Astrophotography diagnosis of IC 1848_Octobre26: Backfocus error
Raw420s02 oct. 2026
The Doc examined this image of IC 1848_Octobre26 (raw, 420s). Estimated overall technical quality: 7/10. 1 defect found: Backfocus error (severity 2/5).
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Info
- Cible
- IC 1848_Octobre26
- Date
- 02 oct. 2026, 19:16
- Lune
- Croissant croissant 41.7% (sous l'horizon)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 2h59m52s · +60°18'51"
Excellent conditions. Bortle 2 is the best case there is, the sky background stays low and uniform, and the Moon, although gibbous in the declared data, is listed as below the horizon at the time of the exposure, so no lunar contribution is possible on this sub. The background measurement confirms it: the gradient plane amounts to only 4 percent amplitude with an R2 of 0.11, meaning no significant oriented gradient. Under these conditions, on an emission target like IC 1848, you can push total integration without worry and even consider OIII or SII, which are far more demanding in transparency and background darkness.
Setup
- Type d'image
- Brut
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI585MM Pro
- Exposition
- 420s
- Phase de lune
- Gibbeuse décroissante (62 %)
- FOV
- 3.36°
A coherent but clearly undersampled setup. At 190 mm focal length with 2.9 micron pixels you sit at 3.15 arcsec/px, and the 1.5 px central FWHM confirms it: stars span one to two pixels, so you are losing resolution the seeing would allow. In exchange, the 3.36 degree field frames IC 1848 with room to spare, which is exactly the intended use for this large emission nebula. On settings, gain 160 and offset 15 produce no dark clipping and no saturated pixels, and the sensor at -20 C is healthy. The 1.75 background-to-black-point margin is low, but that is normal in narrowband at 420 s under a Bortle 2 sky: this is not underexposure, just a very dark background.
The diagnosis in detail
This 420 s sub is technically good. The central PSF is tight (FWHM 1.5/1.35 px, elongation 1.11 against a field floor of 1.06), which means focus is accurate and tracking did not move during seven minutes of exposure: no trace of drift, periodic error or guiding oscillation, and the PA scatter across zones (22 degrees) reflects essentially round stars rather than a coherent elongation.
The only measured defect is edge degradation. FWHM goes from 1.5 px at centre to roughly 2.0 px on average in the corners, symmetrically in all four angles, with oval shapes rather than radial comets. That is the classic signature of field curvature or a backfocus slightly too short or too long. The deviation stays modest and, at 3.15 arcsec/px, the visual impact on the final image will be small, but it is the remaining lever for gaining uniformity across a large sensor.
On the background, the measurement gives an 8 percent radial falloff toward the edges with rotational symmetry (azimuthal anisotropy 0.01, plane/radial dominance 0.13): that is perfectly normal optical vignetting on a pre-flat sub, not a defect. The Soul Nebula's nebulosity matches the DSS reference structure for structure, including the dark globules and the southern edge filaments, so nothing in the background is an artefact.
Priority actions
- Refine backfocus with +/- 0.5 mm test series and compare four-corner FWHM to bring the corner/centre ratio below 1.2
- Shoot flats in the same optical configuration to correct the 8 percent radial falloff measured on this sub
- Capitalise on these Bortle 2 conditions and clean tracking by increasing total integration, or adding OIII and SII layers
