page_title: Point spread function
section_title: Astronomy
hierarchical_section_title: Point spread function / Applications / Astronomy
caption_attribution_description: English: The star Melnick 34 as imaged by Hubble Space Telescope's first Wide Field and Planetary Camera (WFPC-1). The advantages of working in space above Earth's distorting atmosphere are immediately apparent. Atmospheric blurring is gone and many more stars are visible. However, the effects of the Hubble Telescope's spherical aberration also are apparent. In particular there is a four arc second diameter 'skirt' around the bright star which obscures the view of the sky in its vicinity.
context_page_description: The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob in an image that represents a single point object. In functional terms, it is the spatial domain version of the optical transfer function of the imaging system. It is a useful concept in Fourier optics, astronomical imaging, medical imaging, electron microscopy and other imaging techniques such as 3D microscopy and fluorescence microscopy.
  The degree of spreading of the point object is a measure for the quality of an imaging system. In non-coherent imaging systems, such as fluorescent microscopes, telescopes or optical microscopes, the image formation process is linear in the image intensity and described by linear system theory. This means that when two objects A and B are imaged simultaneously, the resulting image is equal to the sum of the independently imaged objects.
context_section_description: In observational astronomy, the experimental determination of a PSF is often very straightforward due to the ample supply of point sources (stars or quasars). The form and source of the PSF may vary widely depending on the instrument and the context in which it is used.
  For radio telescopes and diffraction-limited space telescopes, the dominant terms in the PSF may be inferred from the configuration of the aperture in the Fourier domain.  In practice, there may be multiple terms contributed by the various components in a complex optical system. A complete description of the PSF will also include diffusion of light (or photo-electrons) in the detector, as well as tracking errors in the spacecraft or telescope.
  For ground-based optical telescopes, atmospheric turbulence (known as astronomical seeing) dominates the contribution to the PSF. In high-resolution ground-based imaging, the PSF is often found to vary with position in the image (an effect called anisoplanatism). In ground-based adaptive optics systems, the PSF is a combination of the aperture of the system with residual uncorrected atmospheric terms.
caption: The point spread function of Hubble Space Telescope's WFPC camera before corrections were applied to its optical system.
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