Astrophotography diagnosis of star: Focus miss and bad seeing
Processed
The Doc examined this image of star (processed). Estimated overall technical quality: 3/10. 2 defects found: Focus miss (severity 4/5), Bad seeing (severity 3/5).
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Info
- Cible
- star
- Position
- 0h00m00s · +0°00'00"
The 5 % waning crescent Moon plays no role here: in planetary imaging, light pollution and the Moon have no effect, the planet being far brighter than the sky background. The only relevant condition is turbulence, and it looks unfavorable given the rippled limb and smeared belts. Aim for nights of good stability, with the planet high and near the meridian, and avoid strong jet stream periods by checking a seeing forecast before setting up.
Setup
- Type d'image
- Traitée
- Phase de lune
- Waning crescent (5 %)
- FOV
- 0.6'
The computed 0.01° field confirms a very long focal length configuration, consistent with planetary imaging. That is the right choice for Jupiter, but the sampling looks insufficient or poorly exploited: the disk fills a good part of the frame yet stays poorly resolved, a sign that the limit comes from seeing and focus rather than magnification. Without details on the tube, camera or Barlow, I cannot settle the optimal F/D ratio. A fast planetary camera and video capture would bring far more than any optical change.
The diagnosis in detail
The image shows Jupiter with its cloud belts, captured at very long focal length. The diagnosis is dominated by a single resolution problem, but it has two overlapping causes that must be handled separately.
The first is focus: the planetary limb, which should be a sharp transition over one or two pixels, spreads into a diffuse zone all around the disk, including on the best contrasted parts. This blur is isotropic, with no directional elongation, which rules out a tracking or vibration issue. The second is atmospheric turbulence: the equatorial belts are not merely blurred, they are rippled and unevenly smeared from one area to another, which is the signature of seeing rather than plain defocus.
The final rendering also suggests a single exposure or very few frames rather than a video stack. That is the most limiting point: in planetary imaging, resolution is won in processing, by selecting the calm atmospheric moments among thousands of frames. Without that step, no optical adjustment will compensate.
Priority actions
- Switch to high frame rate video capture, several thousand frames per sequence, rather than a single exposure
- Stack the best 10 to 20 % of frames with AutoStakkert then sharpen with wavelets in RegiStax or AstroSurface
- Refocus carefully in live view, using a Galilean moon or the limb as reference
- Image Jupiter as close as possible to the meridian, on a night of good seeing
- Let the tube cool down 45 to 60 minutes before the session
