What causes faint concentric circles in my astrophotography images, and how can I reduce them?
Asked 8/1/2013
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When processing astrophotography images taken with a Canon 450D at prime focus, I sometimes see a series of faint concentric circles radiating from an off-center point. Darkening the background can hide them, but that also removes faint galaxy detail. What are these rings likely to be, and what can I do to prevent or reduce them during capture or processing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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This looks to me like Newton's rings, an interference pattern that occurs from closely-placed surfaces. They are usually only visible when narrow-band filters are used, or when the light is inherently narrow-band. With wide-band illumination, the rings fall in different places for the different wavelengths of light, so the effect is largely canceled out.
My best guess would be that, in your case, the rings are coming from the various parts of the sensor assembly. I'm not sure how the 450D is arranged, but there's likely an anti-aliasing (AA) filter, IR-blocking filter, and the sensor itself—all flat surfaces in close proximity that could give rise to Newton's rings.
Originally by user2138. Source · Licensed CC BY-SA 4.0
user2138
13y ago
0
Generated from our catalog & community — verify before relying on it.
Faint concentric rings like this are usually caused by either:
- Newton’s rings — an interference pattern from closely spaced flat surfaces in the optical path, such as parts of the camera sensor/filter stack (for example AA or IR-cut filters). These are more noticeable with narrow-band light.
- Glare or stray light — from a bright light source outside the frame, or from light leakage somewhere in the telescope/camera assembly.
To reduce them:
- Check for stray light entering the system and shield the setup from nearby lights.
- Make sure the telescope/camera connection is light-tight.
- If you are using any filters, especially narrow-band ones, they may be contributing to interference patterns.
- If there is a front optical element exposed to off-axis light, use appropriate shielding/dew hood.
If the rings are coming from internal reflections or interference in the sensor/filter stack, they may be difficult to eliminate completely in processing, so prevention during capture is the best approach.
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