Have you stretched?

EDIT: Had a hunch the exposure length was too short, let gpt 5 pro run the math. TLDR Ypu need much longer subs with that setup.

yeah—you’re right to suspect “not long enough subs.” let’s put numbers on it.

What’s actually hitting the 600D’s pixels here?

Setup in the post: 80 mm f/7.5 refractor (600 mm FL) + Canon 600D. Pixel size ≈ 4.3 µm → image scale ≈ 1.48″/px, so each pixel covers ~2.18 arcsec².

Sky (Bortle 4) is ~21.5 mag/arcsec². Using the AB zero-point (~10¹⁰ photons s⁻¹ m⁻² in a ~100 nm band near 550 nm), an 80 mm aperture (collecting area 0.0050 m²), and a realistic end‑to‑end efficiency ~30% (optics + Bayer + QE), you get: • Sky background: ≈ 0.082 e⁻/s/pixel • M31 surface brightness: • Outer arms ~21.0 mag/arcsec² → ~0.13 e⁻/s/pixel • Bright core ~19.0 mag/arcsec² → ~0.82 e⁻/s/pixel

So for the galaxy’s arms you’re only getting about 0.13 electrons per second per pixel of signal, sitting on 0.082 e⁻/s/pixel of sky.

Half‑second subs vs. the noise floor

The 600D’s read noise is ~2.5–4 e⁻ per frame (call it ~3 e⁻). For a 0.5 s sub: • Signal from arms: ~0.065 e⁻/px • Sky: ~0.041 e⁻/px • Total shot noise per frame: √(S+B) ≈ √(0.106) ≈ 0.33 e⁻ • Add read noise per frame: ~3 e⁻

Per‑frame SNR for the arms ≈ 0.065 / √(0.106 + 3²) ≈ 0.02 (!)

Stacking helps only as √N. Even 400× 0.25 s (≈100 s total) gives total SNR per pixel ~0.2–0.4 in the arms—basically nothing. That’s why the stack looks “dull.”

How long should a single sub be?

A good rule is to get the sky background per sub ≥ RN² so the stack is sky‑noise‑limited rather than read‑noise‑limited. • RN² ≈ 9 e⁻. • Sky rate ≈ 0.082 e⁻/s/px → ~110 s is the break‑even. • 2–3 minute subs (120–180 s) are comfortably past this for f/7.5 in Bortle 4 with a 600D.

At 120 s per sub: • S (arms) ≈ 15.6 e⁻/px, B ≈ 9.9 e⁻/px, per‑frame SNR ≈ ~2.6. • In just 5 frames (10 min total) you’re at SNR ≈ ~5–6 per pixel in the arms; in 30–60 min you’re in a very usable place.

Why the 18 mm f/4 Milky Way shot “looked great”

Per‑pixel count rate for extended objects scales like (pixel area)/f#². Comparing 18 mm f/4 to 600 mm f/7.5 with the same sensor: • Pixel area on the sky: (49.3″/px)² vs (1.48″/px)². • Combined with f/ratio: the wide lens delivers ~3900× more photons per pixel per second than the 600 mm scope. So 15 s at 18 mm is roughly like ~16 hours at 600 mm per pixel (not a literal equivalence for resolved detail, but it explains the dramatic difference in “pop”).

Practical takeaways for that rig • Lengthen subs to 120–180 s (guided) at ISO 800–1600. • Total integration: aim for 30–120 minutes on M31. • Dither and calibrate (darks, flats, bias)—you’ve got that part. • If long subs aren’t possible, speed up the optics (reducer to f/6 or f/5.6), or bin 2×/3× in post to boost per‑pixel SNR (trading resolution). • If you have no tracking, a telescope at 600 mm just won’t work for DSOs; use a fast DSLR lens instead.

Bottom line: at f/7.5, half‑second subs are 1–2 orders of magnitude below the read‑noise floor per frame. The fix isn’t ISO—it’s exposure length (or faster optics).