Astrophotography diagnosis of SH2-185: Light pollution gradient, clipped stars and 1 other
ProcessedANTLIA tri band RGB Ultra 261x300s
The Doc examined this image of SH2-185 (processed, ANTLIA tri band RGB Ultra 2, 61x300s). Estimated overall technical quality: 7/10. 3 defects found: Light pollution gradient (severity 3/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- SH2-185
- Site
- Bortle 7 · périurbain pollué (saisi)
Conditions were broadly favorable on the moon side: 4% waning crescent, zero contribution, so the measured gradient is in no way lunar. The limiting factor is Bortle 7: on a faint reflection-plus-emission target like IC 59 / IC 63, a suburban sky crushes background contrast and produces exactly the 48% linear gradient measured here. A tri-band recovers the Ha well, but the bluish reflection component of IC 59 stays heavily penalized by the light pollution. If you can, shoot this target from a Bortle 4-5 site, or simply double the integration to build background SNR.
Setup
- Type d'image
- Traitée
- Télescope
- skywatcher 80ED avec reducteur 0.85x
- Caméra
- ZWO ASI2600MC AIR
- Filtre
- ANTLIA tri band RGB Ultra 2
- Monture
- MLAStro SAL-33
- Exposition
- 61x300s
- Phase de lune
- Dernier croissant (4 %)
A coherent, well-mastered setup. The 80ED at 0.85x (around 510mm) with the ASI2600 gives a wide field of roughly 2.5 x 1.7 degrees: SH2-185 and its two lobes IC 59 / IC 63 sit comfortably around gamma Cas, thoughtful framing. Sampling at about 1.5 arcsec/px is in the comfortable zone, and the measured FWHM of 3.2 px confirms it: you are correctly sampled, nothing wasted. The SAL-33 mount does a flawless job, elongation of 1.07 at center on 300s subs. One remark: the background-to-black-point margin of 1.81 shows the background sits fairly close to the black point in the final image, so watch for shadow clipping in the stretch.
The diagnosis in detail
The diagnosis is easy to state: technically the acquisition is near flawless, and everything left to fix lives in processing and in the observing site. The PSF panel is exemplary, FWHM between 2.9 and 3.3 px across all nine zones, elongation peaking at 1.08 against a field floor of 1.02, PA dispersion of 16.5 degrees and a corner/center ratio of 1.009. In other words: no tilt, no backfocus error, no residual coma despite the reducer, and faultless tracking over 61 subs of 300s.
The real subject is the sky background. The gradient measured at 48% amplitude oriented at 145 degrees, with the plane dominating the radial term by a factor of 5.1, is a pure signature of lateral light pollution, the 4% moon being out of the question. It leads logically into the second finding: a background only weakly lifted off the black point (e_margin 1.81) that lets faint chromatic mottling show through in the dark areas. Both share the same root, insufficient background SNR for a Bortle 7 sky.
The clipped core of gamma Cas is an accepted consequence of 300s subs on a magnitude 2 star, recoverable only through an HDR composite if the rendering bothers you.
Priority actions
- Run a gradient extraction again (GraXpert or DBE) on the linear image to remove the 48% gradient oriented at 145 degrees
- Extend the dew shield and block ground-level light sources to cut stray light at the source
- Push total integration beyond 10h to build background SNR under Bortle 7 and eliminate the chroma mottling
- Add a set of short exposures (5-15s) to rebuild the core of gamma Cas as an HDR blend
