A “sabbatical” project, mostly crafted this month on planes to and from Norway, where I was to travel to Kristiansand with friends from the digital art collective Lab212 to set up a digital art installation. It’s designed with one major constraint: It must fit in the inner pocket of my jacket (well, a typical jacket), excluding the rods.
This is a 3D-printed Dobsonian telescope built around a 76mm/300mm parabolic mirror kit. While there are plenty of mini-scope models on the Internet, I wanted something that looked like a Dobson that would be a little more rugged than it would be through a clothes dryer, but without compromising what matters:
- balance
- smooth movements
- hardness
- retractable
- focusable eyepiece holder
- Minimal style (completely subjective)
hardware
- PETG-CF filament
- 4mm carbon rods
- M3 Screw and M3x4.5×4.5 Heat-Set Insert
- A SPRING
- Nylon screws for leveling both the primary and secondary mirrors
- 4 magnets for secondary
- A little paraffin to lubricate the focuser
- A Lycra light shroud that also helps delay dew formation on mirrors
The focuser follows Analog Sky’s recipe: the tube that receives the eyepiece also carries the movement itself, with a round thread that prints extremely smoothly with very little play. No additional hardware is required – the eyepiece is self-operated by the flexibility of the plastic wings.
The holes for the rods are all straight, which forces them to form an arch, which “locks” the structure in place.
The alt/az movement uses “Teflon pads” (actually gray HDPE or UHMW for furniture legs) with rubber backing, scaled and glued.
Download 3D files on Printables • Discussion on Astrosurf
If you make it, the real trick for ease of mounting is to chamfer the carbon rods with a 1mm chamfer on both ends and seal it with CA glue. View chamfer images in the gallery.
optical testing
Sadly, the results were not good. We were used to buying very nice l/6 or better from Aliexpress until recently, but this is much more accurate. It was very smooth, with a nice edge over Foucault, but it has been improved by over 70%. With the eyepiece I chose, placing it at 30x power, it doesn’t look much, and it retains its “real binocular” status. But this mirror is so small that I wouldn’t make it again – realistically the cost of polishing would exceed the entire project.
Edit by December. 11th :Of course I wasn’t opposed to painting it again. It is now hovering around 0.9 Strehl. The star test with the selected eyepiece shows nice symmetrical defocused stars and I can now find individual spider web threads and distinguish dew drops on a nearby power pole, whereas I did not see spider webs even with the mirror on as it was from the factory. I still need to report a proper “showable” bath with enough interferograms, my last test was 4 interferograms and it had a lot of noise. So that’s great but I have to get it coated now, and working on this small mirror created some challenges in handling it.
All test images below are before reconfiguring
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