Learning how to capture M31 from rainy, Bortle 6 Belfast

I pointed a Seestar S30 Pro at the Andromeda Galaxy from Belfast, where the sky is Bortle 6 and most nights it rains. Below are the processing changes I made, one at a time, and what each one did to the picture.

Stars tightened from 17.0″ to 9.4″. SNR went from 19.41 to 13.86, with 62 minutes against 80.

Stages

Each stage changes one thing. Before is on the left and after is on the right, aligned on the same stars. Drag the line, or tab to it and use the arrow keys.

Stage 00: What the scope gives you

2026-09-07

Before (left): Seestar live stack; 479 fr · 80 min; 17.0″ · SNR 19.4
After (right): First pipeline (8 Sep); 427 fr · 71 min; 14.4″ · SNR 11.1

What changed: I took the 7/8 Sep subs off the scope and ran my first pipeline on them, as it stood on 8 Sep: frame QA, Siril drizzle stacking weighted towards the sharpest frames, and GraXpert's AI background removal.
What it did: Stars shrank from 17.0″ to 14.4″, but SNR at my eight fixed points in the disc fell from 19.41 to 11.07, so the scope's own stack was still the deeper image.

aligned on 302 stars, median residual 0.23 px

Stage 01: Measure, don't guess

2026-09-07

Before (left): First pipeline (8 Sep); 427 fr · 71 min; 14.4″ · SNR 11.1
After (right): 7/8 Sep, current pipeline; 467 fr · 78 min; 15.5″ · SNR 16.5

What changed: A review measured what each step cost, so I kept almost every frame, weighted them by star count instead of sharpness, and replaced the AI background model with a flat plane fit that leaves the halo alone.
What it did: Same night, 467 frames instead of 427: SNR went from 11.07 to 16.48 (+49 %), and stars grew a little, from 14.4″ to 15.5″.

aligned on 285 stars, median residual 0.16 px

Stage 02: The full moon night

2026-09-07, 2026-09-25 · nights 2026-09-07 + 2026-09-25

Before (left): 7/8 Sep, current pipeline; 467 fr · 78 min; 15.5″ · SNR 16.5
After (right): 25/26 Sep, first 529 frames; 529 fr · 88 min; 10.1″ · SNR 10.4

What changed: My second night, 25/26 Sep, was under a 99 % moon. These are the first 529 frames to come off the scope; the rest were still copying.
What it did: Moonlight raised the frame background to 1858 ADU from 1326, and SNR dropped to 10.4 from 16.48 despite more minutes, while stars came out at 10.1″ against 15.5″.

aligned on 281 stars, median residual 0.15 px

Stage 03: Stacking nights

2026-09-07, 2026-09-25 · nights 2026-09-07 + 2026-09-25

Before (left): 7/8 Sep, current pipeline; 467 fr · 78 min; 15.5″ · SNR 16.5
After (right): Two nights combined; 995 fr · 2 h 46 min; 13.3″ · SNR 19.0

What changed: I stacked both nights frame by frame, with each frame weighted by its noise so the moonlit ones can't dilute the dark ones. Before is my best single night by SNR, 7/8 Sep.
What it did: 995 frames and 166 minutes lifted SNR from 16.48 to 18.96 (+15 %), with stars at 13.3″.

aligned on 285 stars, median residual 0.10 px

Stage 04: The sharp night

2026-09-25, 2026-10-16 · nights 2026-09-25 + 2026-10-16

Before (left): 25/26 Sep, first 529 frames; 529 fr · 88 min; 10.1″ · SNR 10.4
After (right): 16/17 Oct alone; 375 fr · 62 min; 9.4″ · SNR 13.9

What changed: On 16/17 Oct the moon was down, gusts peaked at 15 mph against 28, and I got 62 minutes of 10 s subs.
What it did: Stars came out at 9.4″ against 10.1″, the sharpest night so far. SNR reached 13.86 in 62 minutes, where the moonlit night managed 10.4 in 88.

aligned on 281 stars, median residual 0.20 px

The nights

Most nights here are cloudy. These are the clear ones, with Belfast's weather for the hours I was imaging and what the frames recorded.

2026-09-07 23:21–01:14 UTC 7/8 Sep 2026

frames

467 × 10 s

FWHM

16.2″

moon

12 %

gusts

28 mph

cloud

4 %

dropped

22 %

QA out

12

23:21–01:14 UTC · 479 × 10 s · moon 12 % below horizon · cloud 4 % · gusts 28 mph · 22 % dropped
2026-09-25 21:35–23:59 UTC 25/26 Sep 2026

frames

529 × 10 s

FWHM

11.5″

moon

99 %

gusts

28 mph

cloud

6 %

dropped

25 %

QA out

61

21:35–23:59 UTC · 590 × 10 s · moon 99 % up to 34° · cloud 6 % · gusts 28 mph · 24 % dropped
2026-10-16 19:45–21:22 UTC 16/17 Oct 2026

frames

376 × 10 s

FWHM

10.3″

moon

33 %

gusts

15 mph

cloud

3 %

dropped

27 %

QA out

8

19:45–21:22 UTC · 384 × 10 s · moon 33 % below horizon · cloud 3 % · gusts 15 mph · 27 % dropped

Signal against time

With nothing else changed, signal-to-noise grows with the square root of time, which is the faint curve. Points under it lost something along the way. Points over it gained more from processing than the extra minutes alone would give.

Filled points are at or above the √t line through stage 00; hollow ones fall more than 5 % short of it.

SNR by stage
Stage	Minutes	SNR	√t expectation
00 First pipeline (8 Sep)	71	11.1	11.1
01 7/8 Sep, current pipeline	78	16.5	11.6
02 25/26 Sep, first 529 frames	88	10.4	12.3
03 Two nights combined	166	19.0	16.9
04 16/17 Oct alone	62	13.9	10.3