cmb2sphere

Converts Planck temperature data into a height profile on a sphere. Inspired by ['Cosmic sculpture: a new way to visualise the cosmic microwave background'] 1 by Clements, D. L., S. Sato, and A. Portela Fonseca.

I plan to write about how the process works, how to use the script and how to print the resulting file soon(tm). Feel free to remind me if this notice is still up.

Also forgive some bad stylistic choices, the certainly existing bugs and anything that is wrong with the script. As often in "research" there wasn't enough time to do it right... or maybe I was too lazy to improve it further. :p (We really need a standardized note for this)

Installation

This project uses UV for dependency management and includes a Nix flake for reproducible development environments.

Using UV (Recommended)

# Install dependencies
uv sync

# Run the script directly
uv run python cmb2sphere.py output.stl

# With custom parameters
uv run python cmb2sphere.py --fwhm=3 --nside=256 output.stl

Using Nix Flake

# Enter development shell
nix develop

# Dependencies are automatically available
python cmb2sphere.py output.stl

Using Just (Task Runner)

This project includes a justfile with common tasks:

# Show all available commands
just --list

# Complete setup (creates directories and installs dependencies)
just setup

# Download CMB data from Planck mission archive
just download

# Build with custom parameters
just build 5 128 output.stl

# Run tests
just test

Manual Installation with pip

If you prefer pip:

pip install healpy matplotlib numpy scipy numpy-stl docopt-ng

Usage

Download Data

Download the Planck CMB data:

just download

Or manually download https://irsa.ipac.caltech.edu/data/Planck/release_2/all-sky-maps/maps/component-maps/cmb/COM_CMB_IQU-commander_1024_R2.02_full.fits and place it in the "data" subdirectory.

Generate Sphere Mesh

# Using UV
uv run python cmb2sphere.py output.stl

# Using Just
just build 2 128 output.stl

# With custom parameters
uv run python cmb2sphere.py --fwhm=3 --nside=256 custom_output.stl

Output formats

The output format is chosen from the file extension:

• .stl - geometry only (Standard Triangle Language; no color)
• .ply - geometry + per-vertex color, accepted by most full-color print services
• .3mf - geometry + per-vertex color via the 3MF Materials Extension (lib3mf)

For colored output, the temperature scalar is mapped through a matplotlib colormap (default RdBu_r, override with --colormap). Note that trimesh's own 3MF exporter currently drops vertex colors silently, so the 3MF path uses lib3mf directly.

Synthetic data (no FITS required)

For testing, demos, or just trying the pipeline without downloading the ~1 GB Planck FITS file, use --synthetic:

uv run python cmb2sphere.py --synthetic --nside=64 demo.ply

This generates a low-l Gaussian random field with a red power spectrum (C_l ~ 1/(l+1)^2) scaled to a CMB-like ~100 microkelvin RMS. Not physically accurate, but the right shape and amplitude for exercising the pipeline. Reproducible via --seed.

License

The script itself is licensed under the GNU Affero General Public License v3.0 or later (AGPL-3.0-or-later). Please see the COPYING file for details.

If there are questions in your jurisdiction, or if the AGPL is not clear enough on the point, please consider the resulting mesh files under a CC0 or whatever you like. Effectively, do what you want with them, but please include the attributions below — they are requested by the data providers, not by this project.

Acknowledgements

This project visualizes data from ESA's Planck mission, distributed by the NASA/IPAC Infrared Science Archive (IRSA) at Caltech. IRSA asks that two things be included whenever their data is used (see irsa.ipac.caltech.edu/ack.html):

1. The IRSA acknowledgement statement, verbatim:

   This research has made use of the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

2. The canonical paper for the specific data set used. The file shipped via just download (COM_CMB_IQU-commander_1024_R2.02_full.fits) is the Commander component-separation map from Planck Release 2 (2015), whose reference paper is:

   Planck Collaboration. Planck 2015 results. IX. Diffuse component separation: CMB maps. A&A 594, A9 (2016). doi:10.1051/0004-6361/201525936

If you submit to an AAS journal, IRSA also asks that you include IRSA and Planck as facility keywords; see the IRSA page above.

The idea of converting the CMB into a 3D height profile is from Clements, Sato, and Portela Fonseca (2016); see 1.