Astrophotography diagnosis of Sun: Focus miss and oversaturation
Processed
The Doc examined this image of Sun (processed). Estimated overall technical quality: 4/10. 2 defects found: Focus miss (severity 3/5), Oversaturation (severity 2/5).
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Info
- Cible
- Sun
- Position
- 0h00m00s · +0°00'00"
Not enough information for an opinion from the Doc.
Setup
- Type d'image
- Traitée
- Phase de lune
- Waxing crescent (5 %)
- FOV
- 33.0'
With a 0.55° wide field, the Sun (about 0.53°) nearly fills the frame: that is very tight, leaving almost no margin if tracking drifts slightly, and no room for prominences should you move to Ha one day. The current framing is clean, full disk and centered. For white-light solar work this image scale makes sense, but it only delivers with video acquisition: at this scale every single frame is dominated by daytime turbulence. A good full-aperture filter (glass, or fresh well-tensioned film) will also visibly reduce the diffuse halo.
The diagnosis in detail
The core issue is resolution. A solar disk this large in the frame should show photospheric granulation and structured spots with umbra and penumbra. Here the limb is blurred over several pixels, the disk is smooth, and only two or three micro-spots can be guessed. Two causes almost always combine in this case: approximate focus, set on a disk already smeared by turbulence, and daytime seeing itself, which peaks when the ground heats up.
The second axis is color. The uniform orange rendering with a central plateau removes limb darkening, which is the natural dynamic-range cue on a solar disk. This points either to a generous exposure saturating the red channel, or to heavy saturation in processing, often both.
Finally, the extended diffuse halo around the limb is not a stellar PSF artifact but scattered light, typically a marked film filter or atmospheric haze. It lowers global contrast and makes sharpening less effective.
Priority actions
- Switch to video acquisition (SER/AVI) and stack the best frames with AutoStakkert instead of a single exposure
- Redo focus in zoomed live view on the limb or a sunspot, readjusting during the session
- Image early in the morning, over grass or water, to limit daytime turbulence
- Reduce exposure to preserve limb darkening, then partially desaturate in processing
- Check the solar filter's cleanliness and tension, and add a dew shield against stray light
