juliensimon/icrf3-reference-frame
ICRF3 Celestial Reference Frame Credit: NASA/DOE/Fermi LAT Collaboration Part of a dataset collection on Hugging Face. Dataset description The third International Celestial Reference Frame (ICRF3) is the fundamental coordinate reference frame for astronomy, adopted by the International Astronomical Union in 2018. It is defined by extragalactic radio sources (primarily quasars) observed by Very Long Baseline Interferometry (VLBI). This dataset contains… See the full description on the dataset page: https://huggingface.co/datasets/juliensimon/icrf3-reference-frame.
ICRF3 Celestial Reference Frame
<div align="center"> <img src="banner.jpg" alt="The gamma-ray sky as seen by NASA's Fermi telescope" width="400"> <p><em>Credit: NASA/DOE/Fermi LAT Collaboration</em></p> </div>
Part of a [dataset collection](https://huggingface.co/collections/juliensimon/astronomy-datasets-69c24caf2f17e36128946743) on Hugging Face.
Dataset description
The third International Celestial Reference Frame (ICRF3) is the fundamental coordinate reference frame for astronomy, adopted by the International Astronomical Union in 2018. It is defined by extragalactic radio sources (primarily quasars) observed by Very Long Baseline Interferometry (VLBI). This dataset contains structure parameters from Xu et al. (2019) that quantify each source's compactness and positional stability.
The ICRF is the realization of the International Celestial Reference System (ICRS) at radio wavelengths. ICRF3 is based on nearly 40 years of VLBI observations and provides the most accurate positions of extragalactic objects, with median positional uncertainties of ~30 microarcseconds for the defining sources.
The ICRF is conceptually the modern replacement for the FK5 optical fundamental star catalog. While FK5 was limited by the proper motions and parallaxes of nearby stars, the ICRF uses extremely distant quasars whose apparent motions are negligible, providing a quasi-inertial reference frame tied to the large-scale structure of the universe. Source structure — the extended radio jets that cause apparent position shifts — is the dominant systematic error in VLBI astrometry. This dataset includes structure index parameters that quantify each source's compactness, which is critical for selecting calibrators for VLBI observations and for space geodesy applications.
Schema
Quick stats
- 3,417 ICRF3 radio sources
- 283 defining sources (highest positional stability)
- 0 special-handling sources (extended jet structure)
- Median observing sessions per source: 4
- Most-observed source: 439,113 individual VLBI measurements
Usage
from datasets import load_dataset
ds = load_dataset("juliensimon/icrf3-reference-frame", split="train")
df = ds.to_pandas()
# Defining sources — the foundation of the celestial reference frame
defining = df[df["flag_icrf3"] == "D"]
print(f"{len(defining)} ICRF3 defining sources")
# Sky distribution of defining vs. non-defining sources
import matplotlib.pyplot as plt
fig, ax = plt.subplots(figsize=(12, 6))
other = df[df["flag_icrf3"] != "D"]
ax.scatter(other["ra_deg"], other["dec_deg"], s=1, alpha=0.3, label="Non-defining")
ax.scatter(defining["ra_deg"], defining["dec_deg"], s=5, c="red", label="Defining")
ax.set_xlabel("RA (deg)")
ax.set_ylabel("Dec (deg)")
ax.invert_xaxis()
ax.legend()
ax.set_title("ICRF3 All-Sky Distribution")
plt.tight_layout()
plt.show()Data source
https://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=J/ApJS/242/5
Related datasets
Citation
@dataset{icrf3_reference_frame,
title = {ICRF3 Celestial Reference Frame},
author = {Simon, Julien},
year = {2026},
url = {https://huggingface.co/datasets/juliensimon/icrf3-reference-frame},
publisher = {Hugging Face},
note = {Derived from VizieR, Centre de Données astronomiques de Strasbourg (CDS), https://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=J/ApJS/242/5}
}