Astrophotography diagnosis of ic1805: Clipped stars and background chroma noise
Processednarrowband Ha + Oiii
The Doc examined this image of ic1805 (processed, narrowband Ha + Oiii). Estimated overall technical quality: 8/10. 2 defects found: Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- ic1805
- Position
- 18h18m48s · -13°48'00"
You shot under a full Moon, the worst possible context for broadband but entirely workable on an emission target like IC 1805 with Ha and OIII. The Ha copes very well, you can see it: the red ridges are crisp and contrasty. OIII is the weak link, it absorbs the lunar background and carries most of the chroma noise visible in the corners. No instrumental gradient is measured on this background, so calibration handled it well. If you can, keep Ha for moonlit nights and save OIII for moonless ones, you will gain depth directly in the cyan component.
Setup
- Type d'image
- Traitée
- Télescope
- diy114 F4.5
- Caméra
- sv605mc
- Filtre
- narrowband Ha + Oiii
- Monture
- diy
- Phase de lune
- pleine lune
- FOV
- 1.00°
A DIY 114 Newtonian at f/4.5 on a DIY mount producing stars at 1.03 to 1.09 elongation across all nine field zones is a very clean result, collimation and backfocus are clearly dialed in. The 1.00 degree wide field is a bit tight for IC 1805, which spans several degrees: you are framing the central region around Melotte 15, a defensible and well composed choice, but the nebula extends well beyond. For the whole Heart you would need either a mosaic or a shorter focal length. The SV605MC with Ha plus OIII works, though dual-band on a color sensor costs you throughput compared to filtered mono.
The diagnosis in detail
The shape diagnosis is clear: the aberration inspector finds nothing. Central elongation at 1.07 against a field floor of 1.02, horizontal FWHM asymmetry 0.027 and vertical 0.033, corner to center ratio 1.14, PA dispersion at 47.7 degrees which signals round stars with no preferred direction. No tilt, no usable field curvature, no tracking drift. On a DIY mount that is a genuine achievement.
What remains is entirely about post-processing and the signal budget. The cores of the bright stars in the central cluster have gone pure white, with no gradient toward the center: classic of a stretch without highlight protection on a field where the cluster is very luminous. It is cosmetic, but it flattens the richest area of the image.
The second point is the background sky: along the edges and in the top-left corner, the dark background shows colored mottling in patches. The background map confirms this is not a gradient or residual vignetting, the measured profile follows the object itself. So it is chroma noise, driven by an underexposed OIII under the full Moon. More hours on OIII and a background mask when saturating will fix two thirds of the problem.
Priority actions
- Extend integration on the OIII channel, ideally on moonless nights, to bring down the background chroma mottling
- Protect the highlights during stretching (star mask or softened curve) to preserve a gradient in the Melotte 15 star cores
- Apply chroma saturation behind a luminance mask so background noise is not amplified
- Consider a two or four panel mosaic if you want all of IC 1805 at this focal length
