Spectral flux density in the context of Surface brightness

In astronomy, a deep field is an image of a portion of the sky taken with a very long exposure time, in order to detect and study faint objects. The depth of the field refers to the apparent magnitude or the flux of the faintest objects that can be detected in the image. Deep field observations usually cover a small angular area on the sky, because of the large amounts of telescope time required to reach faint flux limits. Deep fields are used primarily to study galaxy evolution and the cosmic evolution of active galactic nuclei, and to detect faint objects at high redshift. Numerous ground-based and space-based observatories have taken deep-field observations at wavelengths spanning radio to X-rays.

The first deep-field image to receive a great deal of public attention was the Hubble Deep Field, observed in 1995 with the WFPC2 camera on the Hubble Space Telescope. Other space telescopes that have obtained deep-field observations include the Chandra X-ray Observatory, the XMM-Newton Observatory, the Spitzer Space Telescope, and the James Webb Space Telescope.

In telecommunications, especially radio communication, spread spectrum are techniques by which a signal (e.g., an electrical, electromagnetic, or acoustic) generated with a particular bandwidth is deliberately spread in the frequency domain over a wider frequency band. Spread-spectrum techniques are used for the establishment of secure communications, increasing resistance to natural interference, noise, and jamming, to prevent detection, to limit power flux density (e.g., in satellite downlinks), and to enable multiple-access communications.

The jansky (symbol Jy, plural janskys) is a non-SI unit of spectral flux density, or spectral irradiance, used especially in radio astronomy. It is equivalent to 10 watts per square metre per hertz.

The spectral flux density or monochromatic flux, S, of a source is the integral of the spectral radiance, B, over the source solid angle:$${\displaystyle S=\iint \limits _{\text{source}}B(\theta ,\phi )\,\mathrm {d} \Omega .}$$

The solar flux unit (sfu) is a non-SI unit of spectral flux density often used in solar radio observations, such as the F10.7 solar activity index. It is equivalent to 10 watts per square metre per hertz (SI), 10 ergs per second per square centimetre per hertz (CGS), and 10 Jansky.