Astrophotography diagnosis of M 16 - Eagle Nebula: no defect detected
RawSVBony SV220 H/O 3nm300s23 mai 2026
The Doc examined this image of M 16 - Eagle Nebula (raw, SVBony SV220 H/O 3nm, 300s). Estimated overall technical quality: 6/10. No significant defect was found on this acquisition.
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Info
- Cible
- M 16 - Eagle Nebula
- Date
- 23 mai 2026, 01:48
- Lune
- Dernier quartier 54.5% (sous l'horizon)
- Site
- Bortle 3 · rural peu pollué (VIIRS)
- Position
- 18h18m52s · -13°53'51"
In Bortle 3 with the Moon below the horizon (54.5%, 127.6 degrees from the target), conditions are excellent for M16. No lunar pollution possible, and a dark class 3 sky gives a naturally low background. With an H/O 3nm filter on an emission nebula, sensitivity to sky background and Moon is minimal anyway: you could image even under a gibbous Moon. Take advantage of this dark sky to accumulate as many subs as possible, the measured gradient stays low (5% amplitude).
Setup
- Type d'image
- Brut
- Télescope
- Askar SQA70 Pro
- Caméra
- ATR585C
- Filtre
- SVBony SV220 H/O 3nm
- Exposition
- 300s
- Phase de lune
- Premier quartier (40 %)
- FOV
- 1.90°
The Askar SQA70 Pro at 336mm f/4.8 gives a 1.90 degree field that frames M16 comfortably with room for the surrounding veil. The 1.78 arcsec/px sampling is ideal for this setup and current seeing, neither over nor undersampled. The ATR585C with the SV220 H/O 3nm is consistent on this emission nebula rich in Halpha and OIII. Setting note: the gain of 1000 is very high, it limits the full well capacity and can clip bright stars; a more moderate gain with slightly longer exposures would better preserve dynamic range. The offset of 650 and cooling to -9.9C are healthy (no dark clipping).
The diagnosis in detail
The PSF panel validates an optically and mechanically sound acquisition. The nine zones show tight FWHM between 3.18 and 3.49px with elongations of 1.07 to 1.11, barely above the OSC camera floor (1.07). The corner/center ratio of 0.988 and near-zero FWHM asymmetries (0.024 H, 0.02 V) rule out tilt, backfocus, coma or astigmatism. The 17.6 degree PA dispersion with a 1.11 center signals no tracking defect: no drift, no oscillation, round stars everywhere. The clean pre-verdict is confirmed.
The real issue is SNR. With a dual-band 3nm filter, a single 300s exposure leaves the background poorly lifted from the black point (e_margin 2.43, Bowley skew 0.3), so nebulosity is still buried in read noise. This is normal and expected on a single sub in narrow narrowband: sigma rejection and summing many subs will radically transform this result. The background map only reveals slight edge darkening (+7% radial, R2 0.74) and a negligible plane gradient (5%), nothing to fix on the acquisition side.
The only setting to reconsider is the gain of 1000, very high for this sensor. It reduces available dynamic range and risks clipping the cores of bright stars in the field; a gain closer to unity gain with slightly longer exposures would offer a better noise/dynamic range tradeoff.
Priority actions
- Stack many subs (20-40+) to raise SNR and fully reveal the Eagle Nebula
- Reconsider the gain of 1000, very high: test unity gain with 480-600s exposures in 3nm
- Keep the dark Bortle 3 sky to accumulate total integration time