Astrophotography diagnosis of M 67: Clipped stars
ProcessedMULTIBAND8×300s30 mai 2026
The Doc examined this image of M 67 (processed, MULTIBAND, 8×300s). Estimated overall technical quality: 8/10. 1 defect found: Clipped stars (severity 2/5).
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Info
- Cible
- M 67
- Date
- 30 mai 2026, 00:00
- Position
- 7h41m48s · -14°49'12"
The 99% moon is the dominant factor of this session: in wide-field color it would normally be crippling for SNR and gradient. The multiband filter saved the day by rejecting most of the lunar sky background, and processing finished the job: the background map measures 0% gradient and 0% residual vignetting. On a target like M67, a star cluster rather than faint nebulosity, the impact stays limited since the interesting signal is concentrated in the stars. That said, to exploit the faint IFN/background of the field you would benefit greatly from a moonless night.
Setup
- Type d'image
- Traitée
- Télescope
- Rokinon 135mm f/2.0 ED UMC
- Caméra
- ZWO ASI533MC Pro
- Filtre
- MULTIBAND
- Monture
- Sky-Watcher Wave 150i Strainwave Mount
- Exposition
- 8×300s
- Phase de lune
- 99%
- Notes
- Logiciels : Photoshop, PixInsight, BlurXTerminator, NoiseXTerminator, StarXTerminator
- FOV
- 1.50°
The Rokinon 135mm f/2.0 on the ASI533MC gives a 1.50deg field, wide for M67 which spans only about thirty arcminutes: the cluster appears small at center, lost in a vast star field. It is a defensible compositional choice (galactic context) but a longer focal length would frame the target tighter. Sampling is comfortable and stars are tight everywhere (FWHM 2.4 to 3.1 px), proof of good focus and good optics/sensor match. The Wave 150i mount holds tracking perfectly over 300s, no measured directional elongation. Coherent and well-mastered setup.
The diagnosis in detail
This image is technically very healthy. The PSF panel confirms round and homogeneous stars across the nine zones: center elongation 1.10 against a field floor of 1.04, FWHM corner/center ratio of only 1.043, negligible vertical asymmetry (0.01) and high PA dispersion at 38.8deg, the signature of round stars rather than coherent elongation. The automatic tilt pre-verdict is not confirmed: none of the §8c.3 thresholds are met (corner/center ratio well below 1.25, no diagonal asymmetry between opposite corners). The 45% sensor-tilt candidate is therefore rejected; the TR/R/BR edges are slightly wider (3.0-3.1 px) than the left corners but too faintly and too symmetrically to call it tilt.
The sky background is exemplary: despite a full 99% moon, the background map measures 0% gradient and 0% vignetting, low R2 values confirming the absence of usable structure. The multiband filter and processing (DBE, GraXpert or equivalent) did a clean job. The strainwave mount tracking is flawless over 300s.
The only point of attention is cosmetic: a few bright stars have clipped cores (pure white without chromatic gradient), notably the star at top right. The visible spikes are synthetic, added in post-processing, and not an optical defect of a reflector.
Priority actions
- Blend in a few short exposures to recover the cores of the brightest saturated stars
- For a better background and the field IFN, plan a moonless session
- Consider a longer focal length if the goal is to frame M67 tighter
