laurence
3c4ae5c77e
Three fixes and one refusal, all found by working on the wide-field narrowband session that would not solve. The detection floor was a fixed 12 pixels, which quietly assumed a well sampled star. At 0.53 arcsec/px with 5.9 px seeing that is right; at 3.5 arcsec/px the stars are undersampled at 1.75 px FWHM and cover a handful of pixels each, so the floor discarded nearly every real star and kept blends and galaxies instead. It now scales with the measured seeing, and that session went from 305 usable detections to 600. Matching now happens over the frame's inscribed circle rather than a cone reaching its corners. On a 4 degree field the old cone covered 33 square degrees of sky against 16 of image, so half the catalogue was not in the picture at all. Where the header records a roll angle - and iTelescope's does - the orientation no longer has to be recovered from scratch. Rotation, scale and parity are applied directly and only the residual pointing error is searched, by histogramming every detection-to-catalogue offset and taking the peak. Asterism matching remains as the fallback for headers that say nothing about orientation. The refusal matters most. With catalogue depth slices added, the wide field produced a "solution" of 25 stars at 2.50 px residual, claiming a centre half a degree from the pointing. A genuine solve on that field matches hundreds of stars and refines to well under a pixel. Accepting it would have silently corrupted every position in the science catalogue, so a solution must now reach 40 stars AND 1.5 px or it is discarded and the session is reported as unsolved. A wrong WCS is worse than no WCS. NGC 2030 improves from 0.31 to 0.12 arcsec on 123 stars under the pointing-based match. NGC 2070 is honestly unsolved.
422 lines
18 KiB
Python
422 lines
18 KiB
Python
"""Stage 4: plate solve the deepest master and copy the WCS to the others.
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iTelescope's frames arrive with a PinPoint HISTORY line but no WCS keywords at
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all, so the astrometry has to be redone locally. A blind solve is unnecessary:
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the header gives the pointing to arcminutes and usually the plate scale to four
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figures, so a catalogue can be fetched for the right patch of sky and matched
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directly.
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The match is asterism-based, which is invariant to rotation and scale - so the
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camera angle never has to be guessed, and a session at an unknown orientation
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solves as easily as one at a known angle. It is NOT invariant to a mirror flip,
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because a similarity transform cannot include reflection, so both parities are
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tried and the one that matches wins.
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The seed match returns only a handful of stars. That is enough to establish an
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approximate solution but too few to trust, so the solution is then used to pair
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every catalogue source with its nearest detection and refit, twice, with a
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shrinking tolerance. The residual that comes out of that is the honest measure
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of whether the solve is real.
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Failure here is not fatal to the pipeline: an unsolved session still produces
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images, it just cannot produce positions.
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"""
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import os
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import astroalign as aa
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import numpy as np
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from astropy import units as u
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from astropy.coordinates import SkyCoord
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from astropy.io import fits
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from astropy.wcs import WCS
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from astropy.wcs.utils import fit_wcs_from_points
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import layout
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import measure as measure_mod
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SEED_STARS = 600
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CATALOGUE_FETCH = 6000 # rich fields need far more than the match uses
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def pointing(header):
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"""Approximate field centre from whatever the header offers."""
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for ra_key, dec_key in (("OBJCTRA", "OBJCTDEC"), ("RA", "DEC"),
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("CRVAL1", "CRVAL2")):
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ra, dec = header.get(ra_key), header.get(dec_key)
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if ra is None or dec is None:
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continue
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try:
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if isinstance(ra, str) and (":" in ra or " " in ra):
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return SkyCoord(ra, dec, unit=(u.hourangle, u.deg))
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return SkyCoord(float(ra) * u.deg, float(dec) * u.deg)
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except Exception: # noqa: BLE001
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continue
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return None
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def plate_scale(header, session=None):
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"""Arcsec per pixel: from the header if present, else from the optics."""
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val = header.get("HIERARCH iTelescopePlateScaleH")
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if val:
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try:
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return float(val)
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except (TypeError, ValueError):
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pass
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focal = header.get("FOCALLEN")
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pixel = header.get("XPIXSZ")
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if focal and pixel:
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# 206265 arcsec per radian; XPIXSZ is already binned microns.
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return 206.265 * float(pixel) / float(focal)
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return session.scale if session else None
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def fetch_catalogue(centre, radius_deg, cache, limit=SEED_STARS, deep=False):
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"""Gaia sources around the pointing, cached so a re-run costs nothing."""
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if os.path.exists(cache):
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z = np.load(cache)
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return z["ra"], z["dec"], z["g"]
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ra = dec = g = None
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mag_cut = 20.5 if deep else 18.0
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# VizieR first, deliberately. It serves the identical DR3 catalogue over a
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# plain HTTP query and answers in under a second. Gaia's own archive is
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# excellent but launch_job_async submits to a job QUEUE and polls for the
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# result, which for a few hundred rows means waiting minutes for work that
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# takes milliseconds - and it hung for ten minutes on a query of 600 stars
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# while the network was demonstrably healthy.
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try:
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from astroquery.vizier import Vizier
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# Ask for far more rows than are needed and pick the brightest
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# locally. VizieR's row_limit truncates the result BEFORE any sort, so
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# asking for 600 rows in a crowded field returns 600 arbitrary stars
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# rather than the 600 brightest - and asterism matching only works if
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# both lists contain the same bright stars. In the LMC field of
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# NGC 2030 that silently produced no match at all.
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v = Vizier(columns=["RA_ICRS", "DE_ICRS", "Gmag"],
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column_filters={"Gmag": f"<{mag_cut}"}, row_limit=50000)
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v.TIMEOUT = 60
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res = v.query_region(centre, radius=radius_deg * u.deg,
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catalog="I/355/gaiadr3")
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if res:
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t = res[0]
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ra = np.asarray(t["RA_ICRS"], float)
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dec = np.asarray(t["DE_ICRS"], float)
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g = np.asarray(t["Gmag"], float)
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except Exception as exc: # noqa: BLE001
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print(f" VizieR unavailable ({type(exc).__name__})")
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if ra is None or len(ra) == 0:
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try:
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from astroquery.gaia import Gaia
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Gaia.ROW_LIMIT = limit
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# Synchronous, not async: no job queue for a query this small.
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job = Gaia.launch_job(f"""
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SELECT TOP {limit} ra, dec, phot_g_mean_mag
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FROM gaiadr3.gaia_source
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WHERE 1 = CONTAINS(POINT('ICRS', ra, dec),
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CIRCLE('ICRS', {centre.ra.deg}, {centre.dec.deg},
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{radius_deg}))
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AND phot_g_mean_mag IS NOT NULL
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AND phot_g_mean_mag < {mag_cut}
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ORDER BY phot_g_mean_mag ASC""")
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t = job.get_results()
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ra = np.asarray(t["ra"], float)
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dec = np.asarray(t["dec"], float)
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g = np.asarray(t["phot_g_mean_mag"], float)
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except Exception as exc: # noqa: BLE001
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print(f" Gaia archive unavailable ({type(exc).__name__})")
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return None, None, None
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order = np.argsort(g)[:limit]
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ra, dec, g = ra[order], dec[order], g[order]
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np.savez_compressed(cache, ra=ra, dec=dec, g=g)
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return ra, dec, g
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def project(ra, dec, centre, scale, parity):
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"""Gnomonic projection to pixel-like coordinates for asterism matching."""
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c = SkyCoord(ra * u.deg, dec * u.deg)
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dx, dy = centre.spherical_offsets_to(c)
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return np.column_stack([dx.to_value(u.arcsec) / scale * parity,
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dy.to_value(u.arcsec) / scale])
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def _roll_angle(header):
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"""Camera angle from the header, if the telescope recorded it."""
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for key in ("HIERARCH RollAngle", "RollAngle", "POSANG", "CROTA2"):
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val = header.get(key)
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if val is None:
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continue
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try:
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return float(val)
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except (TypeError, ValueError):
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continue
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return None
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def match_by_pointing(src, ra, dec, centre, scale, roll, shape, verbose=True):
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"""Pair detections with catalogue stars using a known camera angle.
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Asterism matching exists to recover an UNKNOWN orientation. When the header
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records the roll angle - and iTelescope's does - the whole geometry is
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already known bar a small pointing error, and a direct search is both more
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robust and far faster. Rotation, scale and parity are applied, then the
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residual translation is found by histogramming every detection-to-catalogue
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offset and taking the peak: a real solution puts thousands of pairs in one
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bin, and noise spreads flat.
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"""
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ny, nx = shape
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# Slide down the catalogue's magnitude ranking as well as trying the four
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# geometric conventions. In a field as rich as the LMC the brightest few
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# hundred catalogue stars are far brighter than anything a 60 second
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# narrowband frame records, so the two "brightest N" lists describe
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# different populations and overlap barely at all.
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best = None
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slices = [(0, 400), (200, 800), (600, 1400), (1200, 2400), (2000, 4000)]
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for lo, hi in slices:
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if lo >= len(ra):
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break
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ra_s, dec_s = ra[lo:hi], dec[lo:hi]
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for parity in (1.0, -1.0):
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for sign in (1.0, -1.0):
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th = np.radians(sign * roll)
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cat = project(ra_s, dec_s, centre, scale, parity)
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rot = np.column_stack([
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cat[:, 0] * np.cos(th) - cat[:, 1] * np.sin(th),
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cat[:, 0] * np.sin(th) + cat[:, 1] * np.cos(th)])
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cat_px = rot + np.array([nx / 2.0, ny / 2.0])
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# Every pairwise offset, histogrammed. Limited to the brightest of
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# each list to keep this to a few million comparisons.
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a = src[:400]
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b = cat_px[:400]
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dx = (a[:, None, 0] - b[None, :, 0]).ravel()
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dy = (a[:, None, 1] - b[None, :, 1]).ravel()
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lim = 0.25 * max(nx, ny)
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keep = (np.abs(dx) < lim) & (np.abs(dy) < lim)
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if keep.sum() < 50:
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continue
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bins = np.arange(-lim, lim + 12, 12)
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H, xe, ye = np.histogram2d(dx[keep], dy[keep], bins=[bins, bins])
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iy, ix = np.unravel_index(np.argmax(H), H.shape)
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peak = H[iy, ix]
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if best is None or peak > best[0]:
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off = (0.5 * (xe[iy] + xe[iy + 1]),
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0.5 * (ye[ix] + ye[ix + 1]))
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best = (peak, parity, sign, off, cat_px, lo, hi)
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if best is None:
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return None
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peak, parity, sign, off, cat_px, lo, hi = best
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if verbose:
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print(f" pointing match: catalogue[{lo}:{hi}], parity "
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f"{parity:+.0f}, roll {sign:+.0f}, offset "
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f"({off[0]:+.0f}, {off[1]:+.0f}) px, peak {int(peak)} pairs")
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if peak < 12:
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return None
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shifted = cat_px + np.array(off)
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idx_base = lo
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pairs = []
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for i, (cx, cy) in enumerate(shifted):
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if not (0 <= cx < nx and 0 <= cy < ny):
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continue
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d = np.hypot(src[:, 0] - cx, src[:, 1] - cy)
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j = int(np.argmin(d))
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if d[j] < 18:
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pairs.append((i + idx_base, j))
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if len(pairs) < 10:
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return None
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return np.array([p[0] for p in pairs]), np.array([p[1] for p in pairs])
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def run(session, verbose=True):
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"""Solve the deepest master; write the WCS into every master."""
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masters_dir = os.path.join(session.root, layout.MASTERS)
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deepest = session.filters[0]
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path = os.path.join(masters_dir, f"master-{deepest}.fit")
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if not os.path.exists(path):
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raise RuntimeError("no masters found - run the register stage first")
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with fits.open(path) as hd:
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image = hd[0].data.astype(np.float32)
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header = hd[0].header
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ny, nx = image.shape
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centre = pointing(header)
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scale = plate_scale(header, session)
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if centre is None or not scale:
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print(" no pointing or plate scale in the header - "
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"cannot solve without a blind solver")
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return None
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# Match over the frame's INSCRIBED circle, not its circumscribed one. A
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# cone big enough to reach the corners also reaches well outside the
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# frame - on a 4 degree field that is 33 square degrees of catalogue
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# against 16 of image, so half the catalogue's brightest stars are not in
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# the picture at all and the two "brightest N" lists barely overlap.
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# The inscribed circle is inside the frame whatever the camera angle, so
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# both lists then describe the same patch of sky.
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radius = 0.95 * 0.5 * min(nx, ny) * scale / 3600.0
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if verbose:
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print(f" pointing {centre.to_string('hmsdms')}, "
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f"{scale:.4f} arcsec/px, search {radius:.3f} deg")
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xy, _ = measure_mod.load(session)
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# Detect on the master itself: it is deeper than any single frame.
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from measure import measure_frame
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det = measure_frame(path, max_stars=SEED_STARS * 3)
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src = det["xy"]
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# Same restriction on the detections, so the two lists cover one region.
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r_pix = 0.95 * 0.5 * min(nx, ny)
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inside = np.hypot(src[:, 0] - nx / 2.0, src[:, 1] - ny / 2.0) < r_pix
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src = src[inside][:SEED_STARS]
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if len(src) < 12:
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print(f" only {len(src)} stars in the master - too few")
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return None
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cache = os.path.join(session.root, layout.INTERMEDIATES, "_gaia.npz")
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os.makedirs(os.path.dirname(cache), exist_ok=True)
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ra, dec, gmag = fetch_catalogue(centre, radius, cache,
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limit=CATALOGUE_FETCH)
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if ra is None:
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print(" no catalogue available - skipping the solve")
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return None
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if verbose:
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print(f" {len(ra)} catalogue stars, {len(src)} detected")
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# If the header recorded the camera angle, use it: a direct geometric
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# match is far more reliable than recovering the orientation from scratch.
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roll = _roll_angle(header)
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seed = None
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if roll is not None:
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if verbose:
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print(f" header roll angle {roll:.2f} deg")
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seed = match_by_pointing(src, ra, dec, centre, scale, roll,
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(ny, nx), verbose=verbose)
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if seed is not None:
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ci, ii = seed
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world0 = SkyCoord(ra[ci] * u.deg, dec[ci] * u.deg)
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wcs = fit_wcs_from_points((src[ii, 0], src[ii, 1]), world0,
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proj_point="center", projection="TAN")
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best = "pointing"
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else:
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best = None
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# Matching needs the two lists to contain the SAME stars, and "brightest"
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# does not guarantee that. The detector deliberately rejects saturated
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# cores, so on a well exposed frame the brightest detections are not the
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# brightest stars in the sky - while the catalogue's brightest are. The
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# two top-N lists can therefore barely overlap, which is exactly what
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# happened on the crowded LMC field of NGC 2030.
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#
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# So slide a window down the catalogue's magnitude ranking as well as
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# varying the sample size, and take the best match found.
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if best == "pointing":
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pass
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else:
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best = None
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for offset in (0, 15, 40, 80):
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for n in (120, 200, 60):
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hi = min(offset + n, len(ra))
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if hi - offset < 25 or len(src) < 25:
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continue
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cat_slice = slice(offset, hi)
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for parity in (1.0, -1.0):
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cat_xy = project(ra[cat_slice], dec[cat_slice], centre, scale,
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parity)
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try:
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tform, (cat_m, img_m) = aa.find_transform(
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cat_xy, src[:max(n, 120)], max_control_points=60)
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except Exception: # noqa: BLE001
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continue
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if best is None or len(cat_m) > len(best[0]):
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# Record which catalogue rows matched, in FULL-list terms.
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best = (cat_m, img_m, parity, offset)
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if verbose:
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print(f" catalogue[{offset}:{hi}] parity "
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f"{parity:+.0f}: matched {len(cat_m)} stars")
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if best is not None and len(best[0]) >= 8:
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break
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if best is not None and len(best[0]) >= 8:
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break
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if best is None:
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print(" no match by pointing or asterism - solve failed")
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return None
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if best != "pointing":
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cat_m, img_m, parity, _offset = best
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cat_xy = project(ra, dec, centre, scale, parity)
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idx = [int(np.argmin(np.hypot(cat_xy[:, 0] - px, cat_xy[:, 1] - py)))
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for px, py in cat_m]
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world = SkyCoord(ra[idx] * u.deg, dec[idx] * u.deg)
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wcs = fit_wcs_from_points((img_m[:, 0], img_m[:, 1]), world,
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proj_point="center", projection="TAN")
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# Refine: the seed match is a handful of stars, so use the approximate
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# solution to pair every catalogue source with its nearest detection.
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all_world = SkyCoord(ra * u.deg, dec * u.deg)
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nmatch, resid = (len(seed[0]) if seed is not None else len(cat_m)), None
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for tol in (4.0, 2.0):
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px, py = wcs.world_to_pixel(all_world)
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pairs = []
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for i, (cx, cy) in enumerate(zip(px, py)):
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if not (0 <= cx < nx and 0 <= cy < ny):
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continue
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d = np.hypot(src[:, 0] - cx, src[:, 1] - cy)
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j = int(np.argmin(d))
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if d[j] <= tol:
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pairs.append((i, j))
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if len(pairs) < 20:
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break
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ci = np.array([p[0] for p in pairs])
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ii = np.array([p[1] for p in pairs])
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wcs = fit_wcs_from_points((src[ii, 0], src[ii, 1]), all_world[ci],
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proj_point="center", projection="TAN")
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qx, qy = wcs.world_to_pixel(all_world[ci])
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resid = np.hypot(qx - src[ii, 0], qy - src[ii, 1])
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nmatch = len(pairs)
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# A quality gate, because a WRONG solution is worse than none: every
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# position in the science catalogue would inherit the error, silently. A
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# genuine solve on a field like this matches hundreds of stars and refines
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# to well under a pixel; a spurious alignment matches a couple of dozen and
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# stalls at several. Both conditions must hold.
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MIN_STARS, MAX_RESID_PX = 40, 1.5
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if resid is None or nmatch < MIN_STARS or np.median(resid) > MAX_RESID_PX:
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got = "n/a" if resid is None else f"{np.median(resid):.2f} px"
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print(f" REJECTED: {nmatch} stars at {got} residual "
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f"(need >= {MIN_STARS} stars and <= {MAX_RESID_PX} px). "
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f"Treating as unsolved rather than trusting a doubtful WCS.")
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return None
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cd = wcs.pixel_scale_matrix * 3600.0
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solved_scale = float(np.sqrt(abs(np.linalg.det(cd))))
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rot = float(np.degrees(np.arctan2(cd[0, 1], cd[1, 1])))
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med_resid = float(np.median(resid)) if resid is not None else float("nan")
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field = wcs.pixel_to_world(nx / 2.0, ny / 2.0)
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if verbose:
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print(f" solved on {nmatch} stars, residual "
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f"{med_resid:.2f} px ({med_resid * solved_scale:.2f} arcsec)")
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print(f" centre {field.to_string('hmsdms')}, "
|
|
f"{solved_scale:.4f} arcsec/px, PA {rot:.2f} deg, "
|
|
f"{nx * solved_scale / 60:.1f}' x {ny * solved_scale / 60:.1f}'")
|
|
|
|
whdr = wcs.to_header()
|
|
for filt in session.filters:
|
|
p = os.path.join(masters_dir, f"master-{filt}.fit")
|
|
if not os.path.exists(p):
|
|
continue
|
|
with fits.open(p, mode="update") as hd:
|
|
for card in whdr.cards:
|
|
hd[0].header[card.keyword] = (card.value, card.comment)
|
|
hd[0].header["ASTRSOLV"] = (
|
|
f"Gaia DR3 / {nmatch} stars / "
|
|
f"{med_resid * solved_scale:.2f} arcsec", "local plate solve")
|
|
hd.flush()
|
|
|
|
return dict(nstars=nmatch, residual_px=med_resid,
|
|
residual_arcsec=med_resid * solved_scale,
|
|
scale=solved_scale, position_angle=rot,
|
|
centre=field.to_string("hmsdms"),
|
|
fov_arcmin=(nx * solved_scale / 60, ny * solved_scale / 60))
|