**The Triangulum Galaxy**: Messier 33, a spiral galaxy in Triangulum

Night of 17 October 2026, San Francisco Bay Area

**Equipment**

* Telescope: Celestron 8SE Schmidt-Cassegrain: 203 mm aperture, 2,082.9 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 tonight; aligned from plate-solved photos, both axes driven from that model. Dithered between frames for most of tonight
* Computer: Raspberry Pi 3 running Observatory, in Elixir

**Acquisition**

* Lights: 267 × 15 s at ISO 3200 (66.8 minutes)
* Frames: 342 taken, 267 kept
* When: The night of 17 October 2026, in the evening and after midnight, either side of Saturn; and 13 October 2026, after midnight. About 50 degrees up in the evening, about 80 after midnight
* Sky: Clear and still. A Moon a little under half lit, low in the southwest early on and soon behind the garage
* Dithering: Tonight's first turn held still (the field drifted about 2.5 arcmin). The second moved between frames in a spiral up to 1.5 arcmin. The turn after Saturn moved every 3 frames, small probe steps and then up to 4 arcmin from the start (the last place nearly 5): a bug in the sensor map chose the 4, and it was kept because it widened the field the frames cover. 13 October was not dithered
* Darks: None taken. Hot pixels found from the frames, one map per night (about 20,000 to 22,600 per colour plane tonight, 5,000 to 6,900 on 13 October), 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), the flat tonight's Dumbbell Nebula run chose. On this run's dithered sky, the galaxy's middle masked, it leaves less than the twilight flat alone: 1.3 against 2.8 percent in green after a plane, galaxy light included. The galaxy fills the frame, so no large scale could be built or judged from these frames
* Calibration: Dust that moved since 13 October, measured on each night's own frames (each frame against the first stack put back on its sensor grid, so the galaxy cancels) and divided out. Tonight 10 shadows, each seen in the first turn and the turn after Saturn alone, 0.77 percent of the sensor; among them the new crescent, 11.6 percent deep after the 13 October flat, left at 0.95 percent. 13 October: 19 shallow ones (1.5 to 3 percent), from its odd and even frames, a weaker test because those frames were not dithered
* Field: 36.3 x 21.4 arcmin
* Sampling: 0.776 arcsec per pixel

**Processing**

* Registration: Each frame placed first by its own plate solve (all within 9 sensor px of the star solution), then rotation and shift on the stars, weighted rms at most 0.84 px. The field turned 10.2 degrees across all the frames, 8.7 of it over tonight's turns, because the mount stood about 10 degrees off the pole
* Stacking: RAW colour planes (no demosaic), hot pixels by 3 x 3 median, registered with rotation, then a 3-sigma clipped mean weighted by each frame's clarity and noise. Not done, for the time limit: proper coaddition (1 to 4 percent more signal to noise on stars from 15 s frames), and lining red and blue up with green against the air's prism. One constant per colour per frame, set on the darkest 30 percent of the area every frame covers: the galaxy's faintest level, not the true sky; no surface fitted. Noise 13.7 times lower than one frame (green); with 13 October's frames 10 percent lower than tonight's alone
* Finish: Star colours from Gaia, the camera's colour matrix. Arcsinh stretch; black at the faintest part's own level, gamma 0.9, S-curve 0.3, saturation x 1.25; the dark parts smoothed (bilateral 3 px). 0.2 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/triangulum-galaxy.html

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