In many articles talking about telescopes, they talk often of "bright, unresolved sources".

"bright" usually mean that the magnitude should be less than a certain threshold, (or that the signal to noise ratio should bigger than a given value) but what I don't catch the meaning of "unresolved".

For example :

"unresolved sources" in https://docushare.lsst.org/docushare/dsweb/Get/LPM-17/LSSTSRD_v5%202%203.pdf

"bright unresolved point sources" in the introduction of this paper : https://docushare.lsstcorp.org/docushare/dsweb/Get/Publication-86/

Any idea ?

up vote 6 down vote accepted

"Unresolved" here means a source of light that appears as a point, not as a disc or a cloud.

For example, the planets can all be resolved to discs by even quite moderate telescopes. Galaxies also can be resolved, they don't appear as point sources.

However, stars, quasars, and most asteroids cannot be resolved by amateur equipment. In the context of the LSST paper, resolved sources are "galaxies" and unresolved sources are "stars and quasars".

The photometric measurements for resolved sources (galaxies) have to include several standard magnitudes, such as Petrosian magnitudes, as well as appropriate model magnitudes.

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