Astrophotography diagnosis of IC1396_Askar50P: Backfocus error
Raw300s13 août 2026
The Doc examined this image of IC1396_Askar50P (raw, 300s). Estimated overall technical quality: 7/10. 1 defect found: Backfocus error (severity 2/5).
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Info
- Cible
- IC1396_Askar50P
- Date
- 13 août 2026, 21:36
- Lune
- Pleine lune 98.2% (sous l'horizon)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 21h41m21s · +58°09'59"
Excellent conditions: Bortle 2 with the Moon below the horizon, so no lunar contribution is possible and the sky background is naturally low. That is the ideal context for IC 1396, an extended emission nebula: the Ha signal comes through without fighting an urban background. The measured background sits very close to the black point (margin 1.69), which is normal in narrowband under a dark sky and is not a defect: with a dark sky and a narrow filter you do not easily reach the sky-limited regime, even at 300s. Use these nights to stack many subs and build SNR rather than lengthening the individual exposure further.
Setup
- Type d'image
- Brut
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI585MM Pro
- Exposition
- 300s
- Phase de lune
- Nouvelle lune (0 %)
- FOV
- 3.36°
Setup well matched to the target: 3.36 deg of field comfortably covers IC 1396 and the Elephant Trunk with margin at the edges, framing is good. The ASI585MM Pro at -13C is properly cooled, with no sign of dark current or amp glow. Gain 150 and offset 15 hold up: no dark clipping (0%) and no saturation (0%), so the offset is sufficient and dynamic range is preserved. Stars around 1.3-1.4 px FWHM indicate fairly tight sampling for this short focal length, which explains the sensitivity to small backfocus errors. The only mechanical setting to revisit is the sensor-to-corrector distance.
The diagnosis in detail
This sub is of very good quality. The PSF panel shows a sharp centre (elong 1.31, but on a 1.4/1.1 px PSF, so at fit-noise level for an undersampled monochrome sensor) and above all a steady rise of FWHM from centre to periphery, from 1.43 px at centre to 1.66 px bottom-right. The 29 deg PA dispersion between zones and the absence of clear diagonal asymmetry rule out sensor tilt; the absence of radially oriented comet shapes rules out coma. What remains is field curvature / backfocus, confirmed by the measured pre-verdict.
The amplitude is modest, about 20% corner/centre difference, which is in the improvable rather than blocking range. After stacking and a light deconvolution pass the effect will be barely visible, but fixing the spacing will gain you sharpness across the whole periphery.
On tracking, no defect: no streaks, no S-shaped curvature, no zigzag, on a 300s exposure that is a good sign for the mount. The measured background shows a slight 18% radial falloff toward the edges, simply handled by flats at calibration time, which is normal behaviour for an uncalibrated sub.
Priority actions
- Adjust backfocus in 0.5 mm steps and re-check on a star field with the Aberration Inspector
- Shoot a set of flats at the end of the session to correct the illumination falloff toward the edges
- Accumulate integration time rather than lengthening the single exposure, the Bortle 2 sky allows it
