Astrophotography diagnosis of ic 4628: Sensor tilt, clipped stars and 1 other
Processed15x50
The Doc examined this image of ic 4628 (processed, 15x50). Estimated overall technical quality: 7/10. 3 defects found: Sensor tilt (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- ic 4628
The waning crescent Moon at 15% is a negligible factor, its low illumination imposes no meaningful moon glow on this field. No urban light pollution data is provided, but the nicely dark background and the well-separated red nebulosity of IC 4628 suggest a reasonably transparent sky. The main limitation is not the sky but the very short cumulative exposure (about 12 minutes): under a favorable sky, extending the acquisition would help far more than waiting for a different lunar window.
Setup
- Type d'image
- Traitée
- Exposition
- 15x50
- Phase de lune
- Waning crescent (15 %)
Monochrome camera on a very wide field that frames IC 4628 well while also capturing several open clusters: the composition is coherent and the target is not cut off. The homogeneous FWHM around 4.4-4.5 px at center indicates comfortable sampling and good agreement with the seeing. The only hardware point to watch is the optical train orthogonality, responsible for the slight measured tilt. The background/black-point margin e_margin 1.55 reflects a low background exposure, expected for 50s subs only: nothing abnormal on the settings side, it is the number of subs that needs increasing.
The diagnosis in detail
Optically and mechanically this image is healthy: the PSF panel gives round stars at center (elong 1.01, exactly at the field floor) and a stable FWHM, which validates tracking, focus and seeing together. The only measured shape defect is a slight tilt: the 1.105 corner/center FWHM ratio and the purely horizontal asymmetry (0.101 versus 0.004 vertical) betray a slightly tipped sensor plane, visually confirmed by the stretched star in the bottom-right corner while the bottom edge stays tight. This is an orthogonality adjustment, not an optical quality issue.
The remaining points all stem from the exposure/processing pair. With only 15 subs of 50s, the sky background lacks color signal, hence the fine blue and red speckles scattered around, and the bright cluster star cores are saturated for lack of short frames to rebuild them. None of these defects is structural: they resolve by accumulating more exposure time and handling the stellar highlights better. The clear priority remains extending the integration.
Priority actions
- Greatly increase total integration time (aim for several hours) to clean up the background and chroma noise
- Check and correct optical train orthogonality to remove the slight tilt (bottom-right corner)
- Add short exposures and do an HDR blend to recover the cores of the saturated bright stars
- Apply selective chroma noise reduction on the background during processing
