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I think that you are very harsh on little ST102. It does have some color (a lot if you attempt high power planetary) - but it is very usable even at powers above x20. Following images were taken with ST102 and DSLR attached to it (at full aperture): Only when you zoom in - you can see traces of CA on the edges (granted - this is not very high contrast target): Here is moon shoot with ST102 and DSLR: Yes, there is yellow and blue fringing on limb, but it is rather decent image.

It will be interesting to see what you came up with. When I was thinking about it - here is what I envisioned as a solution: it is based on above principle - spring with two threaded parts. So - two threaded parts - one "attached" to outer focusing ring, while other free - but supported with several thin pins (like 2mm steel pins) - each of which has spring around it.

How about "Screwy Vlad" focuser (it is thread based after all) ? Well, name it as you see appropriate, I won't mind one way or another.

Yes, many 3d printed parts cost insane money from DIY perspective. If one wants rings - it makes more sense to invest in ender 3 and some filament and print them for themselves for same sort of money But I guess if someone runs a 3d printing business, then there is all sorts of overhead / costs - "labor" carrying most of the cost. Second being merchants profits I printed 4 of these for an acquaintance from a local forum for free (it costed me something like less than euro in material and power) https://www.teleskop-express.de/shop/product_info.php/info/p14030_Wega-Clip-Filter-for-1-25--Filters-on-Canon-EOS-APS-C-Cameras.html "Original" with shipping would cost something like 65 euro per piece

Not sure what you are asking? If you want to reuse and modify the design - sure, I here by declare it public domain and you are free to do whatever you want with it. There is FCStd / FreeCad project attached in one of previous posts - so you can load it in FreeCad and modify / export it to suit your needs. How do you plan to do it? I'm rather curious. One way possibly would be to add planetary gear arrangement around second focus ring? But that would make focus rings very thick compared to the rest of assembly?

To which one? Not sure how can I add it to helical one. But I did consider doing anti backlash thing on it with split rings and springs. For regular focuser - one for achromat, I won't bother with dual speed. I'm aiming somewhere in between. Say that with regular focuser, travel is around 20-30mm per turn, and with 1:10 reduction - that is 2-3mm per turn. I'm aiming for 10mm per turn - so it will be somewhere in between of "normal" and reduced. It won't be as slow as reduced when needing to rack considerable distance, yet it will be finer than regular one in focusing.

That can be easily adjusted - focuser travel is draw tube thread length - focusing knob thread length (provided that we opt for 100% engagement all the time). It looks like I have good dimensional stability / accuracy with my 3d printer then ? That is interesting idea. Here is recording of how it works now: focuser_operation.mp4 That strange background music is coming from a TV in the other room - but all you can hear is focuser and my breathing - it is rather quiet in operation, but you can get the sense of it not being smooth from the recording.

I identified such effect with bilinear interpolation and small angle rotation - here is brief explanation of what happens in that particular case: Look at two columns of pixels and very small angle rotation: Red line represents pixel centers after rotation (after rotation it will be vertical, but since we haven't rotated yet - it is tilted and pixels are at their regular positions). In order to rotate - we need to calculate pixel values along that red line. Now for each pair of pixels, if we do linear interpolation between its values - at the bottom we will almost exclusively use left pixel value - it will be like 99%:1% ratio and on the top - it will be other way around 99% of right pixel and only 1% of left. These two extreme cases introduce minimal pixel to pixel correlation, or softening / blur in the image, but as we move along vertical - we progressively move towards 50%:50% case - which perfectly mixes two pixel values or averages them - and that reduces noise the most / adds most blur / cuts off high frequencies the most. Same thing happens in horizontal - except above diagram is rotated 90 degrees. This creates rectangular pattern across the image of different levels of pixel to pixel correlation and thus different levels of blur that will pop up after certain types of processing. Simplest way to see this is to generate random noise, then rotate it by small angle and then clip low values. Standard deviation variation (due to this blur) will produce this rectangular pattern: rectangular pattern wavelength will depend on rotation angle. Above is produced with 2 degrees rotation. If polar alignment is not as good - there will be field rotation over the course of imaging session, and subs from the end of session will be slightly rotated compared to those from beginning. In order to align them for stacking - the will need to be rotated back by small angle - which causes above effet

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.

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.

Well, I open that in FreeCad as a single project , but I guess it's some sort of zip archive with bunch of stuff in it?

@Chriske Here are 5 main parts (in fact - only 4 are needed for focuser - one is M42 end for draw tube): Casing.step Casing cover.step Draw tube end.step Focusing knob.step Draw tube.step These are with built in tolerances for my setup.

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

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.

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! ) 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

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?

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

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: 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).

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. 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.

Ok, here is a brief sketch showing how much room there is: 86mm above is ID of OTA and focuser needs to "slide" in it (It does not need to do that, but it's nice to have focuser as wide as telescope body and not wider). 55mm OD is draw tube. It's made out of aluminum and has 2mm thick wall. It will have 2" 3d printed adapter at the front. 8mm white circle is T8x8 P2 lead screw used for driving the tube back and forth. It is stationary (does not move with tube) and tube has "nut" attached to it and lead screw is threaded thru that screw. Lead screw needs to sit between two tubes and can't touch either of them because there needs to be some space for: a) nut b) telescope housing that goes into OTA tube and holds nuts for OTA attachment screws So it is pretty much set in position where it is. I need to transfer rotation of focusing knob - which is in X direction in above drawing to lead screw (Z direction in drawing) - so I need some sort of bevel gears (or something else) - like those marked in blue. Issue is - lead screw to draw tube distance is set and small and draw tube needs to move freely with respect to lead screw (pass over it back and forth along its length), and I don't think there is enough room to put any sort of sensibly sized bevel gear in there (one that can be 3d printed). Clearance marked with red arrow is not issue - bevel gears will sit outside of the OTA tube, but clearance marked with green is the issue - as draw tube will pass in that space where bevel gear needs to be. One possible solution that I came up with is this: But it has implementation difficulties and those are: - sourcing so small closed loop timing belt (alternative is nice way to make one from open to suit, but I don't know nice and easy method of doing that) - attach timing belt pulley to lead screw - increased number of off the shelf components - which increases total cost of the unit (and I was aiming for cheap solution - this is going to be paired with 80mm achromatic lens from ali express, I'm aiming to provide nice working telescope at lower cost for those that have access to 3d printing).

Sure, here it is: Telescope body.FCStd It contains several bodies - which should be thought of as parts. I did not bother to properly arrange object hierarchy. Only thing that is missing is 1.25" eyepiece adapter at the end - but it screws in via T2 thread. M43 thread on the start connects to binocular lens cell.

Square stars are consequence of nearest neighbor interpolation when zooming in. Look at the same image zoomed in to 500% - but with nearest neighbor interpolation: That is the origin of the myth - most software a decade or two ago used it as default when zooming in (as it is very fast and easy to implement) - and people got impression that data looks like that, but it is actually artifact of interpolation used. But nowadays - nearest neighbor interpolation is no longer default one - so we can rarely see the effect when zooming in.

Thing is - large telescope with mosaic and binning is "as fast" as small telescope for same target resolution. Here is simple way to understand it. Imagine you have 4" telescope and 8" telescope. Let them both be F/5 for the sake of argument (easy calculations. This means that focal length of 8" is double that of 4" (same as aperture). Let's further say that 4" telescope spends 4 hours on target. You'll need 4 panels to cover same FOV with 8" telescope, so you can spend only 1 hour for each panel to get same total imaging time. 8" gathers x4 more light, but since focal length is twice as long - "pixel surface" (sky covered by pixel) is 1/4 that of 4" (sampling rate is halved). Two cancel and 8" is as fast as 4" - but then you bin x2 - and 8" now becomes x4 faster. So each 1h panel will have same SNR as 4h part of image with 4". You end up with same speed between two systems - except the fact that 8" will produce sharper image because of Airy disk component of final FWHM. It is indeed worth doing wide field mosaics with larger scope - it will be equally fast as with smaller instrument (if you target same sampling rate) - or even faster if you go for lower sampling rate, which is fine for wide field - as you've seen - nothing bad comes from under sampling (no square stars and such ).

It also shows that large scopes are excellent wide field instruments Level of detail at that scale is beyond anything lens or small focal length scope could capture.

@Chriske I've considered the idea of using T8x8 P2 lead screw as driving element, and I have a bit of spacing issue. Say that I want to design 2" refractor focuser. Draw tube will be OD 55mm (that is what I can easily source, with 2mm walls, so ID is 51 - enough for 2" eyepiece / 50.8mm to fit in) and I'm hoping to use it on 90mm OD tube, with again 2mm walls - which leaves 86mm of space to fit focuser into OTA. If focuser is 86mm wide and draw tube is 55m wide - we have something like 15.5mm to work with. That is plenty for 8mm lead screw and 3d printed integrated nut, but I don't know how to transfer knob motion onto the shaft. Bevel gears should do the trick - but I can't fit anything sensible in those 15.5mm, or rather lead screw will be like 3mm-4mm away from draw tube - so any bevel gear that I can fit there must have diameter of 16mm max. That is too small. Alternative is to use GT2 timing belt to move bevel gear away from draw tube? Adds complexity and I'm not sure how to connect 8mm lead screw with small gt2 pulley without needing a lathe to drill m5 bore into lead screw. Any ideas?