Processing log · October 2026

The Cave Nebula (Sh2-155), HOO with true-colour stars

How the final was made

The Cave Nebula, Sh2-155 — a bright white and pink rim of glowing gas beside a dark lane of dust, in a field laced with faint red hydrogen and orange, white and blue stars

Target: Sh2-155, the Cave Nebula (Caldwell 9), in Cepheus, about 2,400 light-years away: the bright edge of the Cepheus B molecular cloud, lit by young hot stars of the Cepheus OB3 association. The dark dust lane along the bright rim is the “cave”.

Rig: Celestron Origin, RASA 6″ 335 mm f/2.2, Origin678C colour camera (IMX678, 2 µm, GBRG), Antlia Triband RGB Ultra. ISO 1000, uncooled, 24.0–28.1 °C.

Data: one night, 30 September – 1 October 2026, nine Origin sessions: 1,793 × 15 s shot, 1,751 kept = 7.3 h. Waning gibbous moon.

Where: The Star Farm, Camden, South Carolina — Bortle 4.

The walking-noise story

The first stack came out streaked with “walking noise”: fine lines in the faint background. The imaging-side Claude measured the cause: the Origin’s dither moves the frame only east–west, and the first two sessions had been calibrated with a 15 s dark from June shot at 34.4 °C against 28 °C lights — 37 % too strong, turning every warm pixel into a hole that the one-way dither then dragged into a line. Old darks at the “right” temperature made it worse still (the sensor’s warm pixels change over months). The fix was fresh darks: the Origin’s own darks recaptured the next evening at 27.1 °C, plus the 26 September set at 20.8 °C, each scaled to its session’s temperature (k = 1 + 0.029 × ΔT), and a per-session walking-noise filter. Drizzle and one-pass stacking were tested and did not help.

Stacking (Siril)

Graded every sub (origin_cull.py): 42 culled — 5 streaked, 28 thin (haze), 2 comet-shaped, 7 end-of-night slew. Culls kept in raw/_culled_kept.
Each session calibrated on its own with the freshest matching dark, scaled to temperature, and a bad-pixel map.
Split before debayer: red pixels → Hα, green + blue → OIII (half-size, 2.46″/px). The same subs split again into R, G, G, B for true-colour stars.
Per-session walking-noise filter, sessions merged with star registration on one grid, row/column banding removed, cropped to full coverage (1859 × 1054). RGB star channels balanced on green.

PixInsight (Pete)

Origin HOO template: ChannelCombination, ImageSolver, SPFC, MGC (MARS), SPCC narrowband, BlurX, MLDenoise, OIII lift, GHS, black point, BlurX, StarXTerminator, HDRMT. The template’s three curves crushed the Cave’s faint nebula, so they were replaced with one faint-nebula lift curve (K 0.20 → 0.30), then a black point to sink the speckle in the darkest areas. Stars: balanced RGB stack, GHS, StarX, BlurX stars, ColorSaturation, MMT small-star trim, HT black point; screened back onto the starless. A deeper, more speckled version is kept in finals/versions.

Result

1857 × 1052, signed bottom-left with (Origin). AstroBin gets the true final (p99 147); web/FB/HC&M get a lifted copy (p99 181).

Cave Nebula processing log · serial 870194 · September 30 – October 1, 2026