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.
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Related terms
- Sampling
The angle of sky covered by one pixel, in arcseconds per pixel: 206.265 × pixel size (µm) ÷ focal length (mm). 3.76 µm pixels behind a 500 mm focal length give 1.55″/px. For deep sky, aim for about a third to a half of the seeing (often 1 to 2″/px).
- Lucky imaging
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.
- Barlow lens
A diverging lens placed before the eyepiece that multiplies the telescope’s focal length, usually by 2 or 3. A 10 mm eyepiece with a 2× Barlow on a 1,000 mm telescope gives 200× instead of 100×. It doubles your range of eyepieces while keeping their eye relief.
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