Astrophotography diagnosis of M16: Sensor tilt, clipped stars and 1 other
ProcessedSII/Ha/OIII3x20x600"
The Doc examined this image of M16 (processed, SII/Ha/OIII, 3x20x600"). Estimated overall technical quality: 8/10. 3 defects found: Sensor tilt (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
&w=1920&q=75)
Info
- Cible
- M16
- Site
- Bortle 4 · transition rural/banlieue (VIIRS)
- Position
- 18h18m50s · -13°52'45"
Favorable conditions for this target. Under Bortle 4 with a 30% Moon, using only SII/Ha/OIII narrowband filters makes residual light pollution and lunar contribution nearly irrelevant to SNR: the background map indeed shows no instrumental gradient, the measured variation follows the nebula. M16 being an emission nebula, this is the ideal scenario. The only real limiter is the target's altitude at mid latitudes, which drives seeing: measured FWHM of 5.8 to 7.7 px in the healthy zones suggests average turbulence rather than an optical fault.
Setup
- Type d'image
- Traitée
- Télescope
- Planewave CDK 12,5"
- Caméra
- Moravian C4 monochrome
- Filtre
- SII/Ha/OIII
- Monture
- Paramount MX+
- Exposition
- 3x20x600"
- Phase de lune
- 30%
- FOV
- 49.2'
Coherent, high-end setup for M16. With a 0.82° field, the nebula and its surroundings sit comfortably in frame while keeping the Pillars at a usable scale, which a shorter instrument would not offer. The CDK 12.5' plus Moravian C4 mono and Paramount MX+ combination explains the round stars, elongation 1.00 to 1.02 across eight of nine zones: tracking is not at fault. On exposure, the background-to-black-point margin of 0.98 is low in absolute terms, but that is normal narrowband behavior at 600 s, no underexposure to fix. No dark clipping, offset correct. The one mechanical item to revisit is squaring the imaging train.
The diagnosis in detail
The verdict is clear: the image is very good and the remaining defects are subtle. The PSF plate shows round stars everywhere, elongation between 1.00 and 1.02 across eight zones, ruling out any tracking, guiding, or astigmatism issue. The only measurable deviation is in size, not shape: the bottom edge reaches 11.5 px FWHM and the bottom-left corner 7.1 px, against 5.8 to 5.9 px on the top corners and right edge. This single-axis degradation with a sound center is the classic signature of residual focal-plane tilt, hence the moderate-severity tilt retained.
The rest is processing. The NGC 6611 star cores are clipped to pure white, consistent with 0.16% saturated pixels and 600 s subs with no short complementary set. In the dark areas of the lower-left quadrant, chroma mottling remains, typical of OIII and SII channels still slightly short on SNR when saturation is pushed.
Comparison with the DSS validates all the structures: the lower wisps and the red extensions in the bottom-right corner are genuinely present in the survey, they are not gradient artifacts. Calibration is sound, with no sign of a mismatched flat or dark.
Priority actions
- Correct imaging train tilt using the tip/tilt screws until top and bottom edge FWHM match
- Add a set of short 30 to 60 s subs per filter to rebuild bright star cores in HDR
- Extend integration on the OIII and SII channels to reduce background chroma mottling
- Apply masked chroma noise reduction on the background before raising saturation
