Astrophotography diagnosis of M57: Backfocus error and light pollution gradient
Processedzwo ircut400x20s
The Doc examined this image of M57 (processed, zwo ircut, 400x20s). Estimated overall technical quality: 7/10. 2 defects found: Backfocus error (severity 2/5), Light pollution gradient (severity 2/5).
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Info
- Cible
- M57
- Site
- Bortle 5 · banlieue résidentielle (VIIRS)
- Position
- 18h53m35s · +33°01'44"
Bortle 5 with a 48% gibbous moon: average conditions for a broadband IRcut target. The sky background stays workable thanks to M57's high surface brightness, but these conditions explain the residual 6% gradient and limit contrast on faint extensions and on IC 1296. To push faint signal (M57's outer halo, IFN), a moonless night would be far more favorable. On the bright ring core, the impact stays minor.
Setup
- Type d'image
- Traitée
- Télescope
- C11
- Caméra
- Asi585mc
- Filtre
- zwo ircut
- Monture
- Eq6r pro
- Exposition
- 400x20s
- Phase de lune
- 48%
- Seeing
- 2,5
- FOV
- 1.00°
The C11 at 2800mm with the ASI585 gives very fine sampling (~0.21"/px), heavily oversampled for 2.5" seeing: the ~4.7px central FWHM thus stays modest in arcsec and the EQ6-R tracks very well over 20s. The 1deg field frames M57 comfortably (it is small), with room for IC 1296. One caveat, the corner FWHM (ratio 1.30) betrays a backfocus/curvature to fine-tune. The e_margin of 1.99 shows a background barely lifted: slightly longer subs or more frames would help background SNR.
The diagnosis in detail
The diagnosis is overall very positive. On tracking and on-axis optics, nothing to fault: central elongation (1.08) is just above the field floor (1.03), PA scatter is only 13deg, and stars are point-like at center as the PSF panel confirms. The guided EQ6-R does its job on 20s subs.
The only optical flaw is a symmetric FWHM rise toward all four corners (corner/center ratio 1.30), without radial coma tail or diagonal asymmetry: this is neither coma nor tilt, but field curvature / slightly-off backfocus. It is benign on this small, centered target, but correctable by adjusting spacing.
On the sky background, the measured 6% gradient oriented at 20deg is confirmed as an artifact (the DSS shows no nebulosity in the area), consistent with Bortle 5 and a 48% moon. A clean gradient extraction will remove it. Color is accurate (red ring, blue-green center, no cast), background clean with no marked chroma noise.
Priority actions
- Re-run a gradient extraction (GraXpert/DBE) with sample points off the objects to remove the 6% gradient
- Fine-tune backfocus in 0.5-1mm steps until the corner/center FWHM ratio is below 1.15
- Favor a moonless night to capture faint extensions and IC 1296
