Astrophotography diagnosis of M16: Sensor tilt and clipped stars
ProcessedSII/Ha/OIII3x20x600"
The Doc examined this image of M16 (processed, SII/Ha/OIII, 3x20x600"). Estimated overall technical quality: 8/10. 2 defects found: Sensor tilt (severity 3/5), Clipped stars (severity 2/5).
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Info
- Cible
- M16
- Site
- Bortle 4 · transition rural/banlieue (VIIRS)
- Position
- 18h18m48s · -13°52'31"
Comfortable conditions for this target: in Bortle 4 with a 30 percent moon, an emission nebula shot in SII/Ha/OIII is barely sensitive to sky background, narrowband filters cutting most light pollution and moonlight scatter. It shows in the result, the background stays neutral and the background map detects no instrumental gradient. With 3x20x600 seconds, roughly ten hours, signal to noise is more than sufficient on the faint extensions. You can keep using this kind of window for narrowband, and save moonless nights for broadband targets.
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
- 48.3'
Coherent and very high end setup for M16. The 0.80 degree wide field frames the whole nebula with margin on the extensions, without losing the pillars, which stay well sampled. Center FWHM around 5.7 px is fairly high in pixels, suggesting fine sampling relative to site seeing: on a 12.5 inch CDK, bin 2 in Ha could gain signal to noise with no real detail loss. The SII/Ha/OIII trio is the right choice here and the Paramount MX+ performs perfectly, elongation staying below 1.05 across the field.
The diagnosis in detail
The diagnosis is dominated by a single instrumental point, but a clear one. The Moffat measurement gives a center at 5.65 px and a maximum elongation of 1.04 across the whole field: tracking and guiding are therefore out of the picture, stars are round everywhere. However FWHM varies threefold between the bottom right corner at 3.68 px and the bottom edge at 11.45 px, with vertical asymmetry four times the horizontal one. That is the signature of a sensor plane tilted on the vertical axis, not field curvature, which would degrade all four corners equally.
The rest of the chain is healthy. The background map finds no usable optical profile, the measured variation corresponds to the nebulosity itself, confirmed by the DSS reference showing the same extensions at the same positions. No residual vignetting, no gradient to correct. Processing is restrained, with no plastic smoothing or dark halos around stars, and the SHO palette is coherent.
The only rendering flaw, a few stars of the NGC 6611 cluster at center have white cores with no chromatic gradient. That is minor on a narrowband image and recoverable in processing.
Priority actions
- Correct sensor tilt on the vertical axis, this is the only significant sharpness gain available
- Recheck coplanarity of the whole imaging train after the fix, then run a control aberration inspector
- Slightly soften stellar highlights in processing to recover some gradient in the cores
- Consider bin 2 on the faint channels if site seeing stays above the instrumental limit
