Astrophotography diagnosis of IC 443: Clipped stars and background chroma noise
ProcessedLP60x90s04 janv. 2026
The Doc examined this image of IC 443 (processed, LP, 60x90s). Estimated overall technical quality: 8/10. 2 defects found: Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- IC 443
- Date
- 04 janv. 2026, 00:00
- Position
- 6h16m60s · +22°30'00"
Not enough information for an opinion from the Doc.
Setup
- Type d'image
- Traitée
- Télescope
- Sky-Watcher HAC125DX
- Caméra
- ZWO ASI585MC
- Filtre
- LP
- Monture
- Sky-Watcher Wave 100i
- Exposition
- 60x90s
- FOV
- 1.20°
The HAC125DX + ASI585MC combo frames IC 443 (about 50' across) comfortably in a 1.20 degree field, with decent margin around the remnant. Sampling is very good here, the PSF panel showing FWHM around 1.8-2.2 px, a sign of fine, well sampled stars under the session seeing. The broadband LP filter remains a compromise on a mostly Halpha emission target like IC 443: a dual-band (Halpha/OIII) would clearly boost filament contrast and reduce background chroma noise, especially under light pollution. The Wave 100i mount holds tracking perfectly over 90s.
The diagnosis in detail
On star shape, the Moffat analysis is conclusive: center elong of 1.03 (equal to the field floor), low FWHM asymmetry (H 0.02 / V 0.08), corner/center ratio of 0.99 and PA dispersion of 33 degrees. The PSF panel confirms round, regular stars everywhere, with well fitted cyan ellipses. No tracking, tilt, backfocus, coma or astigmatism defect: the optical train and mount are dialed in. The only slight elong rise in BR (1.12) stays below the significant threshold and is not consistent with a field defect.
The improvable points concern signal and processing, not the hardware acquisition. With 1.5h of integration through a broadband LP filter on a color sensor, background SNR is limited, which shows up as faint chromatic granulation in dark areas (to distinguish from a gradient or walking noise: the pattern is random and colored). Several bright stars, including the orange star below the Jellyfish, have pure white clipped cores, typical of 90s subs without a short layer.
The background map reveals no significant gradient or vignetting (plane amplitude 5%, low R squared, radial symmetry), and the DSS comparison confirms the filamentary structures are the real remnant, not artifacts.
Priority actions
- Increase total integration time (aim for 4-6h) to bring down background chroma noise
- Add short exposures (5-15s) to rebuild the saturated bright star cores
- Consider a dual-band Halpha/OIII filter to better contrast the IC 443 filaments
