Astrophotography diagnosis of M-101: Clipped stars
ProcessedB, G, R, L60x180s05 juin 2026
The Doc examined this image of M-101 (processed, B, G, R, L, 60x180s). Estimated overall technical quality: 8/10. 1 defect found: Clipped stars (severity 2/5).
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Info
- Cible
- M-101
- Date
- 05 juin 2026, 00:00
- Position
- 14h03m12s · +54°21'00"
Not enough information for an opinion from the Doc.
Setup
- Type d'image
- Traitée
- Télescope
- Askar 130PHQ
- Caméra
- ZWO ASI6200MM Pro
- Filtre
- B, G, R, L
- Monture
- iOptron CEM70
- Exposition
- 60x180s
- Notes
- PixInsight
- FOV
- 36.0'
The Askar 130PHQ (1000mm) and ASI6200MM Pro combo is coherent and capable for M101. With a 0.60-degree wide field the galaxy sits nicely in the frame with room for companions (NGC 5474, IFN). Sampling lands around 0.78"/px, slightly oversampled, but the measured FWHM (1.9 to 2.5 px, roughly 1.5 to 2") indicate excellent seeing fully exploited. A monochrome LRGB chain is the reference choice for such a galaxy. The background/black-point margin e_margin=1.27 suggests a fairly low background, normal on a processed image stretched well toward black.
The diagnosis in detail
Star shape is exemplary: the aberration inspector confirms a clean verdict, with center elongation of 1.05 (field floor 1.02), a corner/center FWHM ratio of just 1.04, and negligible horizontal and vertical asymmetries (0.01 and 0.03). The high PA dispersion (57.7 degrees) reflects round stars with no preferred elongation direction, hence no tilt, no backfocus, no coma, and no tracking issue. The perfect fit of the Moffat ellipses on native stars validates these measurements. The abnormal ellipses in zone C correspond to the galactic bulge, not stars, and do not count.
The only improvable point is the clipped core of a few bright stars: at 180s in luminance this is expected and easily fixed with a short HDR layer or star masking during the stretch. Nothing critical. The spiral arm structure, HII regions, and diffuse nebulosity are well rendered, in good agreement with the DSS reference which confirms the authenticity of the faint extensions. No notable gradient or calibration artifact on the sky background. This is an accomplished image needing only a minor adjustment on stellar highlights.
Priority actions
- Add a short-exposure layer (10-30s) to rebuild the saturated cores of bright stars
- Limit highlight clipping in the final stretch to preserve stellar tonal nuances
