NASA's James Webb Space Telescope

Image Quality and PSFs

Simulated JWST point-spread function
A simulation of the the JWST point-spread function (PSF). The left panel shows the central region on a linear scale between 0 and 0.2 in a small field of view (0.6" x 0.6"). On the right, the same PSF is shown in logarithmic grayscale from 1.0e-5 to 1, over a larger field of view (4" x 4"). The Logarithmic scale is useful to identify very faint structures in the PSF that extend far from the center, in particular the effects of the hexagon shaped mirrors.

JWST will be diffraction limited at 2μm, defined as having a Strehl ratio >0.80. JWST will achieve this image quality using using periodic wave-front sensing and control  of the primary mirror. There is a fine guidance sensor in the focal plane and a fine steering mirror to maintain pointing during observations. The observatory and pointing-control system are designed to limit image motion to less than 7 milliarcseconds during observations. The hexagonal segments and 3-legged secondary support structure give the PSF a different shape than traditional telescope designs with a filled circular primary mirror.