Astrophotography diagnosis of Ngc6888: Backfocus error, tracking drift and 2 others
ProcessedAltair ha/oiii 6nm102*300
The Doc examined this image of Ngc6888 (processed, Altair ha/oiii 6nm, 102*300). Estimated overall technical quality: 8/10. 4 defects found: Backfocus error (severity 2/5), Tracking drift (severity 2/5), Background chroma noise (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- Ngc6888
- Site
- Bortle 5 · banlieue résidentielle (saisi)
- Position
- 20h12m16s · +38°22'17"
Very favorable conditions for this target. With a 4 percent waxing crescent moon, lunar pollution is nil, and the stated Bortle 5 is largely offset by a 6 nm dual-band on an emission nebula: the filter cuts most of the urban continuum, which explains the fairly clean background despite the site. NGC 6888 is exactly the target that benefits most from this context, unlike a galaxy or broadband RGB, which would suffer far more under Bortle 5. The 102 subs of 300 s are well sized for a 6 nm filter, and the low background-to-black-point margin (e_margin 1.4) is normal in narrowband, not a sign of underexposure.
Setup
- Type d'image
- Traitée
- Télescope
- Evolux 82ed
- Caméra
- Asi 533mc pro
- Filtre
- Altair ha/oiii 6nm
- Monture
- Proxisky Ragdoll 17 pro
- Exposition
- 102*300
- Phase de lune
- Premier croissant (4 %)
- FOV
- 1.30°
A coherent, well-matched setup. The Evolux 82ED with the ASI533MC Pro yields a 1.30 degree field, which places NGC 6888 comfortably at center while letting the Cygnus H-alpha clouds breathe around it: the framing works, the target is neither lost nor cramped. The Altair Ha/OIII 6 nm filter is the right call on this emission nebula under Bortle 5, and the HOO rendition confirms it. Measured FWHM around 3.7 px at center is sound for this image scale. One hardware reservation: the bottom-right corner degradation suggests the corrector spacing or the tightness of the imaging train deserves a check, on an APS-C sensor it would hurt more, on this format it stays discreet.
The diagnosis in detail
The overall verdict is good: at 8.5 hours of 6 nm integration under a moonless sky, the image holds up and every defect found is fine tuning, not a reshoot.
On optics and tracking, two signals overlap. FWHM goes from 3.73 px at center to 5.13 px in the bottom-right corner, a ratio of 1.35, while the other three corners stay between 3.6 and 3.9 px: the degradation is real but localized, and the affected stars are oval rather than comet-shaped, which points to slightly off corrector spacing or residual field curvature rather than coma. In parallel, the center itself is slightly elliptical (1.13 against a 1.03 floor) with position angles consistent across zones: that part does not come from field optics, it comes from tracking, likely a small drift or flexure on 300 s subs. Both effects add up in the bottom-right corner, which explains why it is the worst.
On processing, the ring ghost in the bottom-left corner is the classic signature of an interference filter and is fixed by filter-to-sensor spacing or by a local retouch. Background chroma noise and clipped star cores are two finishing points: a firmer chrominance smoothing and better highlight control during the stretch would gain a notch in rendition.
Priority actions
- Check the corrector spacing and the tightness of the imaging train to cure the bottom-right corner degradation (FWHM 5.13 px against 3.73 at center)
- Review PHD2 logs and polar alignment to remove the residual 1.13 center elongation
- Shift the ring ghost by changing filter-to-sensor spacing, or retouch it locally on the current image
- Apply targeted chroma denoising on the sky background, stronger on chrominance than on luminance
- Plan a short-exposure set to HDR-recover the clipped star cores
