Astrophotography diagnosis of M63: Noise / underexposure
Processedzwo ircut
The Doc examined this image of M63 (processed, zwo ircut). Estimated overall technical quality: 6/10. 1 defect found: Noise / underexposure (severity 3/5).
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Info
- Cible
- M63
- Site
- Bortle 5 · banlieue résidentielle (VIIRS)
Favorable conditions for this galaxy: a 14% crescent (negligible moon impact) and Bortle 5 remain workable on a broadband target, though a darker sky (Bortle 4 or less) would clearly improve background SNR and recovery of M63's diffuse halo. No lunar gradient or dominant light pollution visible. Use these moonless nights to stack many subs: it's total integration, not conditions, that you're short on here.
Setup
- Type d'image
- Traitée
- Télescope
- C11
- Caméra
- Asi585mc non refroidie
- Filtre
- zwo ircut
- Monture
- Eq6r pro
- Phase de lune
- Premier croissant (14 %)
- Seeing
- 2.5
The C11 (2800mm, f/10) with the ASI585mc (2.9µm pixels) yields about 0.21"/px, heavily oversampled against 2.5" seeing: you gain no real detail and you dilute flux per pixel, which worsens the under-exposure on a faint galaxy. A reducer (f/6.3 or f/7) or 2x binning would raise SNR and bring sampling closer to ~0.4"/px, more consistent. The uncooled camera adds thermal noise: plan temperature-matched darks. The field is narrow but M63 fits; the very off-center framing toward the top right is a choice worth recentering.
The diagnosis in detail
On the instrument side, nothing to fault: the PSF panel is largely inconclusive (TL/TR/R/BR zones empty, fits landing on galactic nebulosity for C and L), but the only cleanly measured star (zone T, elong 1.06, FWHM ~5.3px) is round, and the pre-verdict comes out indeterminate. No tilt, backfocus, coma or tracking-drift signature is detectable, consistent with a properly guided EQ6R-Pro. The background map confirms the measured variation corresponds to the galaxy, not a gradient or vignetting: calibration holds.
The real issue is the exposure/processing pair. The histogram shows 17% dark clipping and a sky-to-black-point margin of 1.05, signs of under-exposure and a black point raised too high. As a result the core and inner arms are pleasing, but M63's outer halo and low-surface-brightness structures are crushed into black.
The priority isn't an optical fix but depth: stack far more, become sky-limited, then stretch the background gently. Given the C11's oversampling, a reducer or 2x binning would help twice over, in SNR and perceived sharpness.
Priority actions
- Greatly increase total integration on M63 (several hours) to bring out the halo and faint arms
- Lengthen sub exposures until the background lifts off the black point (become sky-limited)
- Add a focal reducer (f/6.3-f/7) or bin 2x to gain SNR against the oversampling
- When processing, raise the black point sparingly and rework the stretch before any denoising
