Astrophotography diagnosis of M 57: no defect detected
RawIr-Cut20s23 juin 2026
The Doc examined this image of M 57 (raw, Ir-Cut, 20s). Estimated overall technical quality: 5/10. No significant defect was found on this acquisition.
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Info
- Cible
- M 57
- Date
- 23 juin 2026, 20:23
- Lune
- Croissant décroissant 31% (28.1° d'alt., 91.6° de la cible)
- Site
- Bortle 6 · banlieue dense (VIIRS)
- Position
- 18h53m58s · +32°59'23"
Average but usable conditions. Bortle 6 mainly costs background contrast, which makes an already short sub worse; on M57, a compact and bright planetary nebula, it remains workable. The Moon is at 31% illumination, 28° above the horizon and 91.6° from the target: that wide separation and modest phase make its influence marginal, and indeed no lunar gradient is measured on this sub (background map perfectly flat, 0% amplitude). With the stated 2.5" seeing you are consistent with the measured FWHM. Nothing in the conditions explains the lack of signal: exposure length and sensor temperature are what need fixing.
Setup
- Type d'image
- Brut
- Télescope
- C11
- Caméra
- ZWO ASI585MC
- Filtre
- Ir-Cut
- Monture
- EQ6r pro
- Exposition
- 20s
- Phase de lune
- Gibbeuse croissante (63 %)
- Seeing
- 2.5
- Notes
- F=6.3
- FOV
- 21.8'
Sensible setup in principle, but poorly configured. The C11 at 1755mm gives a 0.36° field, plenty for M57 (a small planetary), and the ASI585MC with 2.9µm pixels yields 0.341"/px: heavily oversampled under 2.5" seeing, so you lose SNR without gaining real resolution. A 2x2 bin or a reducer (your notes mention F=6.3) would bring sampling back into a comfortable range. Gain 120 is reasonable and offset 20 causes no dark clipping (0%). The real black spot is the 30.6°C sensor temperature: far too warm, thermal noise dominates, and darks will have to be shot at exactly that temperature.
The diagnosis in detail
On star shape, the measurements are reassuring. FWHM stays between 1.37 and 1.5 px across eight of nine zones, the corner/center ratio is 0.985 (no edge degradation, so no usable tilt, backfocus or coma), and elongation stays between 1.09 and 1.18, at or below the OSC floor. The 1.24 elongation at center and the 1.91 in zone B are isolated, with a 33° PA scatter: these are fitting artifacts on faint or blended stars in the noise, not a tracking signature. Over 20s at this focal length, the EQ6R is doing its job.
The background is also clean at instrument level: the illumination map shows a 0% radial profile and a 0% amplitude plane, so no measurable vignetting or gradient, which is comfortable for an uncalibrated raw sub.
Everything therefore comes down to the signal budget. With 20s at f/10, a sensor at 30.6°C and 0.341"/px sampling, every photon is spread over too many pixels and buried in high read and thermal noise. M57 shows up, but without its faint extensions or outer halo. This is an acquisition problem, fully recoverable with longer exposures, real cooling and binning.
Priority actions
- Increase sub length to 60-120s (or stack several hundred 20s subs) to lift the background off the black point
- Get the sensor temperature down: cooling at 30.6°C is ineffective, aim for -10°C and shoot darks at the same setpoint
- Bin 2x2 at integration to offset the oversampling (0.341"/px under 2.5" seeing)
- Consider the f/6.3 reducer mentioned in your notes for a large gain in flux per pixel