Astrophotography diagnosis of NGC7822: Residual vignetting, clipped stars and 1 other
ProcessedANTLIA tri band RGB Ultra 250x300s
The Doc examined this image of NGC7822 (processed, ANTLIA tri band RGB Ultra 2, 50x300s). Estimated overall technical quality: 7/10. 3 defects found: Residual vignetting (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- NGC7822
- Site
- Bortle 7 · périurbain pollué (saisi)
Conditions were clearly unfavourable on paper: Bortle 7 with an 80% moon is the worst cocktail for sky background. The Antlia tri-band filter saves the day since NGC 7822 is an emission nebula dominated by Ha and OIII: the narrow bands reject most of the light pollution and moonlight scatter. It still explains the low background to black point margin (1.54) and the residual chroma noise in the background. On an emission target this was the right call; do not attempt broadband RGB under these conditions. If you can, add time on this target during a moonless night to gain depth on the outer nebulosity, which is where SNR suffers most.
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
- 50x300s
- Phase de lune
- 80%
The 80ED reduced at 0.85x with the ASI2600MC is a coherent pairing for NGC 7822: the APS-C field frames the nebula with room for the surrounding clouds, and the target sits nicely in the central third. Sampling yields very tight stars (FWHM around 1.6 px), a sign of slight undersampling: you are pixel limited rather than seeing limited, so stars are compact but there is little headroom for deconvolution. The tri-band is the right filter for this target under Bortle 7. Watch the reducer: it tightens backfocus tolerance and increases edge illumination falloff, hence the importance of flats reshot without touching the optical train.
The diagnosis in detail
On optics and tracking, nothing to complain about: the PSF plate shows round stars everywhere, elongation from 1.09 to 1.20 against a field floor of 1.09, corner to centre FWHM ratio of 0.94 (corners are even slightly tighter than centre) and a PA dispersion of 47 degrees, the signature of round stars with no preferred direction. No tilt, no field curvature, no measurable tracking defect on 300s subs. The optical train is well set and the mount is doing its job.
The three points to address share one cause: the sky background. The illumination map gives an 18% radial falloff from centre to edges with rotational symmetry, so vignetting the flats did not fully absorb. On top of that the background sits very close to the black point (margin 1.54), a direct consequence of Bortle 7 and the 80% moon: usable background signal is thin, which brings out colour mottling in the dark areas as soon as saturation is pushed.
Finally, a few bright star cores are clipped to pure white. This is minor and typical of a slightly heavy final stretch on 300s subs. Handling stars separately in processing is enough to recover the nuances.
Priority actions
- Reshoot a set of flats without dismounting the optical train, then correct the remaining vignetting residual with first order ABE or GraXpert
- Reduce background chroma noise with an inverted luminance mask rather than a global denoise
- Separate the stars in processing to recover the clipped bright star cores
- Add integration time on a moonless night to gain SNR on the outer nebulosity
