Planck: A European Space Agency satellite launched in 2009 to map tiny fluctuations in the cosmic microwave background radiation.

Point spread function (PSF): The point spread function describes the angular response of a telescope to a point-like source of light. Because of atmospheric turbulence and the finite resolving power of a telescope, light from point-like object is spread out in angle.

Reaction wheels: Wheels that provide enhanced guiding and stabilization of a spacecraft.

Redshift: The increase in wavelength of electromagnetic radiation (Doppler shift) caused by the motion of an object as described by the theory of special relativity, or by luminous material in a gravitational field as described by the general theory of relativity. In cosmology, it refers to the fractional increase in the wavelength of a photon received from a distant object, due to the expansion of the universe between emission and reception. Coupled with a cosmological model, redshifts can be used to determine the distance and look-back time of phenomena in the universe.

Reionization: See Epoch of reionization.

Science Definition Team (SDT): Team to help characterize a mission, in this case the WFIRST SDT.

Sloan Digitized Sky Survey (SDSS): Dedicated 2.5-meter ground-based telescope supporting repeated multicolor images of a quarter of the sky.

Space Infrared Telescope Facility (SIRTF): See Spitzer Space Telescope.

Spitzer Space Telescope: NASA’s Space Infrared Telescope Facility, launched in 2003.

Star cluster: A group of stars formed at about the same time.

Very Large Telescope (VLT): Four 8-meter telescopes of the European Space Observatories, located in Chile.

Weak lensing: Fluctuations in the large-scale distribution of matter produce variations in the “shape” of space that alter the path that light takes from a distant galaxy to our telescopes. This gravitational lensing changes the shape of a galaxy and makes it appear more elliptical. By measuring the shapes of large numbers of galaxies, astronomers can detect this “weak lensing” effect and infer the large-scale distribution of matter.

WFIRST: Wide Field Infrared Survey Telescope, the top-ranked large space mission of NWNH.

William Herschel Space Observatory: A European Space Agency satellite launched in 2009 to study the formation of stars and galaxies at far-infrared and submillimeter wavelengths.

XMM-Newton: The European Space Agency’s X-ray Multi-Mirror mission, named after Sir Isaac Newton, is an orbiting X-ray observatory launched in 1999. It features a large collecting area for X-ray spectroscopy.

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