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Topic Review
Photometry
Photometry, from Greek photo- ("light") and -metry ("measure"), is a technique used in astronomy that is concerned with measuring the flux or intensity of light radiated by astronomical objects. This light is measured through a telescope using a photometer, often made using electronic devices such as a CCD photometer or a photoelectric photometer that converts light into an electric current by the photoelectric effect. When calibrated against standard stars (or other light sources) of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects. The methods used to perform photometry depend on the wavelength region under study. At its most basic, photometry is conducted by gathering light and passing it through specialized photometric optical bandpass filters, and then capturing and recording the light energy with a photosensitive instrument. Standard sets of passbands (called a photometric system) are defined to allow accurate comparison of observations. A more advanced technique is spectrophotometry that is measured with a spectrophotometer and observes both the amount of radiation and its detailed spectral distribution. Photometry is also used in the observation of variable stars, by various techniques such as, differential photometry that simultaneously measures the brightness of a target object and nearby stars in the starfield or relative photometry by comparing the brightness of the target object to stars with known fixed magnitudes. Using multiple bandpass filters with relative photometry is termed absolute photometry. A plot of magnitude against time produces a light curve, yielding considerable information about the physical process causing the brightness changes. Precision photoelectric photometers can measure starlight around 0.001 magnitude. The technique of surface photometry can also be used with extended objects like planets, comets, nebulae or galaxies that measures the apparent magnitude in terms of magnitudes per square arcsecond. Knowing the area of the object and the average intensity of light across the astronomical object determines the surface brightness in terms of magnitudes per square arcsecond, while integrating the total light of the extended object can then calculate brightness in terms of its total magnitude, energy output or luminosity per unit surface area.
  • 973
  • 11 Nov 2022
Topic Review
Valence-shell Electron-pair Repulsion Model
There are the following main assumptions of the Valence-shell Electron-pair Repulsion (VSEPR) model. - The arrangement of covalent bonds of the atom centre analyzed depends on the number of electron pairs in its valence shell: bonds and nonbonding pairs as lone electron pairs. - The arrangement of valence electron pairs around the centre considered is to maximize their distances apart. - The non-valence electrons - inner electrons with nucleus (i.e. the core) possess the spherical symmetry (or at least it is in force for the main groups elements). It is worth to note that the intra- and intermolecular interactions influence on electronic and molecular structures in accordance with this VSEPR model.
  • 972
  • 06 Sep 2021
Topic Review
Brite-Constellation
BRITE-Constellation is devoted to high-precision optical photometric monitoring of bright stars, distributed all over the Milky Way, in red and/or blue passbands. Photometry from space avoids the turbulent and absorbing terrestrial atmosphere and allows for very long and continuous observing runs with high time resolution and thus provides the data necessary for understanding various processes inside stars (e.g., asteroseismology) and in their immediate environment. 
  • 970
  • 15 Jul 2021
Topic Review
Low-Energy Electron Microscopy
Low-energy electron microscopy, or LEEM, is an analytical surface science technique used to image atomically clean surfaces, atom-surface interactions, and thin (crystalline) films. In LEEM, high-energy electrons (15-20 keV) are emitted from an electron gun, focused using a set of condenser optics, and sent through a magnetic beam deflector (usually 60˚ or 90˚). The “fast” electrons travel through an objective lens and begin decelerating to low energies (1-100 eV) near the sample surface because the sample is held at a potential near that of the gun. The low-energy electrons are now termed “surface-sensitive” and the near-surface sampling depth can be varied by tuning the energy of the incident electrons (difference between the sample and gun potentials minus the work functions of the sample and system). The low-energy elastically backscattered electrons travel back through the objective lens, reaccelerate to the gun voltage (because the objective lens is grounded), and pass through the beam separator again. However, now the electrons travel away from the condenser optics and into the projector lenses. Imaging of the back focal plane of the objective lens into the object plane of the projector lens (using an intermediate lens) produces a diffraction pattern (low-energy electron diffraction, LEED) at the imaging plane and recorded in a number of different ways. The intensity distribution of the diffraction pattern will depend on the periodicity at the sample surface and is a direct result of the wave nature of the electrons. One can produce individual images of the diffraction pattern spot intensities by turning off the intermediate lens and inserting a contrast aperture in the back focal plane of the objective lens (or, in state-of-the-art instruments, in the center of the separator, as chosen by the excitation of the objective lens), thus allowing for real-time observations of dynamic processes at surfaces. Such phenomena include (but are not limited to): tomography, phase transitions, adsorption, reaction, segregation, thin film growth, etching, strain relief, sublimation, and magnetic microstructure. These investigations are only possible because of the accessibility of the sample; allowing for a wide variety of in situ studies over a wide temperature range. LEEM was invented by Ernst Bauer in 1962; however, not fully developed (by Ernst Bauer and Wolfgang Telieps) until 1985.
  • 969
  • 15 Nov 2022
Topic Review
Thermoporometry and Cryoporometry
Thermoporometry and cryoporometry are methods for measuring porosity and pore-size distributions. A small region of solid melts at a lower temperature than the bulk solid, as given by the Gibbs–Thomson equation. Thus, if a liquid is imbibed into a porous material, and then frozen, the melting temperature will provide information on the pore-size distribution. The detection of the melting can be done by sensing the transient heat flows during phase transitions using differential scanning calorimetry – DSC thermoporometry, measuring the quantity of mobile liquid using nuclear magnetic resonance – NMR cryoporometry (NMRC) or measuring the amplitude of neutron scattering from the imbibed crystalline or liquid phases – ND cryoporometry (NDC). To make a thermoporometry / cryoporometry measurement, a liquid is imbibed into the porous sample, the sample cooled until all the liquid is frozen, and then warmed until all the liquid is again melted. Measurements are made of the phase changes or of the quantity of the liquid that is crystalline / liquid (depending on the measurement technique used). The techniques make use of the Gibbs–Thomson effect: small crystals of a liquid in the pores melt at a lower temperature than the bulk liquid : The melting point depression is inversely proportional to the pore size. The technique is closely related to that of use of gas adsorption to measure pore sizes but uses the Gibbs–Thomson equation rather than the Kelvin equation. They are both particular cases of the Gibbs Equations (Josiah Willard Gibbs): the Kelvin equation is the constant temperature case, and the Gibbs–Thomson equation is the constant pressure case.
  • 966
  • 29 Sep 2022
Topic Review
High-Throughput Screening
HTS involves in vitro, cell- or whole organism- based assays. The most common readouts for biochemical assays in HTS are optical, including absorbance, fluorescence, luminescence, and scintillation.
  • 962
  • 21 Jul 2021
Topic Review
High-Resolution Transmission Electron Microscopy
High-resolution transmission electron microscopy is an imaging mode of specialized transmission electron microscopes that allows for direct imaging of the atomic structure of samples. It is a powerful tool to study properties of materials on the atomic scale, such as semiconductors, metals, nanoparticles and sp2-bonded carbon (e.g., graphene, C nanotubes). While this term is often also used to refer to high resolution scanning transmission electron microscopy, mostly in high angle annular dark field mode, this article describes mainly the imaging of an object by recording the two-dimensional spatial wave amplitude distribution in the image plane, in analogy to a "classic" light microscope. For disambiguation, the technique is also often referred to as phase contrast transmission electron microscopy. At present, the highest point resolution realised in phase contrast transmission electron microscopy is around 0.5 ångströms (0.050 nm). At these small scales, individual atoms of a crystal and its defects can be resolved. For 3-dimensional crystals, it may be necessary to combine several views, taken from different angles, into a 3D map. This technique is called electron crystallography. One of the difficulties with high resolution transmission electron microscopy is that image formation relies on phase contrast. In phase-contrast imaging, contrast is not intuitively interpretable, as the image is influenced by aberrations of the imaging lenses in the microscope. The largest contributions for uncorrected instruments typically come from defocus and astigmatism. The latter can be estimated from the so-called Thon ring pattern appearing in the Fourier transform modulus of an image of a thin amorphous film.
  • 962
  • 15 Nov 2022
Topic Review
Auger Electron Spectroscopy
thumb|A Hanford scientist uses an Auger electron spectrometer to determine the elemental composition of surfaces. Auger electron spectroscopy (AES; pronounced [oʒe] in French) is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science. Underlying the spectroscopic technique is the Auger effect, as it has come to be called, which is based on the analysis of energetic electrons emitted from an excited atom after a series of internal relaxation events. The Auger effect was discovered independently by both Lise Meitner and Pierre Auger in the 1920s. Though the discovery was made by Meitner and initially reported in the journal Zeitschrift für Physik in 1922, Auger is credited with the discovery in most of the scientific community. Until the early 1950s Auger transitions were considered nuisance effects by spectroscopists, not containing much relevant material information, but studied so as to explain anomalies in X-ray spectroscopy data. Since 1953 however, AES has become a practical and straightforward characterization technique for probing chemical and compositional surface environments and has found applications in metallurgy, gas-phase chemistry, and throughout the microelectronics industry.
  • 960
  • 01 Nov 2022
Topic Review
Mooncake
A mooncake (simplified Chinese: 月饼; traditional Chinese: 月餅; pinyin: yuè bǐng; Jyutping: jyut6 beng2; Yale: yuht béng) is a Chinese bakery product traditionally eaten during the Mid-Autumn Festival (中秋節). The festival is for lunar appreciation and moon watching, when mooncakes are regarded as an indispensable delicacy. Mooncakes are offered between friends or on family gatherings while celebrating the festival. The Mid-Autumn Festival is one of the four most important Chinese festivals. Typical mooncakes are round pastries, measuring about 10 cm in diameter and 3–4 cm thick, and are commonly eaten in the Southern Chinese regions of Guangdong, Guangxi, Hong Kong and Macau. A rich thick filling usually made from red bean or lotus seed paste is surrounded by a thin (2–3 mm) crust and may contain yolks from salted duck eggs. Mooncakes are usually eaten in small wedges accompanied by tea. Today, it is customary for businessmen and families to present them to their clients or relatives as presents, helping to fuel a demand for high-end mooncakes. Due to China's influence, mooncakes and Mid-Autumn Festival are also enjoyed and celebrated in other parts of Asia. Mooncakes have also appeared in western countries as a form of delicacy.
  • 960
  • 22 Nov 2022
Topic Review
Scanning Kelvin Probe
In microscopy, a scanning Kelvin probe (SKP) is a non-contact, non-destructive scanning probe microscopy (SPM) technique used to measure the work function of the sample under study. By raster scanning in the x,y plane the work function of the sample can be locally mapped for correlation with sample features. It is predominantly used to measure corrosion and coatings. It is closely related to the Kelvin probe force microscope (KPFM) technique.
  • 960
  • 30 Oct 2022
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