Astrophotography diagnosis of M33: Residual vignetting, noise / underexposure and 2 others
Masterl-QEF22×300s30 oct. 2026
The Doc examined this image of M33 (master, l-QEF, 22×300s). Estimated overall technical quality: 5/10. 4 defects found: Residual vignetting (severity 3/5), Noise / underexposure (severity 3/5), Sensor tilt (severity 2/5), Focus miss (severity 2/5).
&w=1920&q=75)
Info
- Cible
- M33
- Date
- 30 oct. 2026, 19:12
- Position
- 1h33m52s · +30°39'39"
A waning gibbous Moon at 76% is the main constraint of this session. On a galaxy like M33, shot in broadband with a simple l-QEF filter, there is no workaround: lunar sky background directly crushes the SNR of the spiral arms and outer HII regions, and no light pollution filter blocks a lunar continuum. This partly explains the weak signal despite 22 subs of 300s. For this target, aim for a window below 30% illumination, or at minimum a Moon below the horizon during the session. Narrowband emission nebulae would have been a far better choice for that night.
Setup
- Type d'image
- Master
- Télescope
- On-Step
- Caméra
- Poseidon_C
- Filtre
- l-QEF
- Exposition
- 22×300s
- Phase de lune
- Waning gibbous (76 %)
- FOV
- 50.7'
The setup is coherent on paper but oversized for M33. At 1600mm the 0.85° field is too tight for the galaxy, which in practice spills to the frame edges with its outer halo cut off. Sampling at 0.485"/px is heavily oversampled: under typical 2 to 3 arcsec seeing you waste SNR per pixel chasing resolution the atmosphere will not deliver. 2x2 binning would bring you back toward 1"/px, far more comfortable. Gain 125 and offset 25: the gain is reasonable, but the offset is clearly too low, the dark tail of the histogram is clipped at nearly 50%. Sensor temperature at -15°C is healthy.
The diagnosis in detail
The common thread of this image is a signal deficit rather than a serious technical fault. The sky background sits right on the black point (e_margin 1.82, dark clipping 49.6%), the combined consequence of an offset too low that truncates the base of the histogram and of only 1h50 of integration on a galaxy shot under a 76% Moon. The result: M33's arms show up, but drowned in grain, and the HII regions stay timid.
On the instrument side, the PSF panel is reassuring about tracking: elongation of 1.08 at center, PA dispersion of 31°, no directional streaking. The stars are round, simply broad (9.2 px at center, about 4.5 arcsec on sky), which points to imperfect focus or mediocre seeing, amplified by the oversampling. The horizontal FWHM asymmetry (0.217 against 0.101 vertical, top-right corner at 11.75 px versus 8.27 px bottom-left) signs a mild tilt on the right side of the field, real but not crippling at this stage.
Finally, the 41% radial illumination falloff is purely instrumental: near-perfect rotational symmetry (anisotropy 0.05), so a missing or mismatched flat, not a sky gradient nor real nebulosity. It is the easiest point to fix and the one that will make later processing far cleaner.
Priority actions
- Reshoot a properly exposed flat set with matching dark-flats, without changing rotation or spacing, and confirm they are applied at integration
- Increase total integration to 6-10 hours on M33, avoiding nights with strong lunar illumination
- Raise the camera offset to remove the nearly 50% dark clipping of the histogram
- Tighten focus with periodic autofocus (every 30-45 min and on every 1-2°C change)
- Consider 2x2 binning to bring sampling toward 1"/px and gain SNR per pixel
