seiza (Python)

Python bindings for seiza: star detection, WCS fitting, hinted/blind plate solving, satellite prediction, calibration, deconvolution, and batch/live image stacking for astrophotography, implemented in Rust.

pip install seiza

Binary wheels cover Linux (x86_64, aarch64), macOS (universal2), and Windows (x64); each is a single abi3 wheel for every CPython from 3.9 up. Type stubs are included, and computational image operations release the GIL. Input arrays are read in place, without a copy, while the GIL is released: do not mutate an array from another thread until the call returns.

Solve an image

import numpy as np
import seiza

# One-time: download the verified solver catalogs into the shared cache.
paths = seiza.fetch_catalogs()  # Tycho-2 solver + objects, Solar System, transients
catalog = seiza.StarCatalog.open(paths["stars-lite-tycho2.bin"])

# Detect stars in a 2D float32 (or uint8) luma array.
stars = seiza.detect(image_array)

# Hinted solve: approximate center and pixel scale. sip_order=3 also fits
# SIP distortion polynomials when enough matched stars support them.
solution = seiza.solve(
    stars, catalog, width, height,
    ra=150.1, dec=35.2, scale_arcsec_px=2.5, sip_order=3,
)
print(solution)                 # center, scale, matches, RMS
print(solution.rotation_deg, solution.flipped)
ra, dec = solution.wcs.pixel_to_world(100.0, 200.0)

open takes a file, a directory (the right catalog inside is picked — the deepest star catalog wins), or nothing at all. With no argument the standard places are searched: SEIZA_STAR_DATA / SEIZA_BLIND_INDEX, files next to the program, and the seiza setup directories (SEIZA_CATALOG_DIR). These are the same rules as the CLI's --data:

catalog = seiza.StarCatalog.open("data")   # directory
catalog = seiza.StarCatalog.open()         # after seiza setup

Stars can also be plain (x, y, flux) tuples from any other detector — the solver only needs positions and relative brightness:

solution = seiza.solve([(x1, y1, f1), (x2, y2, f2), ...], catalog, w, h,
                       ra=..., dec=..., scale_arcsec_px=...)

Measure stars and sensor tilt

The measurement detector is separate from the fast alignment detector above. It reports HFR, FWHM, SNR, flux, and optional Gaussian/Moffat PSF fits from a mono uint16 frame. The returned coordinates and radii are always in input pixels, including when detection binning is enabled:

measured = seiza.detect_measured_stars(
    image_u16,
    focal_length_mm=550.0,
    pixel_size_um=3.76,
    psf_type="moffat4",
)
cells, tilt = seiza.tilt_analysis(measured)
print(len(measured.stars), measured.average_hfr, measured.average_fwhm)
print(tilt.tilt_percent, tilt.curvature_percent, tilt.worst_corner)

The nine cells cover a 3×3 sensor grid. Fitted star theta and cell mean_theta values are ellipse major-axis orientations in radians over [0, π); theta_coherence describes how consistently stars in that cell share the direction. Tilt and curvature need measurements in all four corner cells, and curvature also needs the center cell.

Blind solve

No position hint, only a plausible scale range. Uses the prebuilt whole-sky pattern index and the deep Gaia catalog:

paths = seiza.fetch_catalogs(["stars-deep-gaia17.bin", "blind-gaia16.idx"])
catalog = seiza.StarCatalog.open(paths["stars-deep-gaia17.bin"])
index = seiza.BlindIndex.open(paths["blind-gaia16.idx"])
solution = seiza.solve_blind(stars, catalog, index, width, height,
                             min_scale_arcsec_px=0.5, max_scale_arcsec_px=15.0)

For faint fields, the optional stars-deep-gaia20.bin catalog reaches Gaia G≤20 (about 9 GB). It is intentionally not included in fetch_catalogs("all"), so request it explicitly with the same G≤16 blind index:

paths = seiza.fetch_catalogs(["stars-deep-gaia20.bin", "blind-gaia16.idx"])
catalog = seiza.StarCatalog.open(paths["stars-deep-gaia20.bin"])
index = seiza.BlindIndex.open(paths["blind-gaia16.idx"])

FITS WCS output

Solutions convert directly to FITS WCS keywords (1-indexed CRPIX, TAN or TAN-SIP projection, CD matrix, and the complete A_p_q/B_p_q/AP_p_q/ BP_p_q set when distortion was fitted):

cards = solution.fits_header_cards()   # dict of keyword -> value
text = solution.fits_header_text()     # 80-column cards ending with END

The header text form is suitable for header-injection APIs — for example Siril's sirilpy scripting interface (set_image_header), which makes a seiza solve usable from a Siril Python script.

Background extraction

Fit a compact background model to a C-contiguous mono (H, W) or RGB (H, W, 3) linear float32 array, inspect it, and then correct the image:

model = seiza.fit_background(stack, model="automatic", degree=2)
print(model.diagnostics)

corrected = model.correct(stack)                 # additive subtraction
illumination_corrected = model.correct(stack, mode="divide")
partial = model.correct(stack, strength=0.6)      # tune without refitting
background = model.render()                      # explicit full-size model

Fitting uses deterministic low-noise sample windows, robust sample rejection, and independent per-channel surfaces. Automatic mode chooses among polynomial degrees from held-out sample errors. Use model="radial_basis" with rbf_smoothing, or set allow_radial_basis=True in automatic mode, for an irregular thin-plate model. RBF is explicit because background samples can share real extended emission. model.correct() allocates only the corrected array; a full-size background exists only after render(). Pass a boolean (H, W) mask to exclude extended objects, dark clouds, registration borders, or source masks:

model = seiza.fit_background(stack, mask=structure_mask,
                             degree=1, samples_per_axis=12,
                             sample_radius=20)
for x, y, values, dispersion, weight, status in model.samples():
    print(x, y, values, status)

The output remains linear and may retain negative or greater-than-one values. Background extraction is not display stretching or color calibration.

Light deconvolution

Apply the same conservative linear-image restoration as the Rust crate and CLI to a C-contiguous mono (H, W) or RGB (H, W, 3) float32 array:

restored = seiza.deconvolve(stack, psf_fwhm=3.1)

psf_fwhm is a measured unsaturated-star FWHM in pixels. The defaults use four damped Richardson-Lucy iterations and blend 35% of the estimate into the input. Pass masked=True for registered images whose missing border samples are NaN: the mask stays in the output and does not darken nearby data. Without it, non-finite samples raise seiza.EngineError. The returned array remains linear float32; no clipping or display stretch is applied. The operation releases the GIL. Inspect identical stretches for noise, rings, saturated-star failures, and field-dependent PSF mismatch before using a stronger iterations or amount.

Image stacking

The wheel includes the same linear calibration, registration, normalization, and online rejection engine as the Rust crate and CLI. Batch stacking accepts FITS paths and writes an unstretched linear float32 FITS result:

options = seiza.StackOptions(
    normalization="local",
    local_tile_size=256,
    maximum_drift_pixels=256.0,
    maximum_drift_fraction=0.15,
)
result = seiza.stack_fits(
    sorted(light_paths),
    "stack.fits",
    options=options,
    bias="master-bias.fits",
    dark="master-dark.fits",
    flat="master-flat.fits",
)
for frame in result.frames:
    print(frame.source, frame.accepted, frame.reason, frame.registration_rms_pixels)

For live integration, construct from a FITS path or a C-contiguous mono/HWC RGB NumPy float32 array. An array-based stacker accepts only already-linear, calibrated arrays through push(). A path-based stacker can use push_fits() for its configured calibration path or push() for caller-prepared arrays. A stacker keeps that input mode after checkpointing. Both methods return a typed admission decision, and a rejected frame never mutates the accumulator:

stacker = seiza.LiveStacker.from_array(reference, options=options)
for frame in incoming_arrays:
    disposition = stacker.push(frame)
    if not disposition.accepted:
        print(disposition.reason)

preview_state = stacker.snapshot()  # immutable copy
linear_mean = preview_state.image
coverage = preview_state.coverage
final = stacker.finish("stack.fits")  # consumes the live accumulator

When the paths are all known — a finished session rather than a live one — push_fits_pipelined() prepares several frames at once while integrating in the order given, so the result is identical to pushing them one at a time. It measured 2.0x faster on local storage and 3.0x with a 300ms read latency:

dispositions, report = stacker.push_fits_pipelined(paths)
print(report)  # PipelineReport(integrated=..., rejected=..., failed=...)

A path that cannot be read, or that repeats one already stacked, comes back in place with accepted false and a reason rather than raising, so one bad path in a night's listing does not lose the rest — check report.failed rather than reading a clean return as success. Pass workers= when the frames arrive over a network, since the library cannot tell a network mount from a local disk, and normalized_full_scale=65535.0 when the set mixes PixInsight XISF frames with 16-bit camera data.

Checkpointing is non-consuming. Reopening preserves the original registration reference, calibration and options, online rejection statistics, coverage, and the FITS/XISF source ledger:

stacker.save_context("m31.seiza-stack")

stacker = seiza.LiveStacker.open_context("m31.seiza-stack")
decision = stacker.push(next_array)
stacker.save_context("m31.seiza-stack")

The context file is versioned, compressed, checksummed, and atomically replaced. It is processing state for resumption; finish("stack.fits") remains the interoperable final image output.

Frames taken after a German-equatorial-mount meridian flip are handled by default. maximum_rotation_degrees limits deviation from either the reference orientation or its 180-degree counterpart; frame diagnostics still report the full fitted rotation.

Snapshot array properties are copies, so Python cannot mutate live Rust state. All expensive FITS, calibration, registration, and integration work releases the GIL.

Color from mono stacks

Aligned mono float32 arrays can be combined without writing intermediate files. Outputs have shape (height, width, 3):

rgb = seiza.combine_rgb(red, green, blue)
lrgb = seiza.combine_lrgb(luminance, red, green, blue,
                          luminance_weight=1.0)
super_lrgb = seiza.combine_lrgb(luminance, red, green, blue,
                                luminance_mode="super")
super_rgb = seiza.combine_rgb(red, green, blue, luminance_mode="super")

sho = seiza.combine_narrowband(ha, oiii, sii, palette="sho")
hoo = seiza.combine_narrowband(ha, oiii, palette="hoo")
foraxx = seiza.combine_narrowband(ha, oiii, sii, palette="foraxx-sho")

Pass crop="bounds" or crop="inscribed" to trim the blank edges that registering one channel onto another leaves behind. bounds keeps the box every channel covers; inscribed keeps the largest rectangle they all cover in full, so nothing stays NaN. seiza.crop_report measures the same thing without composing, and names any channel whose coverage sits far from the others:

report = seiza.crop_report({"red": red, "green": green, "blue": blue})
x, y, width, height = report["region"]
stray = [c["name"] for c in report["channels"] if c["off_center"]]

The default percentile normalization is a quick-look channel match. Pass normalization="none" for already matched inputs. Foraxx inputs must also already lie in [0, 1] in that mode; keep percentile normalization for sensor-unit arrays. RGB, LRGB, additive super-LRGB (L + R + G + B), synthetic super-RGB (R + G + B), the six direct S/H/O permutations, and HOO are linear-light. Super-luminance output can exceed one. Foraxx-SHO/HOO use a stretched working copy as required by the published dynamic formula, so those returned arrays are display-referred. Composition releases the GIL.

Parameterized display stretching

seiza.stretch applies the shared seiza-stretch model to mono (H, W) or RGB (H, W, 3) float32 arrays and returns display-referred float32 without eight-bit quantization:

preview = seiza.stretch(linear, model="percentile-asinh",
                        black_percentile=0.01,
                        white_percentile=0.995, strength=10)
preview = seiza.stretch(linear_rgb, model="auto-mtf",
                        target_median=0.2, shadows_clip=-2.8,
                        color_strategy="luminance-preserving")
preview = seiza.stretch(linear, model="ghs", stretch_factor=4,
                        local_intensity=-1, symmetry_point=0.35,
                        protect_shadows=0.1, protect_highlights=0.8)

Available models are identity, linear, asinh, percentile-asinh, mtf, manual ghs, and auto-mtf; color strategies are linked, unlinked, and luminance-preserving. Analysis and application release the GIL.

Calibration masters use the same bounded-memory two-pass builder:

bias = seiza.build_bias(bias_paths, "master-bias.fits")
dark = seiza.build_dark(dark_paths, "master-dark.fits",
                        bias="master-bias.fits")
flat = seiza.build_flat(flat_paths, "master-flat.fits",
                        bias="master-bias.fits",
                        dark_flat="master-dark-flat.fits")

Each builder reads every input twice, so a master over dozens of frames runs for minutes. Pass cancel= a predicate to stop one early — it is called once per input and raises StackError when it returns true. Ctrl-C is honoured at the same points, without a predicate.

stop = threading.Event()
dark = seiza.build_dark(dark_paths, "master-dark.fits", cancel=stop.is_set)

Image processing primitives

OpenCV-compatible building blocks from seiza-imgproc, for detection pipelines that need OpenCV's exact numerics without the dependency. All operate on 2D single-channel arrays:

import numpy as np
import seiza

image = np.asarray(..., dtype=np.uint8)      # (height, width)

blurred = seiza.gaussian_blur(image, sigma=1.4)      # uint8 or float32
denoised = seiza.median_blur3(image)
edges = seiza.canny(blurred, low=10, high=80)
binary = seiza.otsu_binary(image)
grown = seiza.dilate(binary, shape="rect", ksize=3)

contours = seiza.find_contours(grown)        # list of (n, 2) int32 arrays
areas = [seiza.contour_area(c) for c in contours]

# Edge-aware smoothing and multi-scale structure removal (float inputs).
flat = seiza.dt_filter(guide, src, sigma_spatial=10.0, sigma_color=30.0)
stars_plus_noise = seiza.remove_structures(image.astype(np.float64), layers=4)

Predicted satellite tracks

After a solve, predict which satellites crossed the image while the shutter was open. Predictions come from orbital elements — they are never pixel detections. The exposure must be one continuous shutter-open interval (not a stack's total integration) and needs an observer location:

sats = seiza.SatelliteCatalog.fetch_celestrak()   # cached; ~2h refresh floor
# or offline / historical: seiza.SatelliteCatalog.open("elements.json")

result = sats.tracks_in_footprint(
    solution.wcs, width, height,
    start="2026-07-19T06:12:00Z",     # Unix seconds, RFC 3339, or tz-aware datetime
    duration_s=120.0,
    latitude=42.466, longitude=-71.1516, altitude_m=150.0,
)
for track in result.tracks:           # highest elevation first
    print(track.label, track.max_elevation_deg, track.clipped_segments)

Element records older than seven days are reported in result.stale_elements and skipped rather than silently extrapolated (max_element_age_s=None overrides). CelesTrak rate-limits repeated downloads: keep reusing one cache directory, and check sats.cache_state and sats.warning after fetch_celestrak().

Working on the bindings

This directory is its own cargo workspace, so cargo fmt --all and cargo clippy --workspace at the repository root never reach it. Run its checks from here, which is what CI does:

cd seiza-py
cargo fmt --check
cargo clippy --all-targets -- -D warnings
maturin develop && python -m pytest tests/ -q

Notes

• Solving and detection release the GIL; other Python threads keep running.
• Catalog files are memory-mapped and SHA-256 verified at download time; fetch_catalogs caches under the platform cache directory (override with cache_dir= or SEIZA_CACHE_DIR).
• seiza.StarCatalog.from_stars([...]) builds a small in-memory catalog for tests and synthetic fields.

License

Apache-2.0