![]() ![]() For example, an F8 scope has an Airy disk diameter of 0.00124*8=0.01 mm. For green light, the diameter is 0.00124 mm times the focal ratio. The first is the linear size as if we had an incredibly precise ruler. What is the size of the Airy disk? There are two ways to measure the Airy disk. Make the slit a circular opening and now the fuzzy image of the slit becomes a fuzzy disk surrounded by ever fainter rings. Projected on the wall will be a fuzzy image of the slit (the equivalent of the Airy disk), surrounded with ever fainter straight bands. ![]() Think of the famous slit experiment where light or electrons, anything that has wave properties travel through a slit. Why a discrete spot size and rings? Light diffracts because it has wave properties. Light going through a circular opening forms a spot of light surrounded by ever fainter rings. What is the Airy disk? The smallest spot of light that a perfect lens or mirror can focus is called the Airy disk. This sets up a conflict between lowest power, widest angle viewing and highest resolution viewing.Īs the magnification increases going from maximum exit pupil to minimum exit pupil, how does the eye respond? We can see from the following drawings, that for smaller scale objects like M104, a galaxy in the Virgo Cluster, the eye hangs onto faint nebulosity surprisingly well (though the faintest nebulosity is no longer visible) while the increasing resolution makes for a beautiful view. We typically resolve better than two arc-minutes in the sky, depending on the object to resolve (double stars of equal brightness that are not too bright or dim, double stars of unequal brightness or lunar and planetary detail deep-sky detail depends on object brightness). The eye's resolution is maximum at roughly 2mm pupil, our daytime pupil size. ![]() Filtered views such as OIII and HBeta filters make the sky background fainter with the detectable contrast as low as 0.5-1%, implying objects as faint as 27 mag per arc-sec^2. An exit pupil of 3.5mm as compared to 7mm reduces the object's surface brightness by 0.75 magnitude, rending invisible very faint objects no brighter than the eye's background noise.Īt maximum etendue in dark skies, the dark adapted experienced eye can detect objects of 25.5 mag per arc-sec^2 surface brightness as long as the object is at least several degrees in apparent size. However, etendue can be reduced by increasing magnification which shrinks the telescope's exit pupil. Because of the Law Of Etendue, the surface brightness of an object in the eye cannot exceed that of the object in the sky. The eye's background noise is on the order of 25 to 26 magnitudes per square arc-second. These specs are not that dissimilar to digital cameras: the eye-brain is an amazing detector. For 2% contrast that's 500 rods in the 25,000 that signal at a higher level than the surrounding forest of rods. Two to three degrees apparent size involves 25,000 adjacent rods. The dark adapted eye detects 6-10 photons in certain situations the eye can detect a single photon.Īn eye contains 90 million rods.
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