Astrophotography diagnosis of NGC7331: Noise / underexposure
MasterLuminance83×420s17 août 2025
The Doc examined this image of NGC7331 (master, Luminance, 83×420s). Estimated overall technical quality: 8/10. 1 defect found: Noise / underexposure (severity 2/5).
&w=1920&q=75)
Info
- Cible
- NGC7331
- Date
- 17 août 2025, 21:51
- Lune
- Gibbeuse croissante 69.3% (sous l'horizon)
- Site
- Bortle 4 · transition rural/banlieue (VIIRS)
- Position
- 22h36m49s · +34°13'14"
Favorable conditions. The Moon was below the horizon during the session and sits 80 degrees from NGC 7331 anyway, so no lunar contribution is expected in the background. A Bortle 4 sky is comfortable for a galaxy in luminance: the background stays low and the light-pollution gradient is weak, which the absence of any measured background candidate confirms. On this kind of target, seeing and total integration are the limiting factors, not the sky. With FWHM around 2.4 px, roughly 4.2 arcseconds at your sampling, turbulence was middling that night: targeting steadier windows would gain detail in the bulge and arms.
Setup
- Type d'image
- Master
- Télescope
- Vinzoographe
- Caméra
- Moravian USB Camera 1 (64 bit) (ASCOM)
- Filtre
- Luminance
- Exposition
- 83×420s
- Phase de lune
- Dernier croissant (35 %)
- FOV
- 59.7'
The setup fits the target well. At 876mm and f/4.5 on a 2056x2062 sensor you get a 1 degree field, which puts NGC 7331 and Stephan's Quintet in the same frame, a classic framing pulled off nicely here. Sampling at 1.742 arcseconds per pixel is a good compromise, slightly undersampled relative to the measured FWHM, so the sensor is not your limit. Luminance is the right choice for catching faint halos. On settings, the sensor temperature at -20C is healthy and suppresses dark current. The 420s subs are reasonable, but the histogram shows 0.49% dark clipping: the offset deserves a notch up so nothing is lost in the shadows.
The diagnosis in detail
This master is technically sound, and the measurements confirm it rather than impressions. The PSF panel shows nine consistent zones: FWHM from 2.34 px at center to 2.68 px in the bottom-right corner, a corner/center ratio of 1.138, well below the alert threshold. The horizontal and vertical asymmetries (0.014 and 0.008) rule out sensor tilt, and the symmetry of the four corners with no marked radial broadening rules out a backfocus error. Central elongation, 1.09, is barely above the field floor of 1.02 and below the monochrome floor: no tracking, PE or guiding-oscillation component is detectable over 420s, an excellent result for subs that long.
The background is clean too: the measured profile does not follow an optical law (R2=0.24) and the +30% variation overlays the galactic field itself, so no residual vignetting, no gradient, no amp glow. No banding, no walking noise, no visible dust donuts, which speaks well of your flats and dithering.
The only real remaining lever is depth. With an e_margin of 4.19 and half a percent of pixels clipped on the dark side, you are not quite sky-limited and you are shaving the bottom of the dynamic range. Fixing the offset and extending integration will bring out the outer halo of NGC 7331 and the tidal bridges of the Quintet, which currently sit at the noise limit.
Priority actions
- Raise the camera offset to eliminate the 0.49% of dark-clipped pixels
- Keep integrating in luminance to pass the sky-limited threshold and pull out the faint halos
- Favor better-seeing nights to get below 2.3 px FWHM on the bulge
