The technique behind planetary and lunar photos: record thousands of very short frames, then software keeps only the sharpest, taken in the moments when the air is calm. Often 5 to 20% of the frames are stacked. It goes well beyond what the seeing lets you see at the eyepiece.
Related terms
- Seeing
The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
- Stacking
Adding or averaging many images of the same object, aligned on the stars, to reduce noise. Stacking 16 frames improves the signal-to-noise ratio by a factor of 4. Software such as Siril or DeepSkyStacker does the job.
- Arcseconds per pixel
The unit of sampling. Too large (undersampling) and stars turn square and detail is lost; too small (oversampling) and light is spread over too many pixels, raising noise. For planets, aim for a focal ratio of about five times the pixel size in µm.
← The whole glossary