Astrophotography diagnosis of M16: Backfocus error, clipped stars and 1 other
ProcessedSII Ha OIII3 x 20 x 10 minutes
The Doc examined this image of M16 (processed, SII Ha OIII, 3 x 20 x 10 minutes). Estimated overall technical quality: 8/10. 3 defects found: Backfocus error (severity 2/5), Clipped stars (severity 2/5), Over-denoising (severity 2/5).
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Info
- Cible
- M16
- Position
- 18h18m50s · -13°52'28"
First quarter Moon at 56 percent illumination would hurt in broadband but is largely absorbed here: M16 is an emission nebula shot in SII/Ha/OIII, and narrowband filters reject most of the moonlit sky glow. The measured background is indeed very low (median 0.07 against a 0.02 black point) and no instrumental gradient was detected, the background map attributing the variations to the nebula itself. The background to black point margin of 0.87 is low but perfectly normal for narrowband at 600 s: you are read noise limited, not sky limited, which is the expected regime with a narrow filter. Nothing to change on the planning side.
Setup
- Type d'image
- Traitée
- Télescope
- Planewave CDK 12.5"
- Caméra
- Moravian C4 monochrome
- Filtre
- SII Ha OIII
- Monture
- Paramount MX+
- Exposition
- 3 x 20 x 10 minutes
- Phase de lune
- Premier quartier (56 %)
- FOV
- 49.1'
Very coherent setup for the target. The 0.82 degree field frames M16 with comfortable margin around the complex, the Pillars sit at center and the nebular wing spills out nicely without being cut. A central FWHM of 2.57 px on a CDK 12.5 with the mono C4 points to fine sampling and decent seeing well exploited, the Paramount MX+ holding ten minute subs with no measurable elongation. The one link to tighten is the rear optical train: the 41 percent FWHM growth from center to corners shows the backfocus distance deserves a check, SHO filters included in the budget. The rest of the setup calls for no reservation.
The diagnosis in detail
The strongest aspect of this image is the mount. On 600 s subs, elongation stays between 1.01 and 1.10 across the nine measured zones, with a 42 degree position angle spread that indicates round stars rather than a coherent stretch. There is no drift, no periodic error, no guiding oscillation. Tracking is not the limiting factor.
The limitation lies in the focal plane. Minimum FWHM is at center (2.57 px) and climbs steadily outward to 3.67 px on the left edge, a corner to center ratio of 1.41, symmetrically and with no diagonal asymmetry. That profile signs field curvature, meaning a backfocus to adjust rather than sensor tilt. It is the only genuine instrumental fault here, costing sharpness at the periphery while leaving the image fully usable.
On the processing side the execution is controlled: coherent SHO palette, smooth transitions, no residual gradient since the measurement attributes background variation to the nebula itself. Two restraint points remain: the bright NGC 6611 star cores are clipped to pure white, and the lower left quadrant background has lost all grain, a sign of slightly generous denoising in the shadows.
Priority actions
- Check and correct the rear optical train backfocus distance, SHO filter thickness included, then re-run an aberration inspector test
- Add a short exposure set (30 to 60 s) per filter to rebuild the bright NGC 6611 star cores in HDR
- Reapply denoising with a luminance mask to preserve grain and the faint filaments in the lower left corner
