Astrophotography diagnosis of NGC7331: Sensor tilt
Rawsans filtre60s06 sept. 2026
The Doc examined this image of NGC7331 (raw, sans filtre, 60s). Estimated overall technical quality: 6/10. 1 defect found: Sensor tilt (severity 3/5).
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Info
- Cible
- NGC7331
- Date
- 06 sept. 2026, 20:39
- Lune
- Gibbeuse croissante 77.9% (sous l'horizon)
- Site
- Bortle 6 · banlieue dense (VIIRS)
- Position
- 18h39m04s · +39°02'21"
Conditions are fairly favorable for a galaxy: the Moon is below the horizon and 115 degrees from the target, so no lunar contribution to worry about, which matters since NGC 7331 is a broadband target that would have tolerated neither a gibbous Moon nor close separation. What remains is Bortle 6, which directly penalizes a low surface brightness target: the sky background rises quickly and per-sub SNR stays limited, confirmed by the 2.79 histogram margin. With a reported 2.5 arcsec seeing and 0.44 arcsec/px sampling, you are heavily seeing-dominated: sharpness will come from more integration and strict sub selection, not from more resolution.
Setup
- Type d'image
- Brut
- Télescope
- C11
- Caméra
- ZWO ASI533MC Pro
- Filtre
- sans filtre
- Monture
- EQ6r pro
- Exposition
- 60s
- Phase de lune
- Dernier croissant (25 %)
- Seeing
- 2.5
- FOV
- 22.0'
C11 at f/6.3 on an EQ6R with the ASI533MC: coherent framing, the 0.37 degree field fits NGC 7331 and the Deer Lick group without cutting them, even though the galaxy is pushed toward the bottom-right corner. However, 0.44 arcsec/px is heavily oversampled for 2.5 arcsec seeing, you pay in SNR what you do not gain in detail: 2x2 binning during processing would be more efficient. On settings, gain 100 (unity) is healthy and nothing is saturated, but offset 20 leaves 0.19% dark clipping and should be raised. The sensor at 21.3C is a real weak point: without active cooling you collect needless dark current, enable regulation at -10C.
The diagnosis in detail
The PSF analysis gives a clear verdict: tracking is not to blame. The field center is round (elong 1.09 against a measured OSC floor of 1.03), position-angle dispersion between zones reaches 31.8 degrees, so there is no preferred elongation direction, which rules out drift, periodic error or guiding oscillation on this 60s sub. This is field optics.
The signature is asymmetric and not radial: the top-left corner is twice as bloated as the bottom-left (FWHM 8.0 versus 4.0 px), with elongation 1.25 versus 1.10, while the top-right stays nearly round. A backfocus error would degrade all four corners symmetrically, which is not the case here. The 1.35 corner/center ratio and the 0.27 vertical asymmetry confirm a tilted focal plane, very typical of a long, heavy imaging train at the focus of a C11.
The second point, less severe but structural, is weak signal: at 60s under Bortle 6 the background never reaches the sky-noise-limited regime and NGC 7331's arms stay buried in grain. That is not a flaw of the image itself, it is an acquisition choice to revisit. Fix the tilt first, then lengthen the subs, and overall quality will rise markedly.
Priority actions
- Fix the imaging train tilt, first by re-tightening, then by adjusting a tilt plate until the corner/center FWHM ratio drops below 1.15
- Enable cooling on the ASI533MC at -10C, the sensor is running at 21.3C
- Increase sub length to 120-180s to lift the background off the black point (target e_margin above 5)
- Raise the offset to 30-40 to remove the 0.19% dark clipping
- Accumulate several hours on NGC 7331 and consider 2x2 binning in processing, 0.44 arcsec/px is heavily oversampled
