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Linear motion components for DIY focuser?


vlaiv

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2 minutes ago, Chriske said:

Therefore the drawtube needs to be like this.

I see what you mean, and it is certainly an option.

I was hoping for general purpose focuser design that can be used on other scopes, but you are right. I'm designing for 86mm ID tube - which can only hold up to 80-90mm diameter lenses. Anything larger will go in 96 ID tube (or even larger) - and won't have such spacing issues.

80mm achromat will hardly need larger field than 1.25" offers (at least not many commercial ones come with 2" focuser).

Only issue that I see with using narrower but long draw tube is any sort of vignetting introduced by it.

image.png.3d71542385a5b62175017cb6de8defc0.png

It can even act as aperture stop in some cases, but I guess that is to be calculate on per telescope basis. 80mm F/10 and 80mm F/5 will have quite different beam characteristics.

In any case - this gave me some ideas on where I can "relax" criteria. No need for focuser to fit inside OTA - it can attach to the outside - so all the space up to 86mm can be utilized for lead screw. That coupled with say 45 or 40mm draw tube will give enough space for bevel gears to be used.

 

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image.png.3d71542385a5b62175017cb6de8defc0.png

 

This drawing is something like f/6.5. there you will vignetting indeed.
I 'm pretty shure a refractor that short will have chromatic errors in case of doublet. And as you were talking about cheap scope I'd think of longer FL. So far less Cr-errors and most important far less vignetting.

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1 minute ago, Chriske said:

image.png.3d71542385a5b62175017cb6de8defc0.png

 

This drawing is something like f/6.5. there you will vignetting indeed.
I 'm pretty shure a refractor that short will have chromatic errors in case of doublet. And as you were talking about cheap scope I'd think of longer FL. So far less Cr-errors and most important far less vignetting.

I already have 80/600 lens ordered from AliExpress.

That is F/7.5, so yep, I'll need to do some calculations to see what usable field is.

On the side note, I found these rather cheap:

image.png.1e9d6fc694b6ccbfa77d15598779483f.png

That needs 18mm of space, but I wonder how would I go about attaching it to lead screw?

Possibly 3d printed adapter that "screws" on to lead screw (held in place with some super glue) and has M4 threaded insert on the other side so I can use M4 to bolt above.

Btw, these are like 15e a set (maybe still a bit expensive in grand scheme, 3d printing is still an option).

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22 minutes ago, Chriske said:

15€...???😳😳😳

I buy a set of 10 for just a few €s

Above ones are metal, not injected molded plastic, but I guess nylon ones would do perfect for this application.

Shame I can't source those locally. Maybe on AliExpress?

It is only place where I can sometimes get free shipping, otherwise, shipping alone will be over 15e :D

 

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2 minutes ago, vlaiv said:

Looking at these, I have some confidence that 3d printed versions will work well?

Have you ever tried printing such gears and are they smooth enough?

Well no, for this I need to install a very small nozzle, 0.2 or even 0.15 or so.
I'm used to print very large things with big nozzles, up to 1.5mm, and fairly smooth I might say, but never tried to print very small things.
It needs a complete other set of settings to print these small things compared to my usual work...

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1 minute ago, Chriske said:

If you decide to use these I'll send a few.
OD 18.5mm

Thanks for the offer, but that would sort of defeat the purpose.

I can look at this scope build in two ways:

- first is that I'm building a scope for myself, then I would not mind using metal ones at all - in grand scheme of things, they won't be that expensive, and final scope will still be cheaper - but it will be made by me, which is satisfaction in itself, or

- I'm trying to design low cost alternative for people with access to 3d printer.

If I go by second point, then I'm not sure above will be accessible universally for that kind of money (on the other hand - I can't say that for threaded rod and linear rods either? Can I? I mean - all have access to things like M5 bolts?). I'm sort of starting to get lost here - paralysis by analysis of sorts.

I like the idea of cheap DIY project - but more sophisticated than usual "get PVC pipe" approach. On the other hand, having access to 3d printer - well, that in itself does not put things in affordable class, does it? People seem to charge insane amounts of money for 3d printed parts. TS wants something like 200 euros for set of 3d printed rings (and they use "special PLA" with dimensional stability up to 110C! :D )

https://www.teleskop-express.de/shop/product_info.php/info/p14170_TS-Optics-plastic-tube-rings---made-to-measure-for-tubes-up-to-180-mm-diameter.html

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4 minutes ago, Chriske said:

Well no, for this I need to install a very small nozzle, 0.2 or even 0.15 or so.
I'm used to print very large things with big nozzles, up to 1.5mm, and fairly smooth I might say, but never tried to print very small things.
It needs a complete other set of settings to print these small things compared to my usual work...

So far I've made fine pitch threads and set of worm/worm gear which seems to run smoothly. I did think of getting 0.2mm nozzle and going with that.

Maybe even printing those in nylon (not sure how it will go on 0.2mm nozzle), but that again is putting project out of "cheap DIY" scope.

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1 minute ago, Chriske said:

Another thing, I was able to open a few FC files, but there are to many, don't know which one to pick out of these list of 62 files
Could you please convert the files I need to STP format and send me these instead..?

Thanks

62??

Ok, sure, I'll see to convert those to STP and post them.

 

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8 minutes ago, Chriske said:

There's a extra ring, green in my assembly, what does it do..?

It serves two things:

- it gives additional distance needed for focusing on that particular telescope model, and serves as connection to another bit - 1.25" eyepiece holder

- it acts as a stop for inward draw tube travel

As is, I think there is already stop that prevents inward draw tube travel at the other side of assembly - near M43 thread, but when I designed the thing - I did not know that it will be there, so I designed this part to act as both connector to 1.25" nosepiece adapter and as a stop that prevents further in focus travel against that top housing part.

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6 minutes ago, Chriske said:

Never mind, I just read from your last post :

...but when I designed the thing ...

Yes, I've came up with it.

It is first iteration. I have a few things that I would change though.

First is thread pitch. As is - movement is too fine. There is also some level of backlash - about 1/6th of a turn. I think that 4-5mm per turn is more then enough. It has 18mm of travel, so 4-5 turns would be ok. As is - it needs 18 turns for full motion.

The way I printed it - standing up so that thread is printed fine - body of draw tube and housing both have layer lines that rub one against the other when draw tube is moved - that sort of creates a bit rough feel to the thing.

I did not use any lubricant, so that might help - at least squirt of dry PTFE type or something like that.

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