laurence
c6299f41ab
Preparing to merge this repository into a combined astrophotography repo. session-scripts/ becomes pipeline/ because the scripts import layout.py from their own directory and must stay together, and because 'pipeline' says what it is rather than how it came about. observing/ stays at the top level: observing plans are not processing code.
91 lines
4.2 KiB
Python
91 lines
4.2 KiB
Python
"""Split the finished image into a starless view and a star-only layer.
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The first attempt used a greyscale morphological opening, the classical trick,
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and it failed in a way worth recording: an opening replaces each pixel with a
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local minimum, so on a noisy sky it does not merely delete the star, it digs a
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hole slightly BELOW sky level and leaves a black dot behind. It also left the
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brightest stars standing, because their wings are wider than any structuring
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element small enough to spare the galaxy.
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What works instead is catalogue-and-fill. The stars to remove are taken from
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Gaia DR3 rather than from a blind detection: Gaia is essentially complete for
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stars down to G = 20, which is fainter than anything visible here, so the
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catalogue identifies the foreground almost perfectly. That matters because a
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blind detector cannot tell a Milky Way star from one of Centaurus A's own
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globular clusters or from the blue knots in its dust lane, and removing those
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guts the very structure the image is about. Each Gaia star is painted into a
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mask whose radius follows its magnitude, and the masked pixels are
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replaced by a normalised convolution - a Gaussian blur of the unmasked pixels
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divided by the same blur of the mask itself, which interpolates across each
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star using only real neighbouring pixels. That follows the galaxy's gradient
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instead of flattening it, and leaves no holes.
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The star layer is then simply what was removed. Two honest limitations: the
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extended halos and diffraction spikes of the very brightest stars reach beyond
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any sensible mask radius and leave soft residual glows behind, and anything
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Gaia does not list (background galaxies, the cluster system) stays in the
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starless frame by design. This is a presentation tool, not a measurement.
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"""
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import os
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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 PIL import Image
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from scipy.ndimage import gaussian_filter
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import layout
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OUT = layout.SESSION
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SOURCE = "NGC5128-img-deconvolved.png"
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FILL_SIGMA = 9.0 # interpolation scale for the normalised convolution
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rgb = np.asarray(Image.open(layout.path(SOURCE))).astype(np.float32) / 255
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ny, nx = rgb.shape[:2]
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print(f"{SOURCE}: {nx} x {ny}")
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z = np.load(layout.path("_gaia_deep.npz"))
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with fits.open(layout.path("NGC5128-LRGB.fit")) as hd:
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wcs = WCS(hd[0].header, naxis=2)
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gx, gy = wcs.world_to_pixel(SkyCoord(z["ra"] * u.deg, z["dec"] * u.deg))
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gmag = z["g"]
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on_frame = (gx > -30) & (gx < nx + 30) & (gy > -30) & (gy < ny + 30)
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gx, gy, gmag = gx[on_frame], gy[on_frame], gmag[on_frame]
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print(f"{len(gx)} Gaia stars on the frame, G {gmag.min():.1f}-{gmag.max():.1f}")
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mask = np.zeros((ny, nx), np.float32)
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R = 30
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yy, xx = np.mgrid[-R:R + 1, -R:R + 1]
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rr = np.hypot(xx, yy)
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for x, y, g in zip(gx, gy, gmag):
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# Radius from magnitude: a bright star's visible disc and wings are far
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# wider than a faint one's, and a fixed radius would either miss the
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# bright ones or scour the field around the faint ones.
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r = float(np.clip(30.0 - 1.9 * (g - 8.0), 4, R - 2))
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cx, cy = int(round(x)), int(round(y))
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x0, x1 = max(0, cx - R), min(nx, cx + R + 1)
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y0, y1 = max(0, cy - R), min(ny, cy + R + 1)
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patch = (rr <= r).astype(np.float32)[(y0 - cy + R):(y1 - cy + R),
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(x0 - cx + R):(x1 - cx + R)]
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np.maximum(mask[y0:y1, x0:x1], patch, out=mask[y0:y1, x0:x1])
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mask = np.clip(gaussian_filter(mask, 1.5), 0, 1)
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print(f"mask covers {mask.mean():.2%} of the frame")
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starless = np.empty_like(rgb)
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keep = 1.0 - mask
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denom = gaussian_filter(keep, FILL_SIGMA)
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denom[denom < 1e-3] = 1e-3
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for i in range(3):
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filled = gaussian_filter(rgb[:, :, i] * keep, FILL_SIGMA) / denom
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starless[:, :, i] = rgb[:, :, i] * keep + filled * mask
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stars = np.clip(rgb - starless, 0.0, 1.0)
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Image.fromarray((np.clip(starless, 0, 1) * 255 + 0.5).astype(np.uint8)).save(
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layout.path("NGC5128-img-starless.png"))
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# The star layer is faint on its own; a square-root stretch makes it legible
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# without adding or removing anything.
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Image.fromarray((np.sqrt(stars) * 255 + 0.5).astype(np.uint8)).save(
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layout.path("NGC5128-img-stars.png"))
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print("wrote NGC5128-img-starless.png and NGC5128-img-stars.png")
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