Astrophotography diagnosis of SH2-157, NGC7635, M52: Backfocus error, clipped stars and 1 other
ProcessedANTLIA tri band RGB Ultra 269x300s
The Doc examined this image of SH2-157, NGC7635, M52 (processed, ANTLIA tri band RGB Ultra 2, 69x300s). Estimated overall technical quality: 7/10. 3 defects found: Backfocus error (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- SH2-157, NGC7635, M52
- Site
- Bortle 7 · périurbain pollué (saisi)
- Position
- 23h16m29s · +60°51'53"
New moon at 0%, which is as favourable as it gets: no lunar gradient expected, and the background map indeed shows nothing oriented toward a bright source. Bortle 7 remains the limiting factor, it caps background SNR and partly explains the colour noise visible in the dark areas. The filter choice suits this target: SH2-157 and NGC 7635 emit in Ha and OIII, so the tri-band cuts most of the urban glow. With 5h45 total under this sky you are at the bare minimum for a clean background; doubling integration across several nights would erase the mottling without changing anything else.
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
- 69x300s
- Phase de lune
- Nouvelle lune (0 %)
- FOV
- 3.46°
The reduced 80ED paired with the ASI2600 is very coherent here: 3.46 degrees of field fit the Bubble, SH2-157 and M52 into a single frame with room around them, exactly the composition this target calls for. The central FWHM of 1.8-2.1 px indicates comfortable sampling, neither over nor under sampled for this seeing. The only weak point is the reducer spacing, which leaves measurable field curvature in the corners; that is a spacer problem, not an optical one. On exposure, the background to black point margin of 1.38 is low but normal for tri-band at 300 s, since the filter is precisely what cuts the background: no real underexposure to correct.
The diagnosis in detail
The image is healthy and the diagnosis comes down to a single instrumental defect. Moffat metrics give a sharp centre (FWHM 2.06/1.82 px) and consistently fatter edges, up to 2.70 px at bottom left, a corner to centre ratio of 1.36. This degradation is symmetric with no preferred orientation (PA spread of 38.7 degrees, all elongations below 1.07 outside the centre), which rules out sensor tilt or a tracking fault and points unambiguously to residual field curvature: the spacing behind the 0.85x reducer is not exactly at its nominal value, likely because the Antlia filter thickness was not included in the budget.
The rest is processing. The brightest star cores, particularly the central star of the Bubble, are clipped to neutral white and have lost their tint; the histogram confirms 0.05% saturated pixels. And in the lower left quadrant, where there is almost no nebular signal, the background carries large scale colour mottling that betrays an SNR still on the thin side under this Bortle 7 sky.
No oriented gradient is retained: the background map is dominated by a low confidence plane, and the new moon rules out any lunar contribution. Calibration is clean, no donuts, no banding, no walking noise.
Priority actions
- Adjust the spacing behind the 0.85x reducer accounting for filter thickness, then revalidate corner FWHM on a test frame
- Reduce background colour noise with chrominance-only noise reduction masked away from the nebula
- Add short 10-30 s exposures to rebuild the bright star cores in HDR
- Extend integration beyond 6h under this Bortle 7 sky to durably clean up the background
