Astrophotography diagnosis of NGC7008: Noise / underexposure and clipped stars
ProcessedAntlia Quadri bandes
The Doc examined this image of NGC7008 (processed, Antlia Quadri bandes). Estimated overall technical quality: 7/10. 2 defects found: Noise / underexposure (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- NGC7008
- Site
- Bortle 5 · banlieue résidentielle (VIIRS)
28 percent crescent and Bortle 5: decent conditions for this target. NGC 7008 is a high contrast planetary nebula in OIII and Ha, so the quad band filter handles the residual light pollution and the low moon phase well. The measured background stays very dark with no dominant gradient (plane/radial dominance 0.83, azimuthal anisotropy 0.03), confirming that neither the Moon nor city lighting left a strong signature. The stated 2.5 arcsecond seeing matches the measured FWHM: that, more than the optics, is what limits sharpness here.
Setup
- Type d'image
- Traitée
- Télescope
- C11
- Caméra
- ZWO ASI533MC Pro
- Filtre
- Antlia Quadri bandes
- Monture
- EQ6r pro
- Phase de lune
- Premier croissant (28 %)
- Seeing
- 2.5
The C11 at 2800 mm with the ASI533 gives a very tight field, yet NGC 7008 only spans about 10 percent of the width: the target is small even so, which argues for a final crop or drizzle. Sampling is very fine and FWHM of 5 to 7 px shows clear oversampling under 2.5 arcsecond seeing: bin 2 would gain SNR without losing real detail. The quad band filter is a good match for an emission line rich planetary, and explains the blue violet cast, entirely legitimate here. The EQ6r handles this focal length perfectly, as the tracking metrics confirm.
The diagnosis in detail
The PSF panel is reassuring: central elongation of 1.02 against a field floor of 1.01, PA dispersion of 31.5 degrees, corner to center FWHM ratio of 0.982. In other words the corners are no softer than the center, which contradicts the automatic pre verdict of field curvature or backfocus: I reject it, the backfocus-error candidate at 38 percent confidence has no visual support on the panel, all four corners show round disks. No star shape defect is retained.
What really limits the image lies elsewhere. FWHM of 5 to 7 pixels on a long focal length C11 means massive oversampling under 2.5 arcsecond seeing: stars are round but big, and no processing will recover resolution that was never recorded. The background is clipped tight (median 0.04 against a 0.03 black point, margin 1.89): the grain visible in the empty areas and the nebula outer halo lack signal, a sign that integration should be extended.
Aesthetically the nebula is well rendered, inner structures readable and color consistent with the emission lines. Only a few bright star cores are clipped, a minor point easily fixed with short exposures.
Priority actions
- Extend total integration to strengthen the outer halo of NGC 7008 and lower background noise
- Raise the black point during stretch to stop crushing the background (target median 0.08 to 0.10)
- Switch to bin 2 to match sampling to the real seeing and gain SNR
- Add a few short exposures to rebuild the cores of bright stars
- Crop tighter on the nebula at the end of processing, the C11 field stays very empty around it
