Astrophotography diagnosis of Orion Nebula: Oversaturation, focus miss and 1 other
Processed220×11s15 janv. 2024
The Doc examined this image of Orion Nebula (processed, 220×11s). Estimated overall technical quality: 6/10. 3 defects found: Oversaturation (severity 3/5), Focus miss (severity 2/5), Clipped stars (severity 2/5).
&w=1920&q=75)
Info
- Cible
- Orion Nebula
- Date
- 15 janv. 2024, 00:00
- Position
- 5h35m17s · -5°23'24"
Moon at 21%: low influence, lunar illumination is not a limiting factor here, especially on an emission nebula as bright as M42. The context is favorable; final quality depends more on the winter seeing (which bloats stars a bit) and on exposure strategy than on sky background conditions. No problematic light-pollution gradient is visible, which the agreement with the DSS confirms.
Setup
- Type d'image
- Traitée
- Télescope
- Celestron C6
- Caméra
- ZWO ASI585MC
- Monture
- Celestron NexStar 6SE
- Exposition
- 220×11s
- Phase de lune
- 21%
- Notes
- Logiciels : Siril, GraXpert, ASICap
- FOV
- 1.00°
The C6 (1500mm) with the ASI585MC over a 1.00° field frames M42 tightly but coherently: the core and main wings fit in the frame. Sampling is very fine (2.9µm pixels at 1500mm, sub-arcsecond per pixel), so oversampled: under average seeing you won't exploit that resolution, which partly explains the high FWHM. The NexStar 6SE alt-az tracks well over 11s, a prudent and suitable choice. For the core, you must plan dedicated short exposures.
The diagnosis in detail
The image renders M42 faithfully: the gas wisps, reddish emission zones and tonal nuances are present, and the DSS comparison confirms these structures are real, with no instrumental gradient to correct. On the PSF side, the C center is very sharp (1.8 px) but the edge zones show clearly wider FWHM (5 to 9 px) with moderate elongations and scattered PA: there is no clear signature of tilt, symmetric radial coma or coherent tracking drift. The general softness points instead to slightly short focus, aggravated by severe oversampling and average seeing.
The real weak spot is core processing: the Trapezium is drowned in a pure white plateau, with no gradient or color, while the DSS keeps structure there. This is the classic sign of a clipped core for lack of HDR blending. The brightest stars also suffer, their cores having lost their hue.
The priority is therefore an HDR strategy (short exposures for the core) and a focus refinement. The rest, structure and tracking, is solid for this alt-az setup.
Priority actions
- Capture very short dedicated exposures (1-2s) for the Trapezium core and blend via HDR to recover detail and color
- Refine focus (Bahtinov or autofocus) and refresh it mid-session to tighten the stars
- Protect highlights during the stretch with a luminance mask to avoid clipping the center
- Restore bright-star color via a star mask in post-processing
