astropy-astronomy

# Astropy Astronomy Astronomical computations using Astropy. ## When to Use - Celestial coordinate transforms (ICRS, Galactic, AltAz) - Unit conversions for astronomical quantities - Reading, writing, or inspecting FITS files - Cosmological calculations (distances, ages, lookback times) - Time system conversions (UTC, TAI, TDB, MJD, JD) ## When NOT to Use - Telescope control or instrument automation - Real-time observation planning or scheduling - Image reduction or photometry pipelines (use photutils) - N-body simulations ## Coordinate Transforms ```python from astropy.coordinates import SkyCoord, EarthLocation, AltAz from astropy.time import Time import astropy.units as u coord = SkyCoord(ra=10.684*u.deg, dec=41.269*u.deg, frame='icrs') # M31 coord_str = SkyCoord('00h42m44.3s', '+41d16m09s', frame='icrs') # ICRS to Galactic galactic = coord.galactic print(f"l={galactic.l:.4f}, b={galactic.b:.4f}") # Angular separation c1 = SkyCoord(ra=10.684*u.deg, dec=41.269*u.deg) c2 = SkyCoord(ra=11.0*u.deg, dec=41.5*u.deg) sep = c1.separation(c2) # AltAz (horizontal) coordinates location = EarthLocation(lat=34.05*u.deg, lon=-118.25*u.deg, height=100*u.m) time = Time('2026-03-15 03:00:00', scale='utc') altaz = coord.transform_to(AltAz(obstime=time, location=location)) print(f"Alt={altaz.alt:.2f}, Az={altaz.az:.2f}") ``` ## Unit Conversions ```python import astropy.units as u d = 10 * u.pc print(d.to(u.lyr)) # parsecs to light-years wav = 21 * u.cm freq = wav.to(u.GHz, equivalencies=u.spectral()) wavelength = (13.6 * u.eV).to(u.nm, equivalencies=u.spectral()) ``` ## FITS File Handling ```python from astropy.io import fits with fits.open('image.fits') as hdul: hdul.info() header = hdul[0].header data = hdul[0].data hdu = fits.PrimaryHDU(data_array) hdu.header['OBJECT'] = 'M31' hdu.writeto('output.fits', overwrite=True) ``` ## Cosmological Calculations ```python from astropy.cosmology import Planck18 as cosmo z = 1.0 d_L = cosmo.luminosity_distance(z) # luminosity distance d_A = cosmo.angular_diameter_distance(z) # angular diameter distance age = cosmo.age(z) # age of universe at z lookback = cosmo.lookback_time(z) # lookback time H_z = cosmo.H(z) # Hubble parameter at z ``` ## Time Conversions ```python from astropy.time import Time import astropy.units as u t = Time('2026-03-15 12:00:00', scale='utc') print(t.jd, t.mjd, t.unix) # JD, MJD, Unix print(t.tai, t.tdb) # TAI, TDB scales now = Time.now() ``` ## Quick One-liner ```bash python3 -c " from astropy.coordinates import SkyCoord; import astropy.units as u c = SkyCoord(ra=83.633*u.deg, dec=22.014*u.deg) print(f'Galactic: l={c.galactic.l:.3f}, b={c.galactic.b:.3f}') " ``` ## Best Practices 1. Always attach units to quantities using `astropy.units`. 2. Specify time scale explicitly (`utc`, `tai`, `tdb`). 3. Use `Planck18` as default cosmology unless otherwise specified. 4. Close FITS files or use context managers to prevent resource leaks. 5. Use `SkyCoord` for all coordinate work rather than manual trig.