Astrophotography diagnosis of M15: Clipped stars
Processed50×10s01 oct. 2023
The Doc examined this image of M15 (processed, 50×10s). Estimated overall technical quality: 7/10. 1 defect found: Clipped stars (severity 2/5).
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Info
- Cible
- M15
- Date
- 01 oct. 2023, 00:00
The 94% Moon is the limiting factor of this session: a raised sky background and reduced contrast are expected, which explains the 3.16 e_margin (background poorly lifted off the black point). On a globular cluster, a bright compact stellar target, the impact stays moderate: core resolution does not suffer, unlike a faint nebula target that would have been drowned. Gradient handling here is successful despite these unfavorable conditions. To push deeper on the surrounding star field, aim for a moonless night, but for M15 these conditions remain usable.
Setup
- Type d'image
- Traitée
- Télescope
- Celestron NexStar Evolution 9.25
- Caméra
- ZWO ASI585MC
- Monture
- Celestron NexStar Evolution
- Exposition
- 50×10s
- Phase de lune
- 94%
- Notes
- Logiciels : Photoshop, PixInsight
The NexStar Evolution 9.25 (2350mm focal length) paired with the ASI585MC gives tight sampling, well suited to resolve the dense core of a globular cluster like M15, which fits comfortably in the frame with room to spare. The 10s subs are a sensible choice on an unguided alt-az NexStar mount: they avoid field rotation and drift, and the PSF plate confirms round stars and clean tracking. FWHM around 3 to 4.8 px stays acceptable for this focal length. The short-sub / high-count tradeoff (50x10s) is consistent with this visual setup repurposed for imaging.
The diagnosis in detail
The PSF plate validates clean optics and tracking. Center elongation is 1.05, exactly at the field floor, and PA dispersion between zones is only 9.6 degrees, ruling out any tracking component (no tracking-drift or PE), tilt or coma. The automatic pre-verdict leaned toward field curvature / backfocus (FWHM rising at the 4 corners), but the corner/center ratio is 0.896 (below 1, so the center is actually the widest, due to the cluster nucleus density inflating the measurement), and the candidate confidence is only 28%. I therefore reject backfocus-error: there is no real symmetric radial degradation, the corners are as sharp as the rest. The only true point of attention is clipping of the brightest star cores and the central cluster nucleus, which lose their hue to pure white. This is minor and common on such high-contrast targets, recoverable via a light HDR or restraint on highlights during the stretch. The background is clean, the measured gradient (15%) well controlled despite the 94% Moon.
Priority actions
- Preserve the hue of bright stars via a few very short subs in an HDR composite or by limiting highlight clipping during the stretch
- Reshoot a moonless session to gain depth on the faint star field around the cluster
