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angular sizes of point sources - correct answer

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You've got it! With today's telescopes, most stars are so far away that they appear to be point sources to a telescope detector. Point sources have angular sizes that are smaller than the distortions introduced by the detector. What telescope limitations prevent us from measuring the angular sizes of stars?

optical image of star1 optical image of star2 optical image of star3

Limitations of Point Spread Functions

One distortion light suffers when it is seen through a telescope is called a point spread function. The point spread function of a telescope is the pattern that a point of emission appears as. Each telescope has its own point spread function. The point spread function has the effect of equalizing the size of any objects whose angular size is fewer pixels across than the point spread function.
Info Tell me about point spread functions of telescopes, and show me some examples.

Limitations of Resolution

Telescopes are also limited by their resolution. A telescope's resolution is the angular size of the sky that each pixel represents (this is often a function of wavelength). If the resolution of the telescope is larger than the angular size of the star, it is impossible for the star to look "clearer" than a single pixel.

At best, a distant star that is unresolved will appear as a single pixel (at least the location will be well constrained!). At worst, it will appear as a blurry blob, the telescope's point spread function. In any case, unless the star is lare and nearby (so that its angular size is larger than the telescope's resolution and its point spread function), little information can be gained from an optical image of a star taken by today's telescopes. Perhaps in the future telescope resolution and point spread functions will improve to the point where we can image many stars other than our own Sun!

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A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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