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

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

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

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

from astropy.cosmology import Planck18 as cosmo