Astrophotography defect glossary
42 defects commonly encountered in astrophotography. Click a defect for the full page: visual signature, causes, fixes.
42 defects shown · sort: alphabetical
- Anomalous diffraction spikes
Unequal, curved, or abnormal-count spikes.
Minor0 analyses - Amp glow
Bright uniform halo in one corner or along one edge (readout electronics).
Minor0 analyses - Astigmatism
Stars shaped as + or x depending on focus direction, orthogonal elongation on both sides of focus.
Major6 analyses - Backfocus error
Stars elongated symmetrically at all 4 corners (crescent or radial pointing).
Major9 analyses - Banding
Horizontal or vertical lines strictly aligned with the sensor axes.
Minor0 analyses - Noise / underexposure
Heavy grain, faint structures invisible.
Major8 analyses - Background chroma noise
Red/green/purple mottling visible in the sky background.
Minor10 analyses - Dew / fogging
Diffuse veil starting from one edge, intensifying over time.
Major0 analyses - Framing issue
Subject partially cut off or poorly centered, secondary objects clipped.
Minor0 analyses - Chromatic aberration
Purple/blue fringe around bright stars or colored fringe off-axis.
Minor0 analyses - Cirrus / haze
Wide diffuse halos, inhomogeneous background, variable transparency.
Minor0 analyses - Collimation error
Comma-shaped stars ALL pointing in the same direction (field-wide asymmetry).
Major0 analyses - Coma
Comma-shaped stars pointing radially toward the corners.
Major0 analyses - Dark frame mismatch
Bright hot pixels OR black holes, residual thermal pattern.
Major0 analyses - Tracking drift
Stars trailed in a single direction, identical trail length across the entire frame.
Critical13 analyses - Green cast
Green or yellow-green background and stars, no SCNR applied.
Minor4 analyses - Dust donuts
Residual faint dark fuzzy rings (sensor or filter dust).
Minor0 analyses - Periodic error (unguided)
S-shaped or arc-shaped stars, sinusoidal modulation frame to frame.
Major0 analyses - Clipped stars
Star cores 100% white, with no central chromatic gradient.
Minor16 analyses - Fixed pattern noise
Fixed grid or hatching pattern inherent to the sensor.
Minor0 analyses - Flat frame mismatch
Flat applied but taken under different conditions, visible over-correction.
Major1 analysis - Moon gradient
Gradient oriented toward the Moon (suspend broadband imaging when the Moon is above the horizon).
Major1 analysis - Light pollution gradient
Slow large-scale brightness gradient oriented toward the ground or a nearby town.
Major11 analyses - Filter halo
Wide red or pink ring around bright stars (narrowband imaging).
Minor1 analysis - Dark halos around stars
Sharp black ring around bright stars (over-deconvolution).
Minor0 analyses - Residual hot and cold pixels
Isolated 1-2 px bright or dark spots remaining after calibration.
Minor1 analysis - Stacking misalignment
Doubled or multiplied stars (failed registration).
Major0 analyses - Bad seeing
Point-like but bloated stars (FWHM 3-6 px), turbulent atmosphere.
Minor1 analysis - Focus miss
Uniformly bloated stars, no directional elongation.
Critical3 analyses - Guiding oscillation
Kidney-bean or short zigzag star shapes caused by autoguider over-correction.
Major1 analysis - Pinched optics
Triangular stars (3 lobes) UNIFORM across the entire field, same orientation everywhere.
Major0 analyses - Posterization
Hard tonal banding visible in smooth gradients.
Major0 analyses - Cosmic rays / glitches
Short bright streaks or very bright points, on a single sub only.
Marginal0 analyses - Internal reflection / ghost
Phantom image symmetric about the optical center.
Minor0 analyses - Field rotation
Stars as arcs centered around one point (alt-az mount without field rotator).
Major0 analyses - Over-denoising
Plastic look, artificial star edges, overly smooth texture.
Major4 analyses - Oversaturation
Garish colors, neon nebulosity, loss of subtle gradients.
Minor2 analyses - Sensor tilt
Asymmetric star elongation along a diagonal (two opposite corners sharp, two corners degraded).
Major12 analyses - Satellite trail
Thin lines crossing individual frames (1-3 px wide), continuous (satellite) or dotted (grouped Starlink train, aircraft).
Marginal4 analyses - Wind vibration
Subs sporadically and severely distorted by wind gusts or mechanical shock, non-systematic.
Minor0 analyses - Residual vignetting
Dark corners remaining after calibration (flat absent or mismatched).
Minor4 analyses - Walking noise
Fine diagonal streaks oriented along the direction of drift.
Major1 analysis
The 8 Major Defect Families in Astrophotography
The DocStellar glossary classifies 42 defects into eight families, following roughly the production chain of an astrophotographic image, from telescope to final file. Understanding this taxonomy speeds up diagnosis: a gradient will almost always fall under acquisition or calibration, never under optics.
The eight families
- Optics — defects tied to the telescope, its mirrors, or its lenses: coma, astigmatism, vignetting, chromatic halos. Often correctable through collimation, a field corrector, or a hardware change.
- Mechanics — physical misalignment of elements in the imaging train: sensor tilt, incorrect backfocus, tube flexure. Most are correctable with millimeter-level adjustments.
- Electronics — defects intrinsic to the sensor or readout electronics: amp glow, FPN (Fixed Pattern Noise), banding, hot columns. Calibration frames (darks, bias) eliminate most of them.
- Acquisition — errors related to the imaging protocol: failed tracking, imperfect focus, exposure too short or too long, poor guiding.
- Calibration — bad flats, poorly taken or incorrectly applied darks, missing bias. Consequences: residual gradients, uncompensated vignetting, amplified noise.
- Processing — defects introduced after the fact in post-production: highlight clipping, stretch artifacts, denoising halos, excessive saturation.
- Atmospheric — turbulence (poor seeing), mediocre transparency, dew on the front element. These are managed, not corrected.
- Environmental — urban light pollution, moon gradient, aircraft or satellite trails, vibrations from wind or nearby footsteps. Mitigation comes through site selection, filter choice, or timing decisions.
For each defect, its reference page details the visual signature, probable causes, differential diagnoses, and corrections to consider, illustrated by concrete cases in the public diagnostic gallery.