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Diffraction-limited was used in marketing to describe a telescope that can achieve a resolution that is limited only by its aperture, not by any lack of quality in its optical design or construction. (In reality, for most telescopes, the atmosphere will limit maximum achievable resolution long before aperture size). Most manufacturers no longer use the term as it has been abused and is no longer considered trustworthy.

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WH. A brief explanation is that the so called diffraction spot is that tiny disc in which hopefully all the reflected or refracted light coming from a point source, ie a star, will fall. This spot, (Airey Disc) is measurable, and there is a formula for that purpose. All objectives of the same focal ratio will have the same size of diffraction spot. It will change with focal length, and or objective diameter. The larger the diameter, the smaller the spot. Hence, the bigger the better for resolving detail (Planets etc.) The term Diffraction Limited, I would assume, refers to that tiny disc of focused light, being of the theoretical size it should be for a particular optical system. If it does that, then it can be claimed as Diffraction Limited.

There is probably a better explanation than mine, but I thought I would set the ball rolling for you.

Ron. :)

PS. I thought I was first, but I was headed by Steve and GazOC while I was writing this. Steve is right of course, testing methods of objectives are more sophisticated now.

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