A Beginner’s Guide to Accessing Tycho-2 Data### What is the Tycho-2 Catalogue?
The Tycho-2 catalogue is a widely used astrometric star catalogue produced from observations by the ESA Hipparcos satellite’s Tycho experiment, combined with many ground-based catalogues. It contains positions, proper motions, and two-color photometry (B_T and V_T magnitudes) for about 2.5 million stars, covering the entire sky down to about magnitude 11–12. Tycho-2 improved on its predecessor (Tycho-1) by using a longer time baseline and more reference catalogues, yielding more accurate proper motions and positions.
Why use Tycho-2?
- Broad sky coverage: nearly full-sky catalogue suitable for many observational and calibration tasks.
- Accurate proper motions: useful for studies of stellar kinematics and identification of high–proper-motion objects.
- Photometry included: B_T and V_T magnitudes enable color-based selections and cross-matching with other photometric systems.
- Lightweight compared to modern surveys: easier to download and handle than very large modern sky surveys if you only need bright stars.
What data fields are in Tycho-2?
Key columns you’ll typically find:
- Tycho-2 identifier (TYC1-TYC2-TYC3)
- Right Ascension (RA) and Declination (Dec) at epoch J2000.0
- Proper motions in RA and Dec (mas/yr)
- B_T and V_T magnitudes and their errors
- Number of observations and quality flags
- Cross-identifications with Hipparcos where available
Where to access Tycho-2 data
Primary ways to obtain Tycho-2 data:
- Online catalog services (recommended for most users)
- Vizier (CDS): query and download subsets or full tables.
- ESA/Hipparcos archive: documentation and links.
- FTP/HTTP bulk downloads
- Some archives provide the full catalogue as files for offline use.
- Programmatic access via APIs
- Astroquery (Python), TAP/IVOA services, or custom REST endpoints.
Step-by-step: Downloading Tycho-2 with Vizier (web)
- Open the Vizier website (CDS).
- Enter the Tycho-2 catalogue identifier: “I/259/tyc2”.
- Use query filters to restrict by magnitude, coordinates, or proper motion.
- Choose output format (VOTable, CSV, ASCII) and download selected rows.
Step-by-step: Querying Tycho-2 using Python (astroquery)
Example using astroquery.vizier:
from astroquery.vizier import Vizier from astropy.coordinates import SkyCoord from astropy import units as u Vizier.ROW_LIMIT = 10000 # increase as needed catalog = "I/259/tyc2" coord = SkyCoord(ra=10*u.degree, dec=20*u.degree, frame='icrs') result = Vizier.query_region(coord, radius=0.5*u.deg, catalog=catalog) table = result[0] print(table[:5]) Notes:
- Set ROW_LIMIT to 0 for no limit (careful: large downloads).
- Use filters like Vizier(columns=[“TYC”,“RAJ2000”,“DEJ2000”,“BTmag”,“VTmag”]) to reduce columns.
Using TAP/ADQL queries
If you prefer ADQL (SQL-like) queries against an IVOA TAP service:
SELECT TOP 100 * FROM "I/259/tyc2" WHERE 1=CONTAINS(POINT('ICRS',RAJ2000,DEJ2000), CIRCLE('ICRS', 10.0, 20.0, 0.5)) Run this on a TAP-enabled service (e.g., ESA or some Vizier TAP endpoints).
Cross-matching Tycho-2 with other catalogs
Common tasks:
- Cross-match by position (within an angular radius) with Gaia, 2MASS, SDSS, etc.
- Use astropy.coordinates and astropy.coordinates.match_coordinates_sky or CDS X-Match services for batch cross-matches.
Example (Astropy cross-match):
from astropy.coordinates import SkyCoord from astropy import units as u from astropy.table import Table tyc = Table.read('tycho_sample.vot', format='votable') gaia = Table.read('gaia_sample.vot', format='votable') c_tycho = SkyCoord(ra=tyc['RAJ2000']*u.deg, dec=tyc['DEJ2000']*u.deg) c_gaia = SkyCoord(ra=gaia['ra']*u.deg, dec=gaia['dec']*u.deg) idx, sep2d, _ = c_tycho.match_to_catalog_sky(c_gaia) match_mask = sep2d < 1.0*u.arcsec matches = tyc[match_mask] Photometry conversion: B_T, V_T to Johnson B, V
Tycho magnitudes can be converted approximately to the Johnson system:
- V ≈ V_T – 0.09*(B_T – V_T)
- B – V ≈ 0.85*(B_T – V_T)
These are approximations good for many stars; consult literature for precise transformations for specific spectral types.
Common pitfalls and tips
- Proper motions are given relative to J2000.0 — propagate positions to your epoch if needed.
- Use the catalogue’s quality flags to filter unreliable entries (e.g., low observation counts).
- For high-precision astrometry, cross-check with Gaia DR3/DR4 where available.
- Be mindful of star multiplicity: close binaries can affect photometry and astrometry.
Example workflows
- Calibration: select Tycho-2 stars with 6 < V_T < 10 around your target field for photometric or astrometric calibration.
- Kinematics: build a sample of high–proper-motion stars by filtering proper motion > 50 mas/yr and inspect sky distribution.
- Cross-match for identification: match Tycho-2 with Gaia to get improved parallaxes and radial velocities from other surveys.
Further reading and resources
- Tycho-2 catalogue original paper (for methodology, reductions, and error models).
- Vizier/Catalogue documentation pages (column descriptions, flags).
- Astropy and Astroquery documentation (tools for programmatic access).
- IAU/IVOA pages on TAP/ADQL for advanced queries.
If you want, I can:
- produce ready-to-run scripts tuned to your dataset size or desired sky region,
- show how to bulk-download the full catalogue,
- or help convert Tycho-2 magnitudes to other photometric systems for a given spectral type.
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