Hi HN, Alex here, designer-engineer in the Netherlands. My day job is Repper (a pattern-design tool). Skymap is a side project, built last weekend in 4 days (~32 hours, AI-assisted with Claude Code).

I've followed Anton Petrov on YouTube for years and kept seeing nice pans over galaxy catalogs in research videos, thinking "why can't I just fly through that?" Aparently nobody had built it for the browser yet.

It loads SDSS (~500k galaxies), 2MRS (~45k), and GLADE (~2M) as WebGPU instanced billboards. Past a certain on-screen size each galaxy crossfades from a dot into a procedural oriented disk (catalog axis-ratio + PA from HyperLEDA / 2MASS XSC), then into a real survey image when close enough: SDSS DR18 ImgCutout, or DSS via CDS hips2fits for the rest. There's a cosmic-web filament overlay built offline with DisPerSE (Sousbie 2011), a faint blue lattice over the point field. Striking at supercluster scale.

What I genuinely like about coding with Claude is that I can read about an algorithm in a paper and have it running interactively the same evening. DisPerSE skeletons, Schechter LF corrections, HEALPix re-weighting, K-correction in the fragment shader. Stuff I'd otherwise only have watched a video about.

The interesting texture across these 4 days wasn't bug-fighting, it was iteration: continuous refactoring, guiding Claude away from slop, accepting that scaffolding gets re-done. The renderer wasn't new ground (years of WebGL behind me). Deep-link resolution and tier swaps end up as a useEffect-and-refs web I'd normally write as redux-saga; I went fully agentic this round and you can feel the absence of an orchestrator (ouch).

Live: https://skymap.rulkens.com Repo: https://github.com/rulkens/skymap (MIT) DOI: https://doi.org/10.5281/zenodo.20037028

UX has plenty of rough edges, mobile gestures aren't there yet. I'm not an astronomer (the cosmology math is all Claude-derived against textbook formulas), so if anyone here works in the field and spots something off, I'd love to hear it.

I'm also curious how people grow a side project from a code seed to something that is robust enough to be used in production, especially with agentic coding. I feel I learned a lot about this, but I'm curious about your experience. TBH, I like my code clean, so with the speed this is going I end up with a lot of cringe code that I'd wish would be better.

Show HN: Skymap – a 3D galaxy catalog explorer in the browser via WebGPU