Astrophotography diagnosis of M42: Residual vignetting, oversaturation and 3 others
ProcessedLIGHT POLLUTION FILTER15x60s, 30x30s, 90x10s
The Doc examined this image of M42 (processed, LIGHT POLLUTION FILTER, 15x60s, 30x30s, 90x10s). Estimated overall technical quality: 6/10. 5 defects found: Residual vignetting (severity 3/5), Oversaturation (severity 3/5), Backfocus error (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- M42
- Site
- Bortle 7 · périurbain pollué (saisi)
- Position
- 5h35m32s · -5°22'06"
Average but workable conditions for M42. With a waning gibbous moon at 63% and a Bortle 7 sky, the sky background is high and contrast limited, confirmed by the measured background/black-point margin of 1.84: you are far from a comfortable regime. M42 is the most forgiving target there is, very bright, so the result still holds up. Your light pollution filter helps against urban emission lines but does nothing about the moon. To progress, aim for a window with the moon below the horizon or under 30% phase, and increase integration: 45 cumulative minutes under Bortle 7 with moonlight is what limits your background SNR most.
Setup
- Type d'image
- Traitée
- Télescope
- SEESTAR S50 PRO
- Filtre
- LIGHT POLLUTION FILTER
- Exposition
- 15x60s, 30x30s, 90x10s
- Phase de lune
- Waning gibbous (63 %)
- FOV
- 1.29°
A coherent setup for this target. At 1.29° of field, M42 and the Running Man both fit in frame with margin, the natural S50 framing on Orion, and it is well executed. The multi-exposure strategy (60s, 30s, 10s) is exactly the right idea on a nebula with extreme dynamic range: you are only missing the HDR blending step to exploit it, which is why the core stays burned out. The light pollution filter is relevant under Bortle 7 on an emission target. Since the sensor is factory sealed, the measured field curvature is not on you, a light edge crop is enough.
The diagnosis in detail
The diagnosis splits cleanly into two families. On the instrument side, the PSF measurement is reassuring: the center sits at 3.99 px FWHM with 1.04 elongation, perfectly round stars, so no tracking or guiding issue despite the alt-az mount. The degradation is purely radial, with a 1.288 corner/center ratio and symmetric FWHM rise across all four corners, the signature of field curvature, native behavior for a flat-sensor smartscope. Same logic for the background: plane/radial dominance of 0.09 and anisotropy of 0.05 rule out an oriented gradient and point to clear 48% vignetting left uncorrected in processing.
On the processing side, the main issue is dynamic range management. You acquired three exposure sets, the right method on M42, but the final composite appears dominated by the long subs: the Trapezium is reduced to a white plateau and the bright field stars lose their color. This is entirely recoverable without returning to the field, by redoing the HDR composite from the 10s subs.
Finally, the short integration under a Bortle 7 sky with a 63% moon leaves a noisy background, with chromatic mottling visible in empty areas. That is the only point that genuinely needs more exposure time.
Priority actions
- Correct the vignetting with a division-mode background extraction (GraXpert or DBE) before any other processing
- Redo the HDR composite of the M42 core using the 10s stack to recover the Trapezium
- Increase total integration to 2-3 hours, preferably moonless, to reduce background chroma noise
- Crop about 10% off the edges to remove stars smeared by field curvature
- Soften the highlight stretch to restore color in the bright stars
