Update ICRF3 reference frame: 3,417 sources

README.md

@@ -3,7 +3,7 @@ license: cc-by-4.0
 pretty_name: "ICRF3 Celestial Reference Frame"
 language:
   - en
-description: "The third International Celestial Reference Frame (ICRF3) — the fundamental coordinate reference frame for astronomy, defined by extragalactic radio sources observed by VLBI."
+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 extraga"
 task_categories:
   - tabular-classification
 tags:
@@ -14,39 +14,66 @@ tags:
   - quasar
   - vlbi
   - open-data
+  - tabular-data
+  - parquet
 size_categories:
   - 1K<n<10K
+configs:
+  - config_name: default
+    data_files:
+      - split: train
+        path: data/icrf3_reference_frame.parquet
+    default: true
 ---
 
 # ICRF3 Celestial Reference Frame
 
-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 **4,588** extragalactic radio sources (primarily quasars) observed by
-Very Long Baseline Interferometry (VLBI). This dataset contains **3,417** sources
-with variability and structure parameters from the defining catalog.
+
+<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 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. These sources serve as the fixed reference
-points against which all other celestial positions are measured.
+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.
 
-## Schema
+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.
 
-| Column | Type | Description |
-|--------|------|-------------|
-| `iers_name` | string | IERS designation of the source |
-| `ra_deg` | float64 | Right ascension (degrees, ICRS) |
-| `dec_deg` | float64 | Declination (degrees, ICRS) |
+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.
 
-Additional columns from the catalog are included with snake_case names.
+
+## 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 sources
+- **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
 
@@ -56,37 +83,44 @@ from datasets import load_dataset
 ds = load_dataset("juliensimon/icrf3-reference-frame", split="train")
 df = ds.to_pandas()
 
-# All-sky distribution
-print(f"{len(df):,} ICRF3 reference sources")
-print(f"RA range: {df['ra_deg'].min():.2f} to {df['ra_deg'].max():.2f} deg")
-print(f"Dec range: {df['dec_deg'].min():.2f} to {df['dec_deg'].max():.2f} deg")
+# 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
 
-Xu, M.H., Anderson, J.M., Heinkelmann, R., et al. (2019), "Structure Effects for
-3417 Celestial Reference Frame Radio Sources", ApJS, 242, 5.
-Accessed via [VizieR](https://vizier.cds.unistra.fr/), CDS Strasbourg.
+https://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=J/ApJS/242/5
 
 ## Related datasets
 
-- [pulsar-catalog](https://huggingface.co/datasets/juliensimon/pulsar-catalog) -- ATNF Pulsar Catalogue
-- [open-star-clusters](https://huggingface.co/datasets/juliensimon/open-star-clusters) -- Open Star Clusters
-
-## Pipeline
+- [juliensimon/pulsar-catalog](https://huggingface.co/datasets/juliensimon/pulsar-catalog)
 
-Source code: [juliensimon/space-datasets](https://github.com/juliensimon/space-datasets)
+- [juliensimon/open-star-clusters](https://huggingface.co/datasets/juliensimon/open-star-clusters)
 
 ## Citation
 
 ```bibtex
 @dataset{icrf3_reference_frame,
-  author = {Simon, Julien},
   title = {ICRF3 Celestial Reference Frame},
+  author = {juliensimon},
   year = {2026},
-  publisher = {Hugging Face},
   url = {https://huggingface.co/datasets/juliensimon/icrf3-reference-frame},
-  note = {Based on Xu et al. (2019), ApJS, 242, 5 via VizieR CDS Strasbourg}
+  publisher = {Hugging Face}
 }
 ```
 

data/icrf3_reference_frame.parquet

@@ -1,3 +1,3 @@
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