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 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

Column	Type	Description	Sample	Null %
iers_name	object	Official IERS source designation in B1950 sexagesimal format (HHMM+DDd, e.g. '0002-478'); this is the authoritative name used in geodetic VLBI scheduling, Earth orientation monitoring, and spacecraft navigation	0305+039	0.0%
closure_amplitude_rms_n	float64	RMS of closure amplitude residuals for narrow-field VLBI imaging (dimensionless); measures source compactness — lower values indicate more point-like sources with more stable astrometric positions	0.41	0.0%
closure_amplitude_rms_b	float64	RMS of closure amplitude residuals for broad-field imaging (dimensionless); sensitive to extended jet structure on longer baselines	0.44	0.0%
closure_amplitude_rms_u	float64	RMS of closure amplitude residuals for uniform weighting (dimensionless); intermediate sensitivity between narrow and broad-field	0.47	0.0%
n_closure_amplitude	Int64	Number of closure amplitude measurements used in the structure analysis; more measurements yield more reliable structure diagnostics	8110	0.0%
closure_phase_rms_n_deg	float64	RMS of closure phase residuals for narrow-field imaging in degrees; closure phase is immune to antenna-based errors, making it a robust measure of source asymmetry	18.8	0.0%
closure_phase_rms_b_deg	float64	RMS of closure phase residuals for broad-field imaging in degrees; elevated values indicate resolved jet structure	19.4	0.0%
closure_phase_rms_u_deg	float64	RMS of closure phase residuals for uniform weighting in degrees	22.8	0.0%
n_closure_phase	Int64	Number of closure phase measurements used in the structure analysis	1639	0.0%
n_sessions	Int64	Number of VLBI observing sessions in which this source was observed; well-observed sources have hundreds of sessions over decades	36	0.0%
n_observations	Int64	Total number of individual VLBI observations (delay measurements) across all sessions; high counts indicate heavily-used calibrator sources	1072	0.0%
flag_icrf2	object	Source classification flag in ICRF2: 'D' = defining, 'N' = non-defining, 'S' = special handling; null if not in ICRF2	O	0.0%
flag_icrf3	object	Source classification flag in ICRF3: 'D' = defining (303 sources with the most stable positions), 'S' = special handling (688 sources with extended structure); null for non-defining sources	D	0.0%
icrf2_structure_index	Int64	Source structure index from ICRF2 (1-4 scale); 1 = very compact, 4 = highly extended; used for calibrator selection in VLBI observations	1	0.0%
ra_deg	float64	Right ascension of the extragalactic radio source in degrees, ICRS J2000.0 epoch; range 0-360; defining sources carry positional accuracies of ~30 microarcseconds	47.109265834	0.0%
dec_deg	float64	Declination of the extragalactic radio source in degrees, ICRS J2000.0; range -90 to +90; southern hemisphere coverage is sparser due to fewer southern VLBI stations	4.110916884	0.0%

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

• juliensimon/pulsar-catalog

• juliensimon/open-star-clusters

Citation

@dataset{icrf3_reference_frame,
  title = {ICRF3 Celestial Reference Frame},
  author = {juliensimon},
  year = {2026},
  url = {https://huggingface.co/datasets/juliensimon/icrf3-reference-frame},
  publisher = {Hugging Face}
}

License

CC-BY-4.0