**The Orion Nebula**: Messier 42 and Messier 43, a star nursery in the sword of Orion

Night of 17 October 2026, San Francisco Bay Area

**Equipment**

* Telescope: Celestron 8SE Schmidt-Cassegrain: 203 mm aperture, 2,081.7 mm focal length by plate solve of this stack, f/10.3
* Camera: Sony a6000, stock, at prime focus: APS-C 23.5 x 15.6 mm, 6000 x 4000, 3.9 micron pixels, RAW
* Mount: Sky-Watcher EQ6-R, unguided. Set down about 10 degrees off the pole; aligned from four plate-solved photos, both axes driven from that model. Dithered every three frames
* Computer: Raspberry Pi 3 running Observatory, in Elixir

**Acquisition**

* Lights: 165 × 30 s at ISO 1600 (82.5 minutes; and 44 x 2 s at ISO 400, 88 s, for the bright middle)
* Frames: 241 taken, 209 kept
* When: The night of 17 October 2026, before dawn. 30 to 50 degrees up
* Sky: Clear and still. A Moon a little under half lit, low in the southwest early on and soon behind the garage
* Dithering: Every 3 frames, the long and the short alike: a spiral of steps up to 1.5 arcmin from the start (the first steps up to 1)
* Darks: None taken. Hot pixels found from the frames themselves, one map for the long frames and one for the short, repaired by the 3 x 3 median of their colour plane
* Flats: No flat frames. The hybrid flat of 13 October (large scale from that night's open sky, dust from its twilight frames), as the Dumbbell Nebula and the Triangulum Galaxy used tonight. Its large scale cannot be judged on this run: the nebula's glow is in every frame. The short frames get the same flat
* Calibration: Dust that moved since 13 October, measured on this run's own dithered long frames (each frame against the first stack put back on its sensor grid, so the nebula cancels; each shadow seen in the first and the second half of the run alone) and divided out: 12 shadows, 0.96 percent of the sensor. Among them the new crescent, 12.2 percent deep after the 13 October flat, left at 1.1 percent. Nothing judged within 5 arcmin of the Trapezium, where the nebula is too bright and structured
* Field: 36.5 x 25.2 arcmin
* Sampling: 0.777 arcsec per pixel

**Processing**

* Registration: Each frame placed first by its own plate solve, then rotation and shift on the stars onto one long frame, the short frames too; weighted rms at most 0.64 px for the long frames, 1.62 for the short. The field turned 5.4 degrees over the run. The air's prism: red sat 0.36, 0.51 and blue -0.61, -0.76 sensor px from green, measured and taken out in the one resampling
* Stacking: RAW colour planes (no demosaic), hot pixels by 3 x 3 median, registered on stars with rotation. The long frames by proper coaddition (Zackay and Ofek: each frame weighted by its own measured blur and noise); the old 3-sigma clipped weighted mean made beside it from the same frames gives 1.5 to 2.3 percent less point-source signal to noise and stars 0.5 percent wider: nothing a viewer can see. Noise 12.9 times under one frame, as the square root of 165 says. The short frames by the 3-sigma clipped weighted mean. One constant per colour per frame, set on the darkest 30 percent of the area every long frame covers: the faintest nebula, not the true sky
* High dynamic range: The 2 s stack replaces the 30 s one wherever any 30 s frame came within 7 percent of clipping, in any colour: 1.76 square arcmin round the Trapezium, grown one colour cell, with a smooth ramp in brightness so there is no step. 751 pixels are white, where even the 2 s frames clipped. The scale between the two is measured, not assumed: 62.65 on the nebula (each colour 62.6 to 63.3, the same at every brightness) against the 60 the exposures and ISOs give, and 65.2 on 66 stars. Neither difference is explained: the camera's ISO or shutter not being exactly nominal is a guess, not checked. The join uses the measured 62.65
* Finish: Star colours from Gaia, the camera's colour matrix. Arcsinh stretch (soft 12 DN, white 250,000 DN: one curve from the faint loops to the core); black at the faintest part's own level, S-curve 0.25, saturation x 1.2; the dark parts smoothed (bilateral 3 px). 5.8 percent of the picture is exactly black
* Software: Capture and mount: Observatory (Elixir on a Raspberry Pi 3). Stacking: numpy, OpenCV, rawpy. Plate solving: astrometry.net. Stacked from the camera's RAW frames. Nothing generated

Full write-up: https://bradgessler.github.io/observatory-blog/observations/2026-10-17/orion-nebula.html

All the photography notes: https://bradgessler.github.io/observatory-blog/observations/2026-10-17/orion-nebula.html#photography