Astrophotography diagnosis of N?buleuse de la Trompe d'?l?phant Panneau 4: no defect detected
RawIR Cut60s21 sept. 2026
The Doc examined this image of N?buleuse de la Trompe d'?l?phant Panneau 4 (raw, IR Cut, 60s). Estimated overall technical quality: 6/10. No significant defect was found on this acquisition.
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Info
- Cible
- N?buleuse de la Trompe d'?l?phant Panneau 4
- Date
- 21 sept. 2026, 23:56
- Lune
- Croissant décroissant 22.3% (5.7° d'alt., 81.3° de la cible)
- Site
- Bortle 4 · transition rural/banlieue (saisi)
- Position
- 21h30m39s · +57°31'00"
Favorable conditions. The Moon is at 22.3% illumination and only 5.7 degrees above the horizon, 81.3 degrees from the target: its contribution to the sky background is negligible, consistent with the absence of any measurable lunar gradient. The stated Bortle 4 is comfortable for an emission nebula such as IC 1396, even in broadband with a simple IR Cut. In that context the weak background is not a sky problem but an exposure-time one: you have room to expose much longer without saturating the background. Note that the setup mention of a 78% waxing gibbous contradicts the computed context (22.3%); the latter is authoritative.
Setup
- Type d'image
- Brut
- Télescope
- C11+Hyper
- Caméra
- Poseidon-C PRO
- Filtre
- IR Cut
- Monture
- AZ EQ6
- Exposition
- 60s
- Phase de lune
- Gibbeuse croissante (78 %)
- Notes
- Brut pré traité des DOF
- FOV
- 2.48°
Coherent setup for this panel. At 540mm and f/1.9 the 2.48 degree field suits a mosaic of IC 1396, which far exceeds a single frame; this panel 4 mostly catches star field plus the fringe of nebulosity in the lower left, a signal-poor framing but logical within a mosaic. Sampling at 1.436 arcsec/px is comfortable and consistent with the measured FWHM around 4.5 px, about 6.5 arcsec, which reflects average seeing rather than optics. Settings-wise, gain 125 and a sensor at -20C are healthy; the 60s exposure is the weak link, with offset 50 a bit low given the slight dark clipping.
The diagnosis in detail
The PSF inspection validates both the optical train and the mount. Center elongation is 1.04, the field floor for a debayered color sensor, and no zone exceeds 1.15. The corner/center FWHM ratio of 1.016 is near perfect: corners are as sharp as the center, which rules out field curvature and a backfocus error on a Hyperstar, known to be sensitive on that point. FWHM asymmetry stays at 0.05 horizontally and 0.03 vertically, and a PA spread of 54 degrees signals round stars with no preferred direction, so no drift or guiding oscillation over 60s. On the panel the cyan ellipses closely hug circular disks in all nine zones.
The only lever for improvement is therefore signal to noise. The histogram shows a background at 0.01 with a margin of only 2.04 above the black point: you are not sky-limited, read noise still weighs in the budget, and 0.04% of pixels are already clipped in the shadows. Visually, the lower-left nebulosity is a very faint brownish patch, far from what f/1.9 should deliver.
FWHM around 4.5 px corresponds to roughly 6.5 arcsec: average seeing combined with correct but not optimal focus. An autofocus run during the session and HFR monitoring will help you claw back some sharpness on this axis.
Priority actions
- Lengthen subexposures to 120-180s to reach the sky-limited regime (e_margin >= 5)
- Raise the offset to eliminate the 0.04% dark clipping
- Target several hours of total integration per panel to bring out the nebulosity
- Refocus during the session and monitor HFR, FWHM at 6.5 arcsec leaves some margin