Angstrom

An Angstrom is a unit of length equal to 10^-10 meters, or 0.0000000001 meters.

Charge Coupled Device (CCD)

A CCD is a semiconductor device which converts light (including X-rays) into electrical charges. The charges are then read out using a complex arrangement of electrodes, which also divides the CCD into many small, discrete areas (we call these "pixels" - or picture elements).

electron Volt (eV)

An electron Volt (eV) is a unit of energy. Technically, it is the energy an electron or proton gains when it moves through a potential difference of 1 Volt. Potential difference is the work per unit charge done externally to move a charge from point A to point B without causing a change in potential energy.

This unit is commonly used when referring to the energy of X-rays. Often, the prefix "kilo" is put in front of it. Kilo means 1000, so a kilo-electron Volt or keV is equal to 1000 eV. A Mega-electron Volt, or MeV, is equal to one million eV. A GeV is a Giga-electron Volt - or a billion eV.

High Resolution Spectroscopy

High Resolution means the ability to distinguish small differences in the energies of light. "High" is a somewhat subjective term, and the precise meaning will vary among different areas of astronomy, and with time. Most X-ray astronomers of 1990's would consider the ability to distinguish < 1% difference in photon energies to be 'high resolution'.

High Throughput

High Throughput means being able to measure many X-ray photons. This is achieved by having big areas (either of the detector itself, or of the telescope in front of it) and high efficiency (being able to measure most of the X-rays that hit the instrument).

Microcalorimeter

A microcalorimeter is a thermal device. When an X-ray photon strikes an absorber, (for example HgTe), it deposits its energy, causing a rise in temperature. The temperature rise, which is measured by a thermistor, is approximately proportional to the energy of the X-ray photon. So, by measuring how much the temperature changes, we can determine the energy of the X-ray.

Photon

Although electromagnetic radiation (visible light, IR, X-rays, ...) behaves like waves in many ways, it also comes in packets; the energy of a packet of light is uniquely determined by its wavelength. Photon is the name given to such a packet, and represents the particle-like nature of light.

Spectroscopy

Spectroscopy, the study of spectra, is the measurement of the amount of light as a function of its energy (or wavelength or frequency, all are equivalent), and the associated interpretation. Spectroscopy gives information on the physical conditions of the X-ray (etc.) emitting object, such as temperature, composition, motion, and density.

For more information about spectra, please see our Spectroscopy page.