Astrophotography diagnosis of Ngc6992: Sensor tilt and clipped stars
RawAltair ha/oiii 6nm300s
The Doc examined this image of Ngc6992 (raw, Altair ha/oiii 6nm, 300s). Estimated overall technical quality: 7/10. 2 defects found: Sensor tilt (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- Ngc6992
- Site
- Bortle 5 · banlieue résidentielle (saisi)
- Position
- 20h56m35s · +31°29'06"
A 72% waxing gibbous Moon and Bortle 5 are unfavourable conditions in broadband, but your 6nm Ha/OIII dual-band is doing exactly the job it was built for on a supernova remnant like NGC 6992, a pure emission target. The measured background stays low (median 0.15, no clipping, no global saturation) and the background map shows a 24% plane gradient oriented at 329 degrees, which matches a diffuse sky contribution that stacking with dithering plus gradient removal will erase cleanly. In short, you can keep shooting under this Moon without changing anything, just avoid frames where the Moon would pass very close to the field.
Setup
- Type d'image
- Brut
- Télescope
- Evolux 82ed
- Caméra
- Asi533mc pro
- Filtre
- Altair ha/oiii 6nm
- Monture
- Proxisky Ragdoll 17 pro
- Exposition
- 300s
- Phase de lune
- Gibbeuse croissante (72 %)
- FOV
- 1.35°
The Evolux 82ED with the ASI533MC Pro gives a 1.35 degree field, which frames NGC 6992 correctly but tightly: the lacework fills the full height and the complete complex (NGC 6960 and Pickering's Triangle) would need a mosaic or a shorter focal length. Sampling yields a 4.1 px central FWHM, a bit generous: a sign of mild oversampling combined with seeing, harmless on a single sub. On settings, the background to black point margin of 2.55 is low, but that is normal behaviour for a 6nm filter at 300s and not an exposure fault: do not raise the gain to chase it. The mount holds 300s with no measurable drift, which validates the mount and focal length pairing.
The diagnosis in detail
The frame is healthy overall and usable as is for stacking. The PSF measurement confirms it: the field centre sits at 4.09/3.68 px with an elongation of 1.11, barely above the 1.06 field floor, so there is no tracking component. That matters: over 300s it means the Proxisky mount holds the exposure perfectly at this focal length.
The only real instrumental signal is a residual tilt of the focal plane. The top and left zones climb to elongations of 1.20 to 1.25 with position angles clustered around -20 degrees, while the right column stays at 1.06 to 1.11, nearly perfect. That single-axis asymmetry with a clean centre rules out backfocus (which would be symmetric across all four corners) and rules out coma (which would be radial). The 1.21 corner to centre ratio stays moderate: this is mild tilt, costing a little sharpness on one edge without wrecking anything.
The remaining points match the nature of the image: bright star cores clipped at 300s, and a slightly tilted sky background that post-processing will handle. Nothing here justifies discarding the frame.
Priority actions
- Retighten and check the imaging train between the corrector and the ASI533 to remove the measured left/right asymmetry
- Shoot a short test frame and rerun the aberration inspector to confirm the corner/centre ratio drops below 1.15
- Add a short 30 to 60s series to recover bright star cores with an HDR blend
- Keep dithering between frames so the 24% background gradient can be removed cleanly
