3D printable 6" f/5 compact travel telescope model | Hacker News Reader

3D printable 6" f/5 compact travel telescope model

printables.com

96 pointschantepierre48 comments

Hello HN,

A friend released files for this 6" f/5 telescope conversion kit yesterday.

It is a synthesis of a lot of telescope builds he, I and a few friends made over the last years, as well as general ideas floating in the amateur telescope making community.

It is compatible with most existing 6" f/5 newtonian optics kits. I've tried his model IRL and was pleased with both the movements and the focuser, which is often lacking on commercial offerings (which are also heavier and bulkier).

Unmounted, it forms a quite compact package to hike with or take camping.

It is licensed under CC-BY-NC-SA as he would like to sell parts kit at some point, but the files found on Printables are fully functional.

Do you have any recommendations which optics kits to get and where to get them (preferably available to the EU)? What about a good budget entry eyepiece?

6" f/5 kits sold by GSO are okay, but the best choice is to find any second-hand skywatcher 6" f/5 tube on your leboncoin/craigslist equivalent and scavenge the optics. For industrial optics, they are on average exceptionnally smooth and have a good shape with few defects.

The 6" f/5 optics kit are a bit of an oddity, those we test with our interferometers are consistently good, whereas smaller or bigger diameters are subject to a lot more vatiations.

If buying a used skywatcher dobson is the best way to get good optics, wouldn't I, as a beginner, be better off just using that dobson instead of going the DIY route? Is the DIY telescope only better for portability, or are there other advantages to building it?

The 6" f/5 can often be found cheap and without a mount, but if you are beginning, absolutely find some secondhand dobson before going DIY.

Or better : attend a few nights at a nearby astronomy club, that will allow you to get a feel of the practice, what you can see, what you should expect...

Depending on your location (city? middle of nowhere?) and willingness to drive to darker skies some types of instruments and targets are better than others.

This kind of kit is focused on being compact and lightweight, but that can be an advantage or a non-goal depending on what you want to do.

For budget entry eyepieces, the generic "goldline" eyepieces have pretty good bang for their buck, especially the 9mm/6mm ones. You can find them unbranded online, or the "redline" equivalents.

I'd recommend a quality 20/25mm plossl rather than the gold/red line 15/20mm that display a lot of issues.

For better eyepieces, at f/5, the rubber-top generic eyepieces with color bands sometimes branded as "artesky" are great for their prices too.

If you have a bit more budget, Explore Scientific is generally the overall best quality/price ratio. Avoid baader hyperions, except for the 13mm which hits above its price (second-hand)

Since this is hand tracked, how's the feel of the movement while tracking or slewing in general?

Dobsonian mounts are made to feel great and work with friction between (here) plastic and teflon pads, both in the altitude and azimuth axis. The goal is to have your telescope properly balanced so there is no need to apply force to hold it in position, nor need more force to start a movement than to keep moving, which would lead to jerky motion. Properly balanced, everything must feel smooth and easy.

TIL Bahtinov Mask

https://en.m.wikipedia.org/wiki/Bahtinov_mask

That’s a clever device for focusing a small scope.

Not only a small one, it’s being used in real observatories for some quite big telescopes.

Also for focusing a digital camera for astrophotography.

Great build, but I can't find the README and what is the recommendation if it's not PLA? PETG? (I will give this a try with PLA-CF...)

Yeah his site isn't currently the clearest for that.

Here's a direct link to his build instructions [1] and I've made a temporary by-item BOM [2] while he improves things.

PETG is great, ASA and ABS might be better if you have the capacity.

     [1] https://la3emedim.fr/assets/SMALLEST%20-%20README.pdf  
     [2] https://docs.google.com/spreadsheets/d/1XXZB4jRtvtlDIqlR69C0FRri8x8lZDpxO6ZGnSp3L2Y/view

What about glass or carbon fiber filled PA6?

I don't have enough experience with nylon to assess whether it would be suitable for a full telescope, sorry.

All plastic is going to warp over time. So that probably seems like overkill unless you're planning on abusing it.

That said, I would give PETG-CF a try. I've made parts with that and it's certainly been good, and is definitely better than PETG.

Pla-cf is generally more brittle because of poor layer adhesion due to the fact that carbon fibre doesn't stick to carbon fibre. Pla also doesn't need the additional stiffness CF provides so it's just a gimmicky filament.

Petg is a better option.

PLA-CF isn't a high strength filament, but I do really like it for large functional prints that need to be stiff and look good. It has a nice texture right off the printer.

I 3d printed the 114 hadley telescope years back.

Saturn and Jupiter were easy, but not super clear. Mars was maybe just out of reach even on good nights.

This 6 inch will likely be perfect. Very clever design as well!

The Hadley is the perfect gateway to telescope building.

A 6" packs a lot more punch in a not too much bigger package, and 6" f/5 consumer mirror kits that we test often range from okay (0.77+ strehl) to actually quite good (around 0.92).

You'd get quite an improvement on planets !

I've wanted to build one for a while, but seems theres not much info on the actual optics and which ones are worth getting do you have a list anywhere of where you had good results from?

Hey, I missed your comment. Nowadays the best source for optics is often a secondhand instrument where you scavenge the optics. If we specifically talk about a 6"f/5, most recent industrial tubes have really decent optics.

Buying new optics online, avoid any ad where the price seems surprisingly low. For example you can find 6"f/5 mirrors advertised as lambda/8 precision. There's a lot of abuse with this denomination : some sellers use surface precision, whereas we usually talk about reflected wavefront, so lambda/8 becomes lambda/4.

Then, there are a lot of ads for optics at great prices that aren't specified as parabolic (in the case of newtonian mirrors). Most often those are spherical, which is totally unsuitable except for longer focal ratios (and that quickly becomes impossible as diameter grows).

On the other hand, I recently ordered a 40$ "lambda/8" spherical mirror, and it was a great sphere, so a great starting point to polish it to a parabola.

It'd be great to hike with one; I've heard a some of the Appalachian Mountain Club huts have telescopes.

On the subject of viewing targets, I unexpectedly found Globular Clusters to be really neat. As your eyes adjust, the stars just keep multiplying. And you can easily catch great looking photos.

Seems very cool but all of the photos and videos I could find is about how to build it not the most important thing.. how good it is as an actual telescope how does the moon/stars etc look though it

The actual view has little to do with the build of the telescope, and lots to do with whatever optics you put in it.

The mechanical part can influence the view via "tube currents" (air flowing in the optical path due to heat convection) or by not allowing good heat flow to and from the mirror so it stabilizes quickly.

There's a few more things to consider, but remember it's mostly the optics.

Doesn't the mechanical part need to position the optics with a fair amount of precision to get a good view? Id imagine very slight movements or misadjustments could cause issues.

I don't know much about telescopes :p

You need smooth movement on altitude and azimuth axes, then the telescope must be still and vibration-free to enjoy the view. The other constraint is structural integrity to maintain alignment of the optics. Those two things are the basis of all builds (except for optics!).

Slightly OT: how fragile are 3D printed things? Do they break easily? Do they feel fragile? The rods in thi design, might a person break them by accident? How about intentionally?

The durability depends entirely on print settings (infill percentage, layer height), material choice (PLA, PETG, ABS), and load direction (perpendicular to layers is weakest), but properly printed PETG or ABS parts with 25%+ infill can be surprisingly robust for functional parts like telescope components.

According to the description, the rods are aluminum. Printing rods this long is near impossible and if it was, they definitely wouldn't be rigid enough for this application.

To answer your main question, it depends. There are many materials you can print with, they're strength can differ greatly. Some are very rigid, but somewhat brittle, others are less rigid, but also less brittle.

Then it also depends on the design, the video[0] below is a great video on designing for 3d printing and the considerations needed for strength.

Finally, print settings matter. Temperature can affect layer adhesion. Infill and wall count can affect strength and so can print direction (see video).

3D prints can be pretty strong though. I'm using 3d printed mounts to hang snowboards on my wall, for example. I've printed plenty of other stuff I really couldn't print by hand. And I'm only using FDM printing, with SLA and SLS printing even stronger parts can be printed.

0: https://youtu.be/mziT7KV-fRI

Depends on the material and settings. But for example PETG it's as strong as a water bottle.

That can be a bit misleading, I feel. It depends on how well the layers are fused together, post-processing, or even the orientation of the print. At best, it can be nearly as strong as a water bottle, but I wouldn't expect it to be.

Really depends on what material you're pushing.

PLA is not strong and doesn't like to warm up in the sun, Silk PLA (made to just look nice on the shelf) can even be brittle. But there are better materials.

PETG is entirely reasonable, I store my pots and pans on PETG hooks and they're not oversized. The superior 3d printers can print ABS which is above any and all doubt.

There are also specialized materials, such as carbon infused PETG, and there are engineering materials at 500€/kg with some interesting properties. But the gist is that PLA makes you go "this sucks, must be printed", and PETG makes you go "oh this is fine".

Then there are resin printers that selectively UV cure a liquid to polymerize it. Last I tried that sticky goo it was awfully brittle and borderline unusuable. I heard it improved, but that's why resin printing is not everyday parts but figurines.

Regular PLA is actually stiffer than PETG/ABS, but it's more brittle (so has less impact strength) and a pure formulation has a lower softening temperature. It's generally not suitable for parts that go under the sun, unless it's some other formulation such as some HT-PLA variant, in which case it's actually a good choice for parts that need to be thin and stiff.

Under most cases, you won't get the same inter-layer adhesion with ABS, so while you get better impact resistance, under most circumstances PETG will yield more durable parts that won't delaminate under the same stress conditions. For outside use under the sun, you should use it's cousin ASA.

To respond to the OP.. 3D printed parts can be incredibly durable when printed correctly. The parts need to be designed for 3d-printing in mind, like most other manufacturing methods. A 1cm-thick 15% filled PLA slab that has been printed vertically might be easy to snap in half with two hands, but it becomes almost impossible to break with bare hands when printed horizontal, and requires a saw to be cut when filled to 50%+. And this using consumer-grade printers.

I'm using 3d-printed parts for work and at home, some in use for almost 7 years at this point, and the only telltale sign is the layered look.

The rods in the design are not 3d-printed, which makes sense (most plastics would be too flexible, and 3d-printing a rod is always more expensive).

You can get excellent inter-layer adhesion with ABS, if you crank temperature up and disable cooling.

I print on a Mk3s in a photo tent and have had no trouble eliminating layer failures as an issue.

I've often found PLA to be fine. But small features on a large part can break if dropped.

Of course it all depends on you application.

> PLA is not strong and doesn't like to warm up in the sun

"Strong" is ambiguous. PLA is stiffer than many other thermoplastics (resists bending loads well), but it's not as tough (impact strength is poor/it's brittle).

PLA becomes even stiffer and resists higher temperatures than many other thermoplastics if you anneal it.

You want stiffness in a telescope because deflecting under load moves your optics out of alignment. A telescope has moving masses as well, so stiffness along all axes under load is important.

> that's why resin printing is not everyday parts but figurines.

It's probably more to do with it being messy, smelly, and overly finicky business[0] compared with FDM (but there's zero chance of burning your house down, I suppose.)

[0] levelling the build plate, cleaning the FEP on failures, replacing the FEP, washing the results with IPA (mostly), having to keep the resin warm to get good results, draining the resin when not used for a while, faffing about with orientation and supports is a black art, etc.

The rods are heatshrink tubing wrapped-aluminum. I personnally have built a few partially 3d printed, or full 3d printed (except for a few hardware items and carbon fiber rods) telescopes – it all depends on print preparation skill, correctly drawing for 3d printing, and wise choice of material and print conditions.

If you've never FDM 3D printed anything before, you'll probably find it about on par with most other plastic objects in your life, even if it is technically weaker (for the most part).

Most things aren't particularly fragile - print out a 1" cube in any material at default settings, and you'll be able to stand on it. You probably won't find a way to break it with your bare hands, short of pitching it into a concrete wall.

Haven't printed this telescope but I have printed some pretty big things for example a front rack for my wife's bike.

It's on the 3rd version, the first one broke on small collision (the bike fell over while standing), the 2nd on a big collision (my wife hit a bus stop :/ ). The 3rd version survived several years of daily use including hauling stuff from the mall.

The key is to split the design into parts that need to be strong in 2 axis and can be weak in the 3rd axis (to work around the layer adhesion problems). So you print 4 walls laying flat not standing vertically. That plus including rope or zip ties for elastic connections where needed - and you can make stuff that is plenty strong enough for most daily uses.

I printed with PLA and PETG and both are ok - PETG is less stiff but handles direct sun better.

I’ve just been looking for a telescope that’s backpackable. Thank you for posting this!

Is there some more info? What else do you need, other than a spool of not-PLA? How much does it cost? Where can you source the other materials?

There are a bit more information on his website [1] and I've made a temporary grouped BOM [2] :

    [1] https://la3emedim.fr/assets/SMALLEST%20-%20README.pdf  
    [2] https://docs.google.com/spreadsheets/d/1XXZB4jRtvtlDIqlR69C0FRri8x8lZDpxO6ZGnSp3L2Y/view

Basically anything but PLA will work, PETG will be the best difficulty/cost compromise.

really wish stuff like this was just all correlated on github or something so a community could easily grow for projects like this

I'll push him to tidy this up :)

Why four sets of support struts?

It's a compromise between having a geometry that is rigid enough to ensure the bottom and top planes stay aligned while the telescope slews up and down, while having a practical shape at the bottom (a square) and avoiding struts going into the light path, which happens if you do an hexapod shape without making the base larger to compensate. Since his initial goal was to make this printable on an ender3 build plate, the smaller square shape was preferable.

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