Astrophotography diagnosis of Ic4604: Clipped stars
RawAltaïr triband rgb600s
The Doc examined this image of Ic4604 (raw, Altaïr triband rgb, 600s). Estimated overall technical quality: 5/10. 1 defect found: Clipped stars (severity 2/5).
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Info
- Cible
- Ic4604
- Position
- 5h35m17s · -5°23'24"
The last-quarter Moon at 58% is a real handicap for this target: IC4604 is a REFLECTION nebula, so a faint, blue, broadband signal that suffers directly from moonlit sky glow, unlike an emission nebula. Your background stays low (sky/black-point margin e_margin 2.61) but the lunar veil limits the contrast you can extract. Ideally shoot this target on a moonless night or late once the Moon has set; otherwise you will need a lot of integration to compensate.
Setup
- Type d'image
- Brut
- Télescope
- Sky watcher evolux 82ed
- Caméra
- Zwo asi 533mc pro
- Filtre
- Altaïr triband rgb
- Monture
- Ragdoll 17 pro
- Exposition
- 600s
- Phase de lune
- Dernier quartier (58 %)
- FOV
- 1.50°
The Evolux 82ED with the ASI533 gives about 2 arcsec/px over a 1.5 degree field, comfortable sampling well matched to typical seeing; the measured FWHM around 4px (about 8 arcsec) stays a bit soft, worth watching on focus but with no defect confirmed on the sheet. Framing places the Rho Oph trio low in the field, which is fine. However the triband filter is poorly suited here: IC4604 emits in the broadband continuum (reflection), not in the Ha/OIII/SII lines. A simple L/UV-IR cut, or no filter under a dark sky, would capture this blue signal far better.
The diagnosis in detail
Instrumentally this sub is healthy. The aberration inspector shows no signature of tilt, backfocus, coma, astigmatism or tracking drift: the center elongation (1.11) is barely above the field floor (1.08), the horizontal/vertical FWHM asymmetry is negligible (0.025/0.009) and the 11 degree PA spread confirms round stars. Tracking over 600s is therefore excellent and the optics well set. The slight softness (FWHM ~4px) most likely comes from average seeing or imperfect focus, with no marked consequence.
The only retained defect is the clipping of the bright Rho Oph stars' cores: their centers are white and flat, and stellar color is lost there. On a single sub this is minor and expected for such bright stars in a long exposure.
The real issue is not a defect but the exposure of the useful signal: the reflection nebulosity is barely detectable on this single frame, hampered by the Moon and by a triband filter unsuited to a broadband object. The rest is decided at acquisition, not in processing.
Priority actions
- Accumulate many subs (several hours) to bring out the faint reflection nebulosity, invisible on a single frame
- Reshoot the target under a moonless sky or after the Moon has set
- Swap the triband for an L/UV-IR cut filter (or no filter under a dark sky) suited to a reflection nebula's continuum
- Refine focus to tighten the FWHM below 4px
- Optionally add a short-exposure set to recover the color of the clipped bright stars
