4 HN users recommended this explainer3 months2022–2026score 3.5
Years with an HN recommendation, 2007–2026
Commenters repeatedly recommend it in JWST threads as a great explanation of clever mechanical design that is accurate and long lasting.
Breaking Taps explains how the cryogenic mirror actuators on the James Webb Space Telescope achieve nanometer precision using flexures. It shows a 3D-printed replica and how one motor handles both coarse and fine motion.
The actuator is a genius flexure design that is both mechanically simple and accurate. Breaking Taps 3d-prints a working replica and describes the design. I highly recommend: [this video] The actuator breaking wasn't as concerning after seeing the design.
Explains JWST flexure actuators with a 3D-printed replica
Here a nice video that explains the mechanism that aligns each mirror: [this video] It is amazing that this is done with only a single motor. It works something like this: one direction of the motor sets which axis to align, the other direction sets the alignment of the chosen axis.
Explains how one motor aligns each JWST mirror segment
This video [this video] was posted here [link] about how the actuators work on the James Webb telescope. It's a great watch and shows how well thought out design can produce accurate and long lasting parts.
Shows how JWST actuators achieve accuracy and longevity
I have a related question, how far are you turning the thumbscrews when making adjustments? I think M3 standard thread pitch is 0.5mm, so to a first approximation that almost 1000 wavelengths (if I have the SI units right in my head, and I'm not 3 orders of magnitude out?). I suspect the left/right and up/down adjustments have as fairly high lever ratio, but I can't imagine you could successfully adjust the in/out distance with any precision (not in the 690nm sense anyway)? Is the in/out distance not important so long as the beams are aligned? Dunno if you've seen it, but there's a great youtube video explaining how the actuators to align the James Webb mirrors work: [this video] including a 3D printable version of them: [link] "This is a replica of JWST's mirror actuators. Six of these actuators are paired into a hexapod / stewart platform arrangement and used to control 6 degrees of freedom of each mirror segment (tip, tilt, roll, x, y, z translation)."
Explains JWST mirror actuators and offers a 3D printable version
Clever. I wonder how the mechanical realization looks like, as I can't really wrap my head around it. Reminds me of the JWST actuators which also "overload" a single axis for coarse/fine control: [this video]
JWST actuators overload a single axis for coarse and fine control
Video list and Hacker News comment labels from HackerTube (prompted by @karpathy), released under ODC-By 1.0, built on the open-index/hacker-news archive. Hacker News data is provided by Y Combinator; comments remain the work of their authors. Video titles, channel names and thumbnails belong to their creators and are shown via YouTube. HackerTube Watch is an unofficial edition. Data export of 2026-10-31.
Plays via YouTube's privacy-enhanced embed
6 Hacker News comments
From the HN archive; up to 20 kept per video, favoring different commenters. Recommendations first.
Read more
Citations, criticism and jokes