Core Python library for astronomy and astrophysics workflows that need Astropy APIs, including units/quantities, coordinates, FITS I/O, tables, time systems, WCS, and cosmology. Use when implementing or debugging astronomical data analysis code with Astropy.
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Core Python library for astronomy and astrophysics workflows that need Astropy APIs, including units/quantities, coordinates, FITS I/O, tables, time systems, WCS, and cosmology. Use when implementing or debugging astronomical data analysis code with Astropy.
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LanguagePython
LicenseMIT
📦 Ships with scientific-agent-skills
</> SKILL.md
astropy.SKILL.md
---name: astropy
description: Core Python library for astronomy and astrophysics workflows that need Astropy APIs, including units/quantities, coordinates, FITS I/O, tables, time systems, WCS, and cosmology. Use when implementing or debugging astronomical data analysis code with Astropy.
license: BSD-3-Clause license
compatibility: Requires Python 3.11+ with astropy installed (uv for package installation). Some features (object name resolution, site lookups, remote FITS reads, IERS updates) need network access.
metadata: {"version": "1.2", "skill-author": "K-Dense Inc."}
---# Astropy
## Overview
Astropy is the core Python package for astronomy, providing essential functionality for astronomical research and data analysis. Use astropy for coordinate transformations, unit and quantity calculations, FITS file operations, cosmological calculations, precise time handling, tabular data manipulation, and astronomical image processing.
## When to Use This Skill
Use astropy when tasks involve:
- Converting between celestial coordinate systems (ICRS, Galactic, FK5, AltAz, etc.)
- Working with physical units and quantities (converting Jy to mJy, parsecs to km, etc.)
- Reading, writing, or manipulating FITS files (images or tables)
- Cosmological calculations (luminosity distance, lookback time, Hubble parameter)
- Precise time handling with different time scales (UTC, TAI, TT, TDB) and formats (JD, MJD, ISO)
**See:** `references/wcs_and_other_modules.md` for WCS operations and transformations.
## Additional Capabilities
The `references/wcs_and_other_modules.md` file also covers:
### NDData and CCDData
Containers for n-dimensional datasets with metadata, uncertainty, masking, and WCS information.
### Modeling
Framework for creating and fitting mathematical models to astronomical data.
### Visualization
Tools for astronomical image display with appropriate stretching and scaling.
### Constants
Physical and astronomical constants with proper units (speed of light, solar mass, Planck constant, etc.).
### Convolution
Image processing kernels for smoothing and filtering.
### Statistics
Robust statistical functions including sigma clipping and outlier rejection.
## Installation
```bash
# Reproducible install against the current stable release
uv pip install "astropy==7.2.0"
# Recommended optional dependencies for plotting and common workflows
uv pip install "astropy[recommended]==7.2.0"
# Full optional dependency set for broad astronomy workflows
uv pip install "astropy[all]==7.2.0"
```
Astropy 7.2.0 requires Python 3.11+ and depends on NumPy, PyERFA, PyYAML, and packaging. Use an isolated virtual environment; do not install Astropy with elevated privileges.
Note that the `[recommended]` and `[all]` extras pull in transitive dependencies (matplotlib, scipy, etc.) at unpinned versions. For reproducible production environments, pin the full dependency tree with a lockfile (`uv lock` in a project, or `uv pip compile` for requirements files) and review the resolved versions before deploying.
## Common Workflows
### Converting Coordinates Between Systems
```python
from astropy.coordinates import SkyCoord
import astropy.units as u
# Create coordinate
c = SkyCoord(ra='05h23m34.5s', dec='-69d45m22s', frame='icrs')
# Transform to galactic
c_gal = c.galactic
print(f"l={c_gal.l.deg}, b={c_gal.b.deg}")
# Transform to alt-az (requires time and location)
from astropy.time import Time
from astropy.coordinates import EarthLocation, AltAz
3. **Prefer arrays over loops**: Process multiple coordinates/times as arrays for better performance
4. **Check coordinate frames**: Verify the frame before transformations
5. **Use appropriate cosmology**: Choose the right cosmological model for your analysis
6. **Handle missing data**: Use masked columns for tables with missing values
7. **Specify time scales**: Be explicit about time scales (UTC, TT, TDB) for precise timing
8. **Use QTable for unit-aware tables**: When table columns have units
9. **Check WCS validity**: Verify WCS before using transformations
10. **Cache frequently used values**: Expensive calculations (e.g., cosmological distances) can be cached
11. **Be explicit about network access**: `SkyCoord.from_name()`, `EarthLocation.of_site(refresh_cache=True)`, `EarthLocation.of_address()`, `download_file()`, remote FITS reads, and some IERS time/coordinate transforms can contact external services or update local caches. Avoid sending sensitive target names, addresses, URLs, or proprietary file locations to third-party services. When working with potentially sensitive targets or data locations, confirm with the user before making these network calls.
12. **Pin for reproducibility**: Use pinned versions such as `astropy==7.2.0` for shared environments; update pins intentionally after reviewing release notes.
## Current-Version Notes
- Current stable release researched: Astropy 7.2.0 (released 2025-11-25; verified current as of 2026-06-10)
- Python requirement: 3.11+
- **Astropy 8.0 is at release-candidate stage** (8.0.0rc1, 2026-05-26). Key changes to anticipate:
- The deprecated `astropy.cosmology` submodule shims (`astropy.cosmology.flrw`, `.core`, `.funcs`, `.connect`, `.parameter`) are removed — import everything directly from `astropy.cosmology` (e.g., `from astropy.cosmology import FlatLambdaCDM, z_at_value`)
- `astropy.constants` defaults change from CODATA 2018 to CODATA 2022; pin a constants version via the `astropyconst` science states if reproducibility matters
- NumPy 2.0 becomes the minimum supported version; the 7.2.x LTS branch retains NumPy 1.x support for six months after the 8.0 release
- The built-in test runner (`astropy.test()`, `TestRunner`) is formally deprecated — invoke `pytest` directly
- Recent 7.x deprecations to avoid in new code: passing a table index identifier as the first `.loc` element (`t.loc["b", 2]`) — use `t.loc.with_index("b")[2]` instead (removal planned for 9.0); `astropy.utils.isiterable()` — use `numpy.iterable()`
- Recent 7.0 removals: older deprecated FITS APIs such as `(Bin)Table.update`, `_ExtensionHDU`, `_NonstandardExtHDU`, and the `tile_size` argument for `CompImageHDU`; `CompImageHeader` is deprecated. Avoid those legacy patterns in new examples.
- The recommended optional extras are `recommended` for common plotting/scientific dependencies and `all` only when a broad optional feature set is needed.
## Documentation and Resources
- Official Astropy Documentation: https://docs.astropy.org/en/stable/
- Tutorials: https://learn.astropy.org/
- GitHub: https://github.com/astropy/astropy
## Reference Files
For detailed information on specific modules:
- `references/units.md` - Units, quantities, conversions, and equivalencies
- `references/coordinates.md` - Coordinate systems, transformations, and catalog matching
- `references/cosmology.md` - Cosmological models and calculations
- `references/fits.md` - FITS file operations and manipulation
- `references/tables.md` - Table creation, I/O, and operations
- `references/time.md` - Time formats, scales, and calculations
- `references/wcs_and_other_modules.md` - WCS, NDData, modeling, visualization, constants, and utilities
