galfit

name: galfit description: Guidance and a bundled round-tripping parser for the GALFIT 2-D galaxy image-fitting code (Peng 2002/2010). Use when the task involves GALFIT input/output files (`.galfit`, `.feedme`, `galfit.NN` restart files), GALFIT constraint files, invoking the GALFIT binary, converting GALFIT configs to or from YAML/JSON, or questions about GALFIT profile parameters (sersic, nuker, moffat, king, ferrer, edgedisk, expdisk, devauc, gaussian, psf, sky) and hidden blocks (C0 diskyness/boxyness, Fourier modes, bending modes, coordinate rotation R0-R10, truncation T0-T10).

Use this skill when you need to read, write, analyze, or modify GALFIT config and output files, or to drive the GALFIT binary itself. A fully tested parser is shipped under scripts/.

Quick Start

Confirm what the task needs: reading a .galfit input, reading a galfit.NN restart/output, editing parameters, running a fit, or writing constraint files.
Load only the minimal reference files needed:
- references/index.md always first.
- Profile-parameter questions: references/profile_schemas.md.
- Hidden block questions (C0, B/F/R/T): references/hidden_blocks.md.
- Interpreting a galfit.NN output: references/output_decorations.md.
- Constraint files: references/constraint_grammar.md.
- Running the binary: references/binary_usage.md.
- Debugging weird outputs: references/top10_gotchas.md.
Use the bundled parser to read/write/round-trip. Prefer the parser over hand-rolling regex - it correctly handles every documented form plus the real-world quirks (GALFIT 3.0.7's reserved 6)/7)/8) placeholders on sersic, 1-flag-broadcast Fourier on truncation blocks, envelope composition [*value*], etc.).

Using the Parser

The parser lives at <skill-dir>/scripts/. Two modules:

galfit_io.py - .galfit / .feedme / galfit.NN files.
galfit_constraints.py - constraint files referenced by the G) header.

From Python

import sys
sys.path.insert(0, "<skill-dir>/scripts")

import galfit_io
import galfit_constraints

# Read an input or restart file
gf = galfit_io.read_galfit("galfit.01")
print(gf.header.zeropoint, len(gf.components))

# Inspect a Sersic component
sersic = next(c for c in gf.components if isinstance(c, galfit_io.SersicComponent))
print(sersic.mag.value, sersic.mag.uncertainty)   # uncertainty only in outputs

# Modify and write back (output-decoration stripping by default)
sersic.mag.value = 19.8
galfit_io.write_galfit(gf, "galfit.02")

# Round-trip through YAML for structured config management
yaml_text = galfit_io.to_yaml(gf)
gf2 = galfit_io.from_yaml(yaml_text)
assert galfit_io.to_dict(gf) == galfit_io.to_dict(gf2)

# Constraint files
entries = galfit_constraints.read_constraints("constraints.txt")
for e in entries:
    print(e.components, e.parameter, e.bounds, e.coupling)

From the shell

The parser supplies a convenience CLI for common tasks:

python <skill-dir>/scripts/galfit_cli.py to-yaml galfit.01 galfit.01.yaml
python <skill-dir>/scripts/galfit_cli.py from-yaml galfit.01.yaml galfit.02
python <skill-dir>/scripts/galfit_cli.py summary galfit.01

Design Notes (why the parser is worth using)

Tagged-union component classes: one @dataclass per profile, each with profile-specific fields (e.g. NukerComponent.alpha / .beta / .gamma but SersicComponent.n only). A flat dict[str, float] would lose those semantics on round-trip.
FittedValue captures the four output-file decorations independently so [*value*] (held and suspicious) round-trips correctly.
extra_params on every component absorbs GALFIT's reserved placeholder slots (6) 7) 8) on sersic in 3.0.7 output) - community parsers silently drop these and break round-trip.
HiddenBlocks uses sparse dict[int, ...] for bending and Fourier modes because GALFIT indices are not contiguous (you can have F1, F6, F20 in one block).

Running the GALFIT Binary

Consult references/binary_usage.md before running. Baseline recipe:

# <galfit-binary> <feedme-file>
galfit -imax 50 my_fit.feedme        # bounded iteration count
galfit -o1 my_fit.feedme             # generate model only, no fit
galfit -noskyest my_fit.feedme       # skip internal sky estimate

After a run, read galfit.NN (the highest-numbered restart) with this parser to get structured post-fit parameters plus uncertainties.

References

references/index.md - table of contents for references, load this first
references/profile_schemas.md - parameter tables for all 11 profiles
references/hidden_blocks.md - Z, C0, B/F/R/T and the truncation block
references/output_decorations.md - [], {}, *, (err) semantics
references/constraint_grammar.md - 6 constraint line forms
references/binary_usage.md - invoking the GALFIT binary
references/top10_gotchas.md - distilled pitfalls from GALFIT docs

Scripts

scripts/galfit_io.py - parser for .galfit / .feedme / galfit.NN
scripts/galfit_constraints.py - parser for constraint files
scripts/galfit_cli.py - thin CLI wrapper for common conversions