Astrophotography diagnosis of Wizard Nebula: Backfocus error and clipped stars
ProcessedL-extreme87x300s
The Doc examined this image of Wizard Nebula (processed, L-extreme, 87x300s). Estimated overall technical quality: 8/10. 2 defects found: Backfocus error (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- Wizard Nebula
- Site
- Bortle 4 · transition rural/banlieue (saisi)
- Position
- 18h18m48s · -13°48'00"
Bortle 4 with a 50 percent last quarter Moon is a perfectly workable compromise here, and the result proves it. NGC 7380 is an emission nebula, so the L-eXtreme cuts most of the lunar continuum and residual light pollution, which explains the even background obtained despite the Moon. The background map detects no instrumental gradient either, only the extended nebulosity drives the signal variation. In broadband RGB these same conditions would have been far more punishing. To push further, target waning low Moon or new Moon windows to improve contrast on the OIII parts, which remain the weakest link in this kind of session.
Setup
- Type d'image
- Traitée
- Télescope
- Askar fra400
- Caméra
- zwo asi533 mc pro
- Filtre
- L-extreme
- Monture
- am5n
- Exposition
- 87x300s
- Phase de lune
- Last quarter (50 %)
- FOV
- 48.0'
The FRA400 and ASI533MC Pro combination suits this target well. At 0.80 degrees of field, NGC 7380 fills a good share of the frame without feeling cramped, and the small secondary cloud at lower left fits entirely in, which is thoughtful framing. The sampling of this pairing falls in the comfortable zone for typical seeing, consistent with the measured FWHM around 3.1 to 3.5 px at centre. The L-eXtreme is the right choice under Bortle 4 with the Moon up on an emission target. 87 subs of 300 s, over 7 hours, explain the clean background. The one setting left to refine is optical spacing: that is the last tenth of performance available on this train.
The diagnosis in detail
The image is of good standard and the diagnosis concerns finishing touches, not structural problems. On the PSF side the measurement is clear: the centre is the tightest point in the field (FWHM 3.1/2.6 px) and FWHM rises symmetrically in all four corners, with a corner/centre ratio of 1.29 and near-zero horizontal and vertical asymmetries. That signature, minimum at centre with homogeneous radial degradation, is field curvature tied to spacing, not sensor tilt and not corrector coma. The slightly higher elongation recorded at centre is a measurement artefact: the central thumbnails in the panel land on bright nebulosity, which disturbs the Moffat fit, and no real elongation shows on isolated stars elsewhere in the field.
Tracking is excellent: perfectly pinpoint stars throughout, no trace of drift, periodic error or guiding oscillation over more than seven hours of integration. Calibration holds up well, no measurable instrumental gradient, no residual vignetting, and the background profile matches the real extended structure of the region.
Two cosmetic points remain: a few bright star cores clipped to neutral white, and a fairly tight background-to-black-point margin (e_margin 1.19), which is normal in narrowband even at 300 s but leaves little latitude in the shadows during stretching. Fix the backfocus and you move to a technically faultless image.
Priority actions
- Fine-tune the FRA400 optical train spacing in 0.5 mm steps to bring the corner/centre FWHM ratio below 1.15, including one third of the L-eXtreme filter thickness in the calculation
- Process stars separately with StarXTerminator to soften the clipped cores of the bright stars
- Until the mechanical adjustment is done, apply BlurXTerminator in automatic correction to even out the PSF between centre and corners
- Favour Moon-free windows to strengthen the OIII signal, which remains the weakest component of this composition
