Astrophotography diagnosis of Arcturus: no defect detected
Raw20s18 juin 2026
The Doc examined this image of Arcturus (raw, 20s). Estimated overall technical quality: 6/10. No significant defect was found on this acquisition.
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Info
- Cible
- Arcturus
- Date
- 18 juin 2026, 22:41
- Lune
- Gibbeuse décroissante 80.3% (0.1° d'alt., 68° de la cible)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 14h15m17s · +18°51'21"
Excellent sky: Bortle 2 (VIIRS) yields a very dark background, confirmed by the clean, gradient-free image. The 80% gibbous Moon is right on the horizon (0.1deg) and 68deg from the target, so its influence is negligible here, especially as Arcturus is a bright target insensitive to sky background. No condition constraint on this acquisition; the limiting factor is purely hardware (tilt), not the sky.
Setup
- Type d'image
- Brut
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI585MM Pro
- Exposition
- 20s
- Phase de lune
- Premier croissant (15 %)
- FOV
- 1.70°
Sampling of 1.595"/px at 375 mm focal length with the ASI585's 2.9um pixels is very comfortable, well matched to typical seeing, neither over- nor under-sampled. The 1.70deg field is wide for a single star, consistent with this being a collimation/tilt test on Arcturus rather than a target. Gain 200 on the ASI585 is reasonable, offset 15 with no dark clipping (0.01%) is fine, sensor temp -17C is healthy. The 20s exposure gives e_margin 2.02, background barely lifted off the black point: normal for a short test sub, to be lengthened for a real faint target.
The diagnosis in detail
The diagnosis is dominated by a single, clearly measured fact: sensor tilt. The aberration panel shows strictly lateral degradation, the left edge presenting stars elongated to elong 1.76-1.86 with a doubled major-axis FWHM (6.5-7 px), while the right edge stays sharp (elong 1.06-1.20, ~3 px) and the top/bottom edges are round. This side-to-side gradient, without radial symmetry, rules out backfocus and coma (which would degrade all four corners symmetrically) and points to mechanical tilt of the sensor plane. The automatic 'indeterminate' pre-verdict stems from the central fit corrupted by saturated Arcturus (elong 20), to be ignored.
The rest of the image is healthy. The large diffuse halo and messy spikes around Arcturus are the expected behavior of a negative-magnitude star saturating the sensor on 20s: scattered light and blooming, not an optical defect or reflection. The Bortle 2 background is clean, with no light-pollution gradient or walking noise (single sub anyway). The 12% radial falloff at the periphery is normal optical vignetting on an uncalibrated sub, to be handled with flats, not a residual defect.
In practice this is a usable test frame for tuning the hardware: the one useful action is to correct the tilt before committing to a run on a faint target.
Priority actions
- Correct sensor tilt with a push-pull tilt plate, equalizing elongation between left and right edges, checked via the aberration inspector until elong < 1.2 everywhere
- Repeat the test on a faint star field (not saturated Arcturus) to judge central star shape
- Once tilt is fixed, shoot flats to cancel the 12% edge vignetting