astropy

Use astropy.coordinates.SkyCoord to create coordinates in one frame, then transform with attributes like .galactic or .transform_to(). For observer-dependent AltAz frames, also supply an EarthLocation and observation Time.

Open files with astropy.io.fits.open() inside a context manager, access HDUs by index or name, and read data as NumPy arrays. Write new files with hdu.writeto() or the fits.writeto() convenience function.

Build SkyCoord objects from each catalog's RA and Dec columns, then call match_to_catalog_sky() to get indices and angular separations. Filter matches by a separation threshold such as 1 arcsecond.

Yes, astropy.cosmology provides built-in models like Planck18 and WMAP9 plus custom FlatLambdaCDM definitions. Methods include luminosity_distance, angular_diameter_distance, age, lookback_time, and z_at_value for inverse lookups.

Astropy 7.2.0 requires Python 3.11 or newer and depends on NumPy, PyERFA, PyYAML, and packaging. Install it in an isolated virtual environment with a pinned version for reproducibility.

High-precision UT1 and polar-motion values use IERS Earth-rotation data, which Astropy may auto-download. Set iers.conf.auto_download = False for offline or privacy-sensitive runs, and avoid SkyCoord.from_name() with confidential target names.