Astrophotography diagnosis of IC 63: Focus miss
Masterrgb ultra 23150×10s04 sept. 2026
The Doc examined this image of IC 63 (master, rgb ultra 2, 3150×10s). Estimated overall technical quality: 3/10. 1 defect found: Focus miss (severity 3/5).
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Info
- Cible
- IC 63
- Date
- 04 sept. 2026, 21:25
- Lune
- Gibbeuse croissante 56.9% (sous l'horizon)
- Site
- Bortle 3 · rural peu pollué (VIIRS)
- Position
- 0h59m45s · +60°56'26"
Conditions were favorable: Bortle 3 with the Moon below the horizon, so no lunar pollution was possible during this session. For IC 63, a mixed reflection and emission nebula sitting right next to Gamma Cassiopeiae, a dark sky is exactly what is needed, since the difficulty comes mainly from the glare of the neighboring star rather than from sky background. The background map confirms this: no oriented gradient, no radial falloff, the background is flat. With a sky of this quality you can lengthen sub exposures considerably without saturating the background, which is the most direct improvement available here.
Setup
- Type d'image
- Master
- Télescope
- Skywatcher250-Mbx
- Caméra
- Uranus-C
- Filtre
- rgb ultra 2
- Exposition
- 3150×10s
- Phase de lune
- Dernier quartier (45 %)
- FOV
- 57.8'
A powerful setup, but poorly configured for this target. At 1200 mm and 0.498"/px you are heavily oversampled for typical seeing: the measured 8.7 px FWHM corresponds to 4.3 arcseconds, and binning 2 would be healthier. The 0.96 degree field frames IC 63 correctly, but Gamma Cassiopeiae demands careful reflection management. Settings are where it goes wrong: 10 second subs at gain 210 over 3150 frames leave the background glued to the black point (zero margin, 62% dark clipping), so you are read noise limited rather than sky limited. Finally, a sensor at 22°C is not cooled: dark current is needlessly high.
The diagnosis in detail
Three findings converge. The first is purely practical: the submitted image is clipped to white, probably an automatic stretch applied to an already normalized master, so only the outline of the stacked area remains visible. That jagged outline is normal, the signature of generous dithering over thousands of frames, and is not a defect.
The second is optical and instrumental: the PSF is broad and round. The center measures 8.7/7.7 px, an elongation of 1.14 that sits at the monochrome floor, so tracking is fundamentally fine. Left and bottom edges show elongations of 1.25 and 1.26, but with inconsistent position angles between them (4 degrees versus -70 degrees) and on few noisy stars: this forms no usable tilt, backfocus or astigmatism pattern, so I draw no conclusion there.
The third is the exposure regime. At 10 seconds under Bortle 3, the background is not lifted off the black point: you accumulate read noise instead of digging into signal, a waste for a 250 mm mirror under such a dark sky.
Priority actions
- Provide a properly stretched, non clipped preview so background and structures can be analyzed
- Lengthen sub exposures to at least 60 seconds to reach a sky limited regime under Bortle 3
- Refocus and enable periodic autofocus to bring FWHM below 3 arcseconds
- Turn on sensor cooling at -10°C instead of running at 22°C
- Consider 2x2 binning given the oversampling at 0.498"/px
