Astrophotography diagnosis of Ngc6888: Sensor tilt and clipped stars
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. 2 defects found: Sensor tilt (severity 3/5), Clipped stars (severity 2/5).
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Info
- Cible
- Ngc6888
- Site
- Bortle 5 · banlieue résidentielle (saisi)
- Position
- 20h15m24s · +38°21'00"
Very favorable conditions for this target. With a 4% waxing crescent, the Moon is a non-factor: no lunar contribution to the background, and the measured gradient (9% amplitude, oriented 298 deg) stays weak and more radial than directional. The declared Bortle 5 is largely offset by the 6 nm Ha/OIII filter, which cuts most of the urban light pollution: NGC 6888 is an emission target, exactly the ideal use case for this filter. The 102 subs of 300 s, 8.5 h in total, give enough signal to noise to bring out the surrounding Ha field and the blue OIII shell, which is the hardest part. Nothing to criticize on the sky side.
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
- 1.20°
Setup perfectly matched to the target. The Evolux 82ED gives a 1.20 deg field, leaving NGC 6888 (about 18 arcmin) comfortably centered with room for the surrounding Ha field, very well showcased here. The ASI533MC Pro with its 3.76 um pixels puts sampling in the comfortable zone for typical seeing, consistent with the measured 3.0 px central FWHM. The 6 nm Ha/OIII is the right choice on an emission nebula under Bortle 5. On the histogram side, the background to black point margin of 1.28 and Bowley skew of 0.61 indicate a background barely lifted off the floor, but at 6 nm narrowband and 300 s that is normal and not an exposure fault. No dark clipping, so the offset is correct.
The diagnosis in detail
The dominant issue is instrumental rather than processing related. The aberration inspector shows an excellent center (FWHM 3.01/2.93 px, elong 1.03, right at the field floor) but a corner/center ratio of 1.478, which is substantial: corners lose nearly 50% of their sharpness. The signature is single-axis (vertical asymmetry 0.072 versus 0.005 horizontally) with a top-right corner clearly more elliptical (1.27) than the adjacent top edge (1.06). That profile does not match symmetric field curvature, which would degrade all four corners equally, but does match a tilted focal plane.
The two other points are minor and relate to processing or secondary optics. A few bright stars have clipped cores, an expected consequence of a strong stretch on 300 s subs, recoverable through separate star processing. The circular ring in the lower-left quadrant, sharp and with no central star, is most likely a filter reflection: visually anecdotal, but worth identifying so it does not return on future sessions.
Everything else is clean: the background shows no walking noise or banding, the Ha nebulosity matches the real structure of this Cygnus field, and the bicolor palette is well controlled.
Priority actions
- Fix the imaging train tilt: even cross-pattern tightening, removal of compression-ring adapters, then a comparative FWHM measurement of the four corners
- Add a short-exposure set to rebuild the clipped cores of bright stars
- Identify the surface causing the circular reflection in the lower left by comparing two sessions with different field rotations
