Astrophotography diagnosis of M16: Background chroma noise
Processed60x600s
The Doc examined this image of M16 (processed, 60x600s). Estimated overall technical quality: 8/10. 1 defect found: Background chroma noise (severity 2/5).
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Info
- Cible
- M16
- Position
- 18h18m48s · -13°48'00"
With a first crescent at 35% illumination and an emission target imaged in narrowband (SHO), the lunar conditions have almost no impact: narrowband effectively filters moonlight and the background stays clean. This is exactly the right session choice to exploit a partial moon. The stated 1.5'' seeing is consistent with the measured FWHM around 2.6-3.0 px, a sign of a steady night and well-used sampling. Nothing to fault on the conditions side, the session was run smartly.
Setup
- Type d'image
- Traitée
- Télescope
- Askar 103Apo
- Caméra
- ZWO2600mm
- Exposition
- 60x600s
- Phase de lune
- Premier croissant (35 %)
- Seeing
- 1.5"
- FOV
- 1.00°
The Askar 103Apo paired with the ZWO 2600MM mono yields about a 1.0 degree wide field, which frames M16 comfortably: the nebula and its diffuse envelope fit with margin, and the associated open cluster is included. The sampling puts the measured FWHM around 2.6-3.0 px, a good compromise without oversampling. 60x600s on a cooled mono is a solid base; pushing the OIII and SII exposures a bit further would help reduce the residual chroma noise seen in the background. The setup is perfectly suited to this target.
The diagnosis in detail
The PSF panel confirms tracking and optics of a very high level: eight of nine zones show elongation between 1.01 and 1.04, meaning round stars at the field floor. The automatic 'tracking' pre-verdict relies solely on the center C (elong 1.15, coherent PA), but the center vignettes land on the bright nebulosity of the Pillars, which contaminates the Moffat fit: the cyan ellipse does not enclose an isolated star but a blend of star plus nebular signal. I therefore reject the tracking-drift candidate: the central elongation is a measurement artifact, not real drift. The corner/center ratio of 1.063 and negligible FWHM asymmetry confirm a healthy PSF across the whole field.
The SHO processing is well controlled: the Hubble palette is balanced, the bright core keeps its blue-white gradient without a flat clip, and the dark structures (globules, trunks) are nicely outlined. The only improvable point is slight chroma noise in the dark sky background, especially toward the corners, typical of a blend where the weak channels (OIII/SII) lack a bit of signal.
The background map reveals no instrumental gradient or vignetting: the +50% variation corresponds to the extended nebular structure, not a calibration flaw. Overall an excellent image.
Priority actions
- Apply selective chroma noise reduction on the sky background with a mask protecting the nebular structures
- Increase integration time on OIII and SII to clean up the weak channels
