Astrophotography diagnosis of IC63: Clipped stars and noise / underexposure
ProcessedChroma LoGlow285x90s
The Doc examined this image of IC63 (processed, Chroma LoGlow, 285x90s). Estimated overall technical quality: 8/10. 2 defects found: Clipped stars (severity 2/5), Noise / underexposure (severity 2/5).
&w=1920&q=75)
Info
- Cible
- IC63
- Site
- Bortle 4 · transition rural/banlieue (saisi)
- Position
- 21h39m12s · +67°54'00"
Conditions were favourable: no moon and Bortle 4 make a clean context for a target like IC 63, which mixes Ha emission with blue reflection around Gamma Cas. The dark sky shows in the result, no instrumental gradient or directional light pollution was measured on the background. In that context the LoGlow filter is not essential, it cuts some broadband flux useful to the reflection component, but it remains a sensible safety net. With a sky this clean, the real lever is no longer light pollution but sub length and total integration time.
Setup
- Type d'image
- Traitée
- Télescope
- Askar fra400
- Caméra
- zwo asi533 mc pro
- Filtre
- Chroma LoGlow
- Monture
- am5n
- Exposition
- 285x90s
- Phase de lune
- 0%
- FOV
- 1.20°
The setup suits the target well. The FRA400 gives 1.20 degrees of field width, which places IC 63 and IC 59 comfortably with Gamma Cas inside the frame and no crowding. Measured FWHM around 3.9 to 4.3 px at centre suggests sampling slightly tighter than the night's seeing, but nothing damaging: stars stay small and well defined. The optical plane is exemplary, with a corner to centre FWHM ratio of only 1.10 and no significant asymmetry, so backfocus and sensor orthogonality are correctly set on this train. The one setting worth revisiting is sub length: 90s with a cutting filter under Bortle 4 leaves the background too close to read noise (e_margin 1.24).
The diagnosis in detail
On the instrument side there is nothing to criticise. Moffat measurements give an elongation of 1.02 at centre against a field floor of 1.01, a corner to centre FWHM ratio of 1.10, and horizontal and vertical asymmetries below 0.01. In other words, no tilt, no backfocus error, no tracking drift: the optical train and the AM5N did their job perfectly over 285 subs. The background map confirms that the +74% variation corresponds to the real nebulosity in the field, not to residual vignetting or a gradient.
The two points of attention come from acquisition and stretch. At 90s per sub under a LoGlow filter, Gamma Cas saturates heavily and its core comes out neutral white, losing the blue tint that gives this star its character. At the same time, the margin between sky background and black point is only 1.24, a sign that the background is not truly sky-limited and that read noise still weighs in, which shows as the grain visible in the empty areas at the bottom of the field.
The fix is simple: lengthen the subs and add a short dedicated set for the cores. The rest of the chain is already there.
Priority actions
- Lengthen subs to 180 or 300s to reach a genuinely sky-limited regime and reduce background grain
- Shoot a short-exposure set (5 to 10s) on Gamma Cas and blend the cores in HDR to recover their blue tint
- Raise the black point slightly during the stretch so the faint background tones are not crushed
- Add total integration time to bring out the faint wisps at the bottom of the field
