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Astrophotography diagnosis of M 33 HDR 180s: Noise / underexposure and sensor tilt

MasterL-Pro180s13 déc. 2025

The Doc examined this image of M 33 HDR 180s (master, L-Pro, 180s). Estimated overall technical quality: 4/10. 2 defects found: Noise / underexposure (severity 3/5), Sensor tilt (severity 2/5).

Annotated image
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Info

Cible
M 33 HDR 180s
Date
13 déc. 2025, 22:43
Lune
Gibbeuse croissante 70.3% (sous l'horizon)
Site
Bortle 3 · rural peu pollué (VIIRS)
Position
1h33m52s · +30°39'44"

Conditions were excellent for this low surface-brightness galaxy: Bortle 3 and a 70% gibbous Moon below the horizon, so no lunar pollution and minimal gradient (measured plane 8%, R² 0.03). For a broadband RGB target like M33, this is exactly the kind of night you want. The limiting factor is therefore not the sky but cumulative exposure time: the background is too lightly exposed (e_margin=2) to bring out the disk and IFN. With such a site, you can afford longer and especially many more subs.

- the Doc

Setup

Type d'image
Master
Télescope
TS-Optics 61 EDPH
Caméra
ATR585C
Filtre
L-Pro
Exposition
180s
FOV
2.33°

The TS 61 EDPH with the ATR585C gives a 2.33° field that frames M33 comfortably with margin around it, very good framing. Sampling is fine and tracking holds well (central FWHM ~2.4 px, round stars). The L-Pro filter is consistent for a broadband galaxy under a good sky. The only hardware caveat is a slight field flatness asymmetry on the left side (TL/L FWHM higher than R/BR), to watch via the optical train tightening. With this setup, the real lever is accumulating subs, not the hardware.

- the Doc

The diagnosis in detail

On tracking and optics this master is healthy: the center is round (elong 1.13 against a field floor of 1.08) and PA dispersion stays moderate. The panel however shows that the left zones (TL, L) are noticeably more spread (FWHM 2.76-2.84) than the right ones (R, BR around 1.7-2.0), a horizontal asymmetry of 0.278. This looks more like a left-side field flatness defect than a clear diagonal tilt, hence low severity: to confirm and fix via optical train tightening.

The real blocker is exposure. The histogram shows dark clipping at 97.31% and a background/black-point margin of only 2, a clear signature of under-exposed integration. On M33, a diffuse low surface-brightness galaxy, the disk and IFN stay buried in noise, and the comparison with the DSS confirms a large amount of missing signal in the spiral arms.

The paradox is that conditions were ideal (Bortle 3, Moon below horizon). The lever is therefore neither sky nor hardware but duration: stacking many more subs and checking the offset to avoid clipping shadows will make all the difference.

Priority actions

  1. Accumulate much more total integration to lift M33 out of the noise
  2. Check the offset at capture and the black point at stacking to stop the dark clipping (97%)
  3. Verify parallelism/tightening of the optical train to reduce the left-side FWHM asymmetry
  4. Treat the weak gradient with GraXpert/DBE then a progressive stretch

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