Description
An internal reflection (or "ghost") is a phantom image of a bright source (a bright star, and sometimes the Moon or a planet), created by stray reflections between the optical surfaces of the imaging train (lenses, filter, sensor window, field corrector).
The ghost appears as a faint, blurred replica of the source, often symmetric about the optical center: if the bright star is in the upper left, its ghost forms in the lower right.
It can take the shape of a disk, a ring, or a colored patch, and shifts within the field when you reframe, since it depends on the geometry of the source relative to the optical axis.
This is an optical defect. It is distinct from a filter halo (a concentric ring centered on the star itself), from dust donuts (dark and fixed), and from clipped star cores.
Visual signature
A luminous phantom patch appears at a distance from a bright source, generally diametrically opposite with respect to the center of the image.
The ghost is a dimmed replica of the source: a disk, ring, or colored blob, sometimes with internal structure (a reflection of the aperture or diaphragm).
Its position depends on the framing: moving the bright star within the field moves the ghost in mirror fashion, which distinguishes it from a real object or a fixed dust particle.
It is most visible when the source is bright and when the optical train contains many surfaces (filter, corrector, sensor window).
Differential diagnosis
Not to be confused with a filter halo: the halo is a concentric ring centered on the bright star itself, whereas the ghost is a displaced image, often on the opposite side of the center.
Distinct from dust donuts: donuts are dark, fixed on the sensor, and independent of stars; the ghost is bright, linked to a bright source, and moves with the framing.
To be separated from a real object: a ghost is absent from the DSS reference image and moves in mirror fashion when reframing, unlike a true galaxy or nebula.
Not to be taken for a cosmic ray: the latter is a bright point or streak present on only one frame, whereas the ghost repeats on all frames of the same composition.
Distinct from anomalous diffraction spikes: spikes radiate from the star itself (secondary spider, sensor microlenses), whereas a ghost is a displaced replica at a distance from the source.
Probable causes
- Reflections between optical surfaces (lenses, corrector, sensor window)
- A very bright star, the Moon, or a planet inside or near the field
- A reflective filter or one mounted perpendicular to the axis
- Optical surfaces lacking an effective anti-reflection coating
- An imaging train with many interfaces
- A framing that places a bright source in a ghosting position
Course of action
- Shift the framing to move the bright star out of the field
- Move the ghost away from the target by recomposing the image
- Identify and replace the offending reflective surface
- Prefer optics and filters with anti-reflection coatings
- Avoid imaging right next to the Moon or a very bright planet
- Clone-stamp out a ghost that falls on background sky
- Check the tilt and mounting of filters
The Doc's advice
The ghost is light bouncing between your optical surfaces before landing where it should not. First reflex: find the source. Look for the very bright star (or the Moon) and check the point diametrically opposite with respect to the center -- your phantom is usually there. The simplest fix is framing: shift the composition to move the offending star out of the field or to push the ghost away from your target. On the equipment side, track down the culprit surface (filter, reflective sensor window) and favor anti-reflection coatings. In post-processing, if the ghost falls on background sky, a clone stamp makes it disappear cleanly.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
What is a ghost or internal reflection?
It is a spurious phantom image of a bright light source, produced by multiple reflections between the optical surfaces of the imaging train (lenses, corrector, filter, sensor window). A fraction of the light from the bright star bounces before reaching the sensor and forms a dimmed replica, often diametrically opposite with respect to the optical center. The ghost can be a disk, a ring, or a colored patch, and it depends directly on the position of the source within the field.
How do I know whether a patch is a ghost or a real object?
Three checks. First, a ghost is absent from the DSS reference image of the same area: a real object (galaxy, nebula) appears there. Second, a ghost moves in mirror fashion when you reframe or when the bright source changes position, whereas a real object stays fixed relative to the stars. Third, a ghost is almost always linked to an identifiable bright source located on the opposite side of the center. If all three criteria point to a reflection, it is not an object in the sky.
How do I prevent internal reflections?
The simplest method is framing: shift the composition to move the very bright star out of the field, or to place the ghost far from your target. On the equipment side, prefer optics and filters with good anti-reflection coatings and check how filters are mounted (a filter perfectly perpendicular to the axis promotes reflections). Also avoid imaging right next to the Moon or a very bright planet. Reducing the number of surfaces in the imaging train also helps.
Can a ghost be removed in post-processing?
It depends on where it falls. If it forms over a region of sky background, a simple clone stamp or local retouching makes it disappear cleanly. If it overlaps the target (nebula, galaxy), the task is much harder because you have to reconstruct the masked signal without damaging the object. In that case, prevention at acquisition through better framing is preferable. A faint ghost can sometimes also be reduced by working on local levels.