I have very high light pollution so nice photos, but valuable as test.

/Lars

[Astrofriend]

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

[Astrofriend]

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

[Astrofriend]

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

[Pepper]

Lars, have you looked at 3D resin printers? My brother just bought one and the detail is excellent. Seems alot more robust than a typical 3D printed part.

[Astrofriend]

Hi Pepper,
I have watch them on YouTube and looks to very good. But in my case, if I need higher precision I think I do the prototypes with my printer and then hand it over to someone who has a deicated 3D-printer with high precision or even a metal printer.

What is the maximum size your brother can print with his resin printer ?

/Lars

[Astrofriend]

A second version of the design of the focus chassi, now with a flex joint to act as a hinge instead of the earlier sliding roller bearings.

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page10-project-focus-friction.html

/Lars

[Pepper]

His current printer doesn’t have a huge print size. Can’t remember exactly but I know it was smaller than I required.

[Astrofriend]

Hi Pepper,
No problem, my 3D-printer can take 230 mm diameter and 280 mm high at center, almost twice the size that is needed.

http://www.astrofriend.eu/3d-printing/3d-printer-setup/01-3d-printer-setup.html

/Lars

[Astrofriend]

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

[Astrofriend]

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

[Astrofriend]

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

[Astrofriend]

Today I installed the puch pull screws for the focuser drawer tube. As always when going from theory to practise there will be some complications. I had to do the holder of the screws adjustable to have it center the nuts in the timing gears. But that was an easy task.

Have a look at the bottom of the page to see the progress:

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

/Lars

[Joshua Bunn]

Nice work, Lars. I can see you have put a lot of thought into it and the stepper motor side of things to.

[Astrofriend]

Thanks Joshua,

During the summer I spend my time on the car and not very much progress on my astronomy projects.

Most of the nechanical on the foucser is today ready for a test. But the stepper motor isn't finished yet.

Latest progress:
http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page15-project-focus-friction.html

At the botton a link to the stepper motor driver project. A lot of new things to learn until I can manage this.

/Lars