Astrophotography diagnosis of m16 eagle: no defect detected
MasterHa900s22 avr. 2024
The Doc examined this image of m16 eagle (master, Ha, 900s). Estimated overall technical quality: 7/10. No significant defect was found on this acquisition.
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Info
- Cible
- m16 eagle
- Date
- 22 avr. 2024, 09:37
- Lune
- Croissant décroissant 2.5% (12.1° d'alt., 80.7° de la cible)
- Site
- Bortle 3 · rural peu pollué (VIIRS)
- Position
- 18h19m01s · -13°52'14"
Excellent conditions. The Bortle 3 site offers a very dark background, ideal for Hα as well as broadband. The Moon at 2.5% illumination, barely 12 degrees above the horizon and over 80 degrees from the target, has no meaningful impact: the lunar factor can be ignored. In Hα narrowband, the measured background sitting close to the black point (e_margin 1.41) is perfectly normal at 900s, as the filter cuts most of the sky flux; this is not a true underexposure. These conditions allow stacking long subs without compromise, which explains the quality of the nebular signal captured on M16.
Setup
- Type d'image
- Master
- Caméra
- ASI Camera (2)
- Filtre
- Ha
- Exposition
- 900s
- Phase de lune
- Gibbeuse croissante (98 %)
- FOV
- 2.10°
Setup consistent with the target. At 479mm focal length, the 2.10 degree field frames M16 well with surrounding nebulosity and margin all around, without cramping the subject. The 0.82 arcsec/px sampling is aggressive (oversampled) for typical seeing: the measured FWHM of about 5 px (~4 arcsec) is limited by turbulence and optics, not by the sensor. A 2x bin during processing would bring sampling closer to optimal and improve SNR with no real loss of detail. The Hα filter is the right choice for this rich HII region and to tolerate even a less favorable site. Nothing to revise on the hardware side.
The diagnosis in detail
This Hα master of the Eagle Nebula is technically sound. The aberration inspector is conclusive: across the nine zones, elongation stays between 1.05 and 1.25, with the center at 1.10 for a field floor of 1.05, a corner/center FWHM ratio of 1.01, and negligible horizontal and vertical asymmetries (0.005 and 0.084). The cyan ellipses faithfully hug round, isolated stars, with no coherent directional pattern (PA dispersion 33 degrees), which rules out tilt, backfocus, coma, astigmatism and any tracking drift. The collimation and sampling of the whole optical train are therefore correct.
On the sky background, the measured variation (+51%) is attributed to the extended nebular structure and not to an instrumental defect (non-optical profile, R2=0.76): there is no residual vignetting, no light-pollution gradient, no flat mismatch to report. The histogram confirms the absence of saturation (0%), so no clipping of the core or the bright stars despite their apparent on-screen brightness, a mere effect of the preview stretch.
The only area for improvement is stellar sharpness: a FWHM of about 4 arcsec reflects average seeing on an oversampled rig at 0.82 arcsec/px. This is not a defect but a resolution limit; a light bin during processing would optimize the rendering. The acquisition needs no correction, only processing will bring out this good signal.