Astrophotography diagnosis of M 16: no defect detected
RawH300s11 août 2025
The Doc examined this image of M 16 (raw, H, 300s). Estimated overall technical quality: 7/10. No significant defect was found on this acquisition.
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Info
- Cible
- M 16
- Date
- 11 août 2025, 00:02
- Lune
- Croissant croissant 3.4% (35.1° d'alt., 65.5° de la cible)
- Site
- Bortle 3 · rural peu pollué (VIIRS)
- Position
- 18h18m48s · -13°48'24"
Excellent conditions for this target: Bortle 3 sky and a 3.4% crescent moon (negligible) despite being 35° high and 65° away. In Hα the moon has almost no impact, and Bortle 3 guarantees a very clean background with excellent potential SNR on an emission target like M16. No reservations about the conditions, they are nearly ideal for stacking a long Hα series without parasitic gradients.
Setup
- Type d'image
- Brut
- Télescope
- SkyWatcher 150 PDS Quattro
- Caméra
- ZWO ASI585MM Pro
- Filtre
- H
- Exposition
- 300s
- FOV
- 1.23°
Coherent and well sized setup for M16. The Quattro 150 PDS (600mm) with the ASI585MM Pro yields a 1.23° field that frames the Eagle Nebula and its cluster nicely, with room for the surrounding structure. The sampling places the measured FWHM around 3 px, meaning healthy tracking and optics without troublesome oversampling. Gain 252 is high but consistent with the ASI585 for Hα (favoring low read noise), offset 15 produces no dark clipping (0%) and the -9.9°C temperature is healthy. The 300s Hα exposure gives a modest background (e_margin 2.02) but that is normal for narrowband, the nebular signal takes priority.
The diagnosis in detail
This Hα sub of M16 shows no defect to report. The aberration panel confirms nominal behavior: center elongation of 1.06 against a field floor of 1.04, FWHM ranging between 2.96 and 3.44 px across zones, corner/center ratio of 1.157 and low asymmetries (0.05). The automatic 'clean' pre-verdict is visually confirmed, the cyan Moffat ellipses fit round stars well from center to corners, with no signature of tilt, backfocus, coma or astigmatism. The 30° PA dispersion combined with sub-floor elongation rules out any tracking or guiding issue.
The background map confirms that the +30% variation follows the real nebulosity of the Eagle (R²=0.22, non-optical profile), so there is no vignetting or instrumental gradient to correct. Comparison with the DSS validates the nebular structure and pillars.
The only thing to keep in mind for acquisition is not a defect but a setting: the e_margin of 2.02 indicates a background poorly lifted off the black point, which is expected for Hα at 300s under Bortle 3. It is the total integration time (number of subs) that will build the final SNR, not lengthening the single exposure.
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