NGC 602 — three bands, one decision#

NGC 602 — IR/R/B in purple/orange/blue, Lab screen compositing, with legend and combo swatch

NGC 602 — IR/R/B in purple/orange/blue, Lab screen compositing (dark canvas)

NGC 602 is a young star-forming region in the SMC with freely downloadable multiwavelength FITS from the Chandra OpenFITS page. The IR, R-band, and B-band frames already share a pixel grid, so there is no crop / reproject step — this page is about the color decisions.

You will learn how to:

build a three-color session with a custom palette (purple / orange / blue, not just RGB primaries),
choose between classic RGB summing and Lab / screen compositing — and see the difference on identical layers,
finish and ship the figure: legend, combo swatch, mosaic, PNG / FITS / reproducible script.

Companion notebook: NGC 602 — same pixel grid. Full mode comparison grids: Color-space compositing suite. Concepts: Color compositing.

The whole pipeline in one block#

import multicolorfits as mcf

s = mcf.McfSession(n_panels=3)
s.load_files(
    ['./ngc602_ir.fits', './ngc602_optical_R.fits', './ngc602_optical_B.fits'],
    colors=['#BE599E', '#DEA215', '#77C0F9'],   # purple / orange / blue (POB)
    labels=['IR', 'R', 'B'],
)
s.compose.combine_mode = 'lab'       # the one decision this page teaches
s.compose.combine_blend = 'screen'
s.compose.gamma = 2.2
s.compose.show_legend = True         # which color = which band
s.compose.show_combo_swatch = True   # honest preview of overlap mixing

rgb = s.render_combined()
fig, ax = mcf.make_combined_figure(s, combined=rgb)
fig.savefig('n602_POB_lab.png', dpi=150, bbox_inches='tight')

Why a custom palette at all? With more than three layers — or three layers that should not scream pure red/green/blue — hex colors per band keep the image informative and attractive. The combo swatch (bottom right of the hero) shows how the chosen colors actually mix under the current compositing mode, so the legend never lies.

RGB vs Lab — same layers, same colors#

The single most consequential switch is combine_mode. Classic RGB sums channels: fast, familiar, higher-contrast — but the sum hard-clips at white, so the very brightest overlaps lose their hue all at once. Lab / screen blends lightness in a perceptual space: hue transitions are smoother and nothing clips abruptly, at the cost of lifting the whole field — screen blending always brightens, so the Lab panel below reads paler and more luminous overall.

NGC 602 with identical layers combined as classic RGB (left) vs Lab/screen (right)

NGC 602 — RGB vs Lab/screen (dark chrome)

Look at the bright rim and the central cluster: RGB keeps the deep-black sky and punchy saturation but snaps to white where the shells overlap; Lab/screen trades some of that contrast for graceful, hue-preserving gradients through the bright regions. Neither is wrong — RGB matches decades of published three-color images. If you like Lab’s smoothness but not the extra lift, try a different blend (combine_blend = 'max' or 'mean' instead of 'screen'). Full grids across modes and blends: Color-space compositing suite.

Tip

White background for publication? Don’t invert layer colors (the legacy hex_complement + inverse=True trick). Just set s.compose.combine_background = 'white' — or 'transparent' for slides (see M74 — transparent cutouts for slides & stamps).

Show your work — component mosaic#

A conference-ready answer to “what went into this color image?” — the combined hero plus each colorized band on the shared WCS:

fig, axes = mcf.make_component_mosaic(
    s, components='top', max_per_line=3, ticks='plain')
fig.savefig('n602_mosaic.png', dpi=150, bbox_inches='tight')

NGC 602 component mosaic — IR/R/B strip above the combined hero panel

Component titles are automatically tinted with each layer’s color. Layouts, tick modes, and packing: Figures and mosaics.

Note

Rotated WCS: these OpenFITS frames sit ~90° from north-up, which can make default WCS tick placement look odd. Either accept it (the mosaic above does), reproject to a north-aligned header (make_simple_header + CROTA2 = 0 + reproject_image), or use Align layers… in the GUI with target icrs (needs [reproject]).

Ship it#

fig.savefig('n602_POB_lab.pdf', dpi=300, bbox_inches='tight')  # paper
s.save_rgb_fits('./n602_POB_lab.fits')     # combined cube + provenance HISTORY
print(s.export_script())                   # full-fidelity reproduction script
s.save_state('./n602_session.json')        # resume in GUI or scripts later

# ... or polish interactively and export from the browser GUI:
# mcf.gui(session=s)

NGC 602 — three bands, one decision#

The whole pipeline in one block#

RGB vs Lab — same layers, same colors#

Show your work — component mosaic#

Ship it#

Classic low-level API (v2-compatible)#