Astrophotography diagnosis of ngc7023: Clipped stars and background chroma noise
ProcessedChroma LoGlow135x240s
The Doc examined this image of ngc7023 (processed, Chroma LoGlow, 135x240s). Estimated overall technical quality: 8/10. 2 defects found: Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- ngc7023
- Site
- Bortle 4 · transition rural/banlieue (saisi)
- Position
- 21h01m40s · +68°10'31"
Favorable conditions: 0% moon and a Bortle 4 sky, exactly what a reflection nebula like NGC 7023 needs, since it works in broadband and gains nothing from a narrowband filter. The LoGlow, a gentle light pollution filter, fits this sky level and does not distort the blue reflection tones or the browns of the IFN. With 9h of integration under these conditions the faint outer signal comes through properly, as confirmed by the dark cirrus at the bottom and on the right. Pushing the IFN further is a matter of more time, not better conditions.
Setup
- Type d'image
- Traitée
- Télescope
- Askar fra400
- Caméra
- zwo asi533 mc pro
- Filtre
- Chroma LoGlow
- Monture
- am5n
- Exposition
- 135x240s
- Phase de lune
- 0%
- FOV
- 1.58°
A well matched setup: the FRA400 gives a 1.58 degree field, enough to place the Iris at center with plenty of dark nebulosity around it, which is the right framing for this target. Sampling with the FRA400 and ASI533 pair falls in the comfortable zone for typical seeing, and the measured FWHM between 5.1 and 6.1 px mostly reflects turbulence and stacking rather than an optical defect. The 240s subs are consistent with a guided AM5N under a Bortle 4 sky; the measured background to black point margin of 1.13 reflects the final stretch, not underexposure at capture.
The diagnosis in detail
On the instrumental side there is nothing to fault. The PSF sheet shows round stars everywhere: elongation runs from 1.06 in the bottom right corner to 1.10 at center, a spread below the measurement floor, and FWHM stays in a narrow 5.1 to 6.1 px band from center to corners. The corner to center ratio is therefore close to 1, which rules out sensor tilt, a backfocus error or residual field curvature. The scatter in position angles with no dominant direction confirms there is no tracking drift: the AM5N held its 240s subs without trouble.
The two points to work on are therefore in post-processing. The cores of the brightest stars, including the one lighting the nebula, are clipped to neutral white and have lost their color index. In the darkest background areas, chroma mottling remains in clumps, a sign that chrominance was processed less than luminance.
The rest is at a good level: the fine blue structure of the Iris is preserved, the dark globules are readable, and the background is neither plastic nor posterized. With targeted work on those two points the image moves up a class.
Priority actions
- Reduce sky background chroma noise with a treatment dedicated to chrominance, leaving luminance untouched
- Add a short set of 10 to 30s subs to rebuild bright star cores and restore their color
- Protect highlights during the final stretch with a luminance mask
