Astrophotography diagnosis of NGC7000: Noise / underexposure, flat frame mismatch and 2 others
ProcessedANTLIA tri band RGB Ultra 211x240s
The Doc examined this image of NGC7000 (processed, ANTLIA tri band RGB Ultra 2, 11x240s). Estimated overall technical quality: 6/10. 4 defects found: Noise / underexposure (severity 3/5), Flat frame mismatch (severity 2/5), Background chroma noise (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- NGC7000
- Site
- Bortle 7 · périurbain pollué (saisi)
Conditions were favorable on the lunar side: a 12% waxing crescent, so lunar contribution is negligible and you can rule out any moon pollution in this background. The real limiting factor is the Bortle 7 sky of the Paris region: the sky background rises fast and eats the SNR on the faint extensions of NGC 7000. Your tri-band filter is exactly the right answer to that sky, cutting most urban lighting while passing Ha and OIII, but it does not make up for lack of time. At this level of light pollution, budget 4 to 6 hours minimum on the target to cleanly bring out the filaments and the Pelican's background.
Setup
- Type d'image
- Traitée
- Télescope
- skywatcher 80ED avec reducteur 0.85x
- Caméra
- ZWO ASI2600MC AIR
- Filtre
- ANTLIA tri band RGB Ultra 2
- Monture
- MLAStro SAL-33
- Exposition
- 11x240s
- Phase de lune
- Premier croissant (12 %)
- Notes
- Tiret bleu ciel sur tous les bruts. En ville en IIe de France...
The combination is coherent. The 80ED with a 0.85x reducer gives a wide field that frames the NGC 7000 / Pelican complex well: the composition fits within the ASI2600MC APS-C sensor, margins are reasonable, nothing awkwardly cut. The resulting sampling is comfortable for a lowland sky and the measured FWHM stays uniform (4.2 to 5.6 px depending on zone, corner/center ratio 1.05), a sign the reducer's backfocus is properly set. The Antlia tri-band is the right choice in town on an emission target. On the acquisition side, 240s is a healthy duration with this filter: the low background/black-point margin comes from the narrow filter, not from underexposure, so do not shorten the subs, add nights instead.
The diagnosis in detail
The optical verdict is reassuring: the aberration inspector reports an elongation of 1.04 at center against a field floor of 1.02, a corner/center FWHM ratio of 1.053 and position angles scattered over 58 degrees, all pointing to round stars everywhere. No trace of tilt, misset backfocus, astigmatism or tracking drift. On an 80ED with a reducer, that is a clean result validating both the optical train and the SAL-33 mount.
What truly limits the image comes down to two related causes. First, the amount of signal: 44 minutes under a Bortle 7 sky on an extended nebula is little. Grain remains visible in the dark areas and colored mottling persists in the background, two symptoms of the same photon shortage. Second, background calibration: the measurement shows a 3 sigma illumination dipole oriented toward the bottom-right, with a poorly modeled residual plane, typical of a flat that no longer quite matches the lights' configuration.
Finally, a few bright stars have cores clipped by the stretch, losing their color. That is the easiest point to fix, purely a processing matter.
Priority actions
- Accumulate 4 to 6 hours of integration on NGC 7000, across several nights, to bring down noise and colored mottling
- Reshoot a set of flats without disassembling the optical train, checking sensor and filter orientation
- Redo the background processing with a gentle gradient extraction (GraXpert or DBE degree 1) after calibration
- Protect highlights during the stretch, or add short exposures, to recover the color of stellar cores
- Enable dithering between subs to break the spatial correlation of noise
