http://www.lbto.org/LBT%20Website%20...e%20optics.htm
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1. Due to the nature of light a pinpoint star produces an Airy disk, surrounded by a Airy Pattern.
2. All stars great and small produce the same linear diameter Airy disk for the same f ratio ie a 50mm f5 and a 500mm f5 produce the same sized Airy disk.
The limitations are not due to the Airy disk but the receptors ability to resolve and record the disk ( and the outlying pattern)
When you analyse a short exposure image, the star is still the Airy disk in diameter but with a much lower peak intensity. (See Star #4)
A brighter star (or more exposed star) has the same Airy disk diameter but a higher peak intensity (See Star #2)