Astrophotography diagnosis of ngc7009: Clipped stars and posterization
Processed300x1s
The Doc examined this image of ngc7009 (processed, 300x1s). Estimated overall technical quality: 7/10. 2 defects found: Clipped stars (severity 2/5), Posterization (severity 2/5).
&w=1920&q=75)
Info
- Cible
- ngc7009
- Position
- 18h53m35s · +33°01'44"
With a 0% moon there is no lunar contribution to the background, and nothing in the image betrays a gradient: the night was well chosen. That said, on a planetary nebula like NGC 7009 the surface brightness is so high that sky conditions matter little, seeing and sampling drive the result. The 300 by 1 second strategy fits that lucky imaging logic: it freezes turbulence and lets you keep only the best frames. If you want more on the ansae and the faint outer shell, add a second series of much longer subs and blend the two.
Setup
- Type d'image
- Traitée
- Exposition
- 300x1s
- Phase de lune
- 0%
- FOV
- 18.0'
The 0.30 degree wide field indicates a long focal length, the right choice for a planetary under one arcminute across. The image proves it: the inner disc and the ansae are separated, so sampling is adequate. The downside is that the nebula occupies only a tiny fraction of the frame, which visually dilutes the subject. On the settings side, the background to black point margin is zero with 0.13% shadow clipping: with 1 second subs the background inevitably sits in the read noise, but here it is mainly the processing that pinned it to black. Allow a more generous black point at the stretch.
The diagnosis in detail
The acquisition is sound. Stars are pinpoint from centre to corners, with no directional elongation or field deformation, which is expected with 1 second exposures: neither tracking drift nor periodic error has time to show. The nebula itself is well resolved, with the inner ring, the central cavity, a hint of the central star and the two reddish lateral ansae, a sign that the seeing kept during frame selection was good.
The two points to fix belong to post processing, not to capture, and they share one origin: a stretch tuned to hide the noise of a stack of very short subs. By raising the black point you gained a clean background but also erased the faint outer extensions of the nebula and cut the stellar halos dead. By pushing the highlights you drained the colour from the cores of the few bright stars in the field.
The good news is that all of this can be replayed from the master: start again from the linear stack, stretch more gently with a black point that lets the background breathe, and process stars and nebula separately. A tighter crop on NGC 7009 would complete the job of showing off the resolution you achieved.
Priority actions
- Redo the stretch from the linear stack, keeping the sky background above the black point with no shadow clipping
- Separate stars and nebula (StarXTerminator) to process stellar cores without blowing them out
- Crop tighter on the nebula to exploit the resolution achieved
- Add a longer exposure series and blend it with the short stack to recover the ansae and outer halos
