Astrophotography diagnosis of Nébuleuse: Focus miss, noise / underexposure and 3 others
ProcessedUV ir cut
The Doc examined this image of Nébuleuse (processed, UV ir cut). Estimated overall technical quality: 5/10. 5 defects found: Focus miss (severity 3/5), Noise / underexposure (severity 3/5), Bad seeing (severity 2/5), Background chroma noise (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- Nébuleuse
- Position
- 19h59m36s · +22°43'16"
First quarter at 59% illumination is a real handicap for broadband acquisition with a plain UV/IR cut: the sky background rises, contrast on M27's faint parts drops and signal to noise collapses on the outer extensions. The reported seeing of 3.8 is frankly average and explains part of the star bloat on a 305 mm whose theoretical resolving power is far better. In practice M27 is a very bright planetary nebula so the Moon does not ruin the session, but reaching the outer handles calls for a moonless night, or a narrowband filter that rejects lunar scatter.
Setup
- Type d'image
- Traitée
- Télescope
- Skywatcher Dobson Flextube 305/1500
- Caméra
- Pegasus astro smarteye
- Filtre
- UV ir cut
- Monture
- Dobson
- Phase de lune
- Premier quartier (59 %)
- Seeing
- 3.8
- FOV
- 24.0'
A 305/1500 gives a 0.40 degree field, very comfortable for M27: the nebula spans about a fifth of the width, well centered, with room for the faint extensions. The aperture is a genuine asset in light collection on this bright target. The real limit is the Dobson mount: with no equatorial tracking or derotator you are stuck with short subs and exposed to field rotation, which caps integration and part of the sharpness. The UV/IR cut suits a planetary in natural colors, but under a 59% Moon a dual-band OIII/Ha would be far more effective on this emission target while keeping the two lines that give M27 its color.
The diagnosis in detail
The diagnosis comes down to one word: resolution. Stars are round, with no directional elongation or corner asymmetry, so nothing to blame on tracking or on the 305 mm optics as such. They are however uniformly wide and soft across the whole field, center included, which points to imperfect focus compounded by the reported 3.8 seeing. The two reinforce each other, and the result is that close pairs the DSS separates easily stay merged here.
The second family of problems is photometric. The nebula is grainy in its teal areas, the sky background shows discreet colored mottling, and the outer handles clearly visible in the DSS reference do not appear at all. The common denominator is insufficient integration time, made worse by the 59% Moon lifting the background with a plain UV/IR cut. A few bright stars also have cores clipped to pure white, a leftover of a slightly heavy stretch.
The good news: framing, color balance and the dumbbell morphology are faithful. It is a sound base, it lacks exposure time and rigorous focus.
Priority actions
- Redo focus with a Bahtinov mask and recheck it during the session
- Stack far more integration on M27, ideally several hours
- Schedule a moonless session, or add a dual-band OIII/Ha filter
- Sort subs by FWHM and discard the most turbulent ones before stacking
- Redo the stretch while protecting the cores of bright stars
