Astrophotography diagnosis of N?buleuse de la Trompe d?l?phant Panneau 4: Noise / underexposure
MasterIr/Uv cut51×60s22 sept. 2026
The Doc examined this image of N?buleuse de la Trompe d?l?phant Panneau 4 (master, Ir/Uv cut, 51×60s). Estimated overall technical quality: 6/10. 1 defect found: Noise / underexposure (severity 3/5).
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Info
- Cible
- N?buleuse de la Trompe d?l?phant Panneau 4
- Date
- 22 sept. 2026, 00:03
- Lune
- Croissant décroissant 22.2% (4.9° d'alt., 81.3° de la cible)
- Site
- Bortle 4 · transition rural/banlieue (saisi)
- Position
- 21h30m11s · +57°31'14"
Favorable conditions, under-exploited. The lunar context gives a 22.2 % crescent only, 4.9 degrees above the horizon and 81.3 degrees from the target: negligible contribution, no lunar gradient expected, and the background measurement confirms no instrumental or light-pollution gradient is detectable. With the entered Bortle 4, you had a clean night to accumulate signal with confidence. Under those conditions nothing justified staying at 60 s subs: read noise still dominates and you give away signal-to-noise for nothing. A night like this deserves 180 s subs and several cumulative hours.
Setup
- Type d'image
- Master
- Télescope
- C11+Hyper+Uranus
- Caméra
- Poseidon-C PRO
- Filtre
- Ir/Uv cut
- Monture
- AZ EQ6
- Exposition
- 51×60s
- Phase de lune
- Gibbeuse croissante (78 %)
- Notes
- 51' de stack 60s
- FOV
- 2.48°
Setup coherent on paper, calibrated one notch too short on exposure. At 540 mm and 3.76 um, the 1.436"/px sampling sits in the comfortable zone, and the measured 3.7 px FWHM gives roughly 5.3" of effective seeing: decent without being exceptional. The 2.48 degree field amply contains IC 1396A. Gain 125 and offset 50 are healthy, sensor temperature at -20C is spotless. The histogram, however, shows a background only 1.93 above the black point with 62.8 % dark clipping: in broadband IR/UV cut that is not normal, the 60 s sub is too short. On an emission target like this one, a dual-band would bring a lot.
The diagnosis in detail
The instrumental diagnosis is short because there is nothing to fault. The Moffat measurements are among the cleanest you can read: major-axis FWHM between 3.63 and 3.82 px across the nine zones, under 6 % spread from center to corners, corner/center ratio 0.995, horizontal asymmetry 0.003 and vertical asymmetry zero. No tilt, backfocus, coma or astigmatism signature. Central elongation is 1.10 against a field floor of 1.05, with 8.3 degrees of PA dispersion, which stays within measurement noise: the AZ-EQ6 tracks perfectly over 60 s. The background map detects no usable optical profile (R-squared 0.69), and the measured variation corresponds to real nebulosity in the field, not vignetting.
The problem is entirely one of exposure budget. With 51 cumulative minutes of broadband on an Ha region, the master is structurally underexposed: the background median sits 1.93 above the black point, well below the sky-limited threshold, and 62.8 % of pixels are clipped at the bottom. In practice read noise still outweighs sky noise, and every sub wastes signal-to-noise. That is why the Elephant's Trunk reads as a ghostly silhouette and the Ha wisps across the rest of the panel are nearly invisible.
The practical consequence: this master will not survive an aggressive stretch without the grain exploding. More signal is needed before any processing. As for framing, check it against your mosaic plan: if panel 4 is meant to be stitched, the Trunk's corner position is intentional and that point falls away.
Priority actions
- Lengthen subs to 120-180 s to reach the sky-limited regime (background-to-black-point margin >= 5)
- Accumulate 4 to 6 hours on this panel before attempting any processing
- Try a dual-band or Ha filter, the IR/UV cut alone does not do justice to an emission nebula
- Check panel 4 overlap against your mosaic plan
