Astrophotography diagnosis of Pacman nebula: Backfocus error, clipped stars and 1 other
ProcessedHOO3x30x600s
The Doc examined this image of Pacman nebula (processed, HOO, 3x30x600s). Estimated overall technical quality: 8/10. 3 defects found: Backfocus error (severity 3/5), Clipped stars (severity 2/5), Over-denoising (severity 2/5).
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Info
- Cible
- Pacman nebula
- Site
- Bortle 5 · banlieue résidentielle (VIIRS)
- Position
- 0h52m28s · +56°38'41"
Conditions were fine for this target despite a 65% waxing gibbous moon and Bortle 5 skies. NGC 281 is a strong emission nebula, and narrowband HOO makes light pollution and moonlight nearly irrelevant: the measured background is clean (black point 0.02, median 0.04) with no detectable gradient, confirming the filter strategy absorbed those conditions. The background-to-black-point margin of 1.29 is low but perfectly normal in narrowband at 600 s, this is not underexposure. On the same night, a broadband target (galaxy, cluster) would have suffered badly: keep narrowband for high moon phases and save RGB for moonless nights.
Setup
- Type d'image
- Traitée
- Télescope
- CDK 12,5"
- Caméra
- Moravian C4 monochrome
- Filtre
- HOO
- Monture
- Paramount MX+
- Exposition
- 3x30x600s
- Phase de lune
- Gibbeuse croissante (65 %)
- Seeing
- 1.5"
- FOV
- 49.5'
High-end and very coherent setup for NGC 281. The 12.5 inch CDK with the Moravian C4 gives 0.83 degrees of field, framing the Pacman with generous margin all around: comfortable composition, no risk of clipping the target. A central FWHM of 2.39 px under the reported 1.5 arcsecond seeing indicates sound sampling, neither problematically over nor under sampled. HOO is the right call on this HII region, rich in Ha with a well defined central OIII, which your image renders clearly. The Paramount MX+ holds perfectly over 600 s, with no drift or oscillation signature. The only link to refine is mechanical: the spacing between corrector and sensor.
The diagnosis in detail
The diagnosis is dominated by a single instrumental issue, cleanly isolated by the measurements. The field center is excellent: FWHM 2.39/2.18 px and elongation 1.10, exactly the field floor for a monochrome camera. That rules out tracking, guiding and focus. By contrast FWHM climbs to 3.8 at the top edge, 4.0 at bottom right and 4.2 at the bottom edge, a corner/center ratio of 1.65. This rise is radial and broadly symmetric, with no oriented comet shape and no clear diagonal asymmetry, which points to field curvature or mis-set backfocus rather than tilt or coma.
The slight vertical asymmetry (0.127 versus 0.07 horizontally) is worth watching: after optimizing the spacing, if top and bottom edges remain unbalanced, a small mechanical tilt component is hiding behind the curvature. Work in that order, backfocus first, flatness second.
On the processing side, the DSS comparison validates every visible structure: the dark globules, the central absorption lane and the diffuse red extension are all real, with no gradient artifact to report. The two reservations are minor: a few bright star cores are clipped to pure white, and the OIII channel looks somewhat smoothed across the central cyan areas.
Priority actions
- Adjust backfocus in 0.3 to 0.5 mm steps and re-validate with the aberration inspector until the corner/center ratio drops below 1.25
- Check optical train flatness if the vertical asymmetry persists after the spacing is optimized
- Shoot a short exposure set to recover bright star cores via HDR combination
- Ease off denoising on the OIII channel and drive it with an SNR mask
