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Photometry (astronomy)
 
 
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Photometry is a technique of astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 concerned with measuring
Measurement

Measurement is the process of assigning a number to an attribute according to a rule or set of rules. The term can also be used to refer to the result obtained after performing the process....
 the flux
Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.*In the study of transport phenomena , flux is defined as the amount that flows through a unit area per unit time....
, or intensity of an astronomical object
Astronomical object

s are significant entity, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence....
's electromagnetic radiation
Electromagnetic radiation

Electromagnetic radiation takes the form of wave propagation waves in a vacuum or in matter. EM radiation has an electric field and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy Wave propagation....
. Usually, photometry refers to measurement over large wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
 bands of radiation
Radiation

In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body....
; but, when not only the amount of radiation but its spectral distribution are measured the term spectrophotometry is used.

Methods
The methods used to perform photometry depend on the wavelength regime under study.






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Photometry is a technique of astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 concerned with measuring
Measurement

Measurement is the process of assigning a number to an attribute according to a rule or set of rules. The term can also be used to refer to the result obtained after performing the process....
 the flux
Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.*In the study of transport phenomena , flux is defined as the amount that flows through a unit area per unit time....
, or intensity of an astronomical object
Astronomical object

s are significant entity, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence....
's electromagnetic radiation
Electromagnetic radiation

Electromagnetic radiation takes the form of wave propagation waves in a vacuum or in matter. EM radiation has an electric field and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy Wave propagation....
. Usually, photometry refers to measurement over large wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
 bands of radiation
Radiation

In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body....
; but, when not only the amount of radiation but its spectral distribution are measured the term spectrophotometry is used.

Methods


The methods used to perform photometry depend on the wavelength regime under study. At its most basic, photometry is conducted by gathering radiation
Radiation

In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body....
 in a telescope
Telescope

A telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century....
, perhaps passing it through specialized optical filter
Filter (optics)

Optical filters, generally, belong to one of two categories. The simplest, physically, is the absorptive filter, while the latter category, that of interference or dichroic filters, can be quite complex....
s, and then capturing and recording the light energy with a photosensitive instrument. The set of passbands
Passband

In brief, the passband is the range of frequencies or wavelengths that can pass through a electronic filter without being attenuated....
 (fdvsbbfd a photometric system
Photometric system

In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used....
.

Historically, photometry in the near infrared
Infrared

Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves ....
 through long-wavelength ultra-violet was done with a photoelectric photometer, an instrument that measured the light intensity of a single object by directing its light on to a photosensitive cell. They have largely been replaced with CCD
Charge-coupled device

A charge-coupled device is an analog signal shift register that enables the transportation of analog signals through successive stages , controlled by a clock signal....
 cameras which can simultaneously image multiple objects, though photoelectric photometers are still used in special situations, such as where high time resolution is required.

CCD Photometry


When using a CCD camera to conduct photometry there are a number of possible ways to extract a photometric measurement (e.g. the magnitude
Apparent magnitude

The apparent magnitude of a celestial body is a measurement of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the Earth's atmosphere....
 of a star) from the raw CCD image. The observed signal from an object will typically be smeared (convolved
Convolution

In mathematics and, in particular, functional analysis, convolution is a mathematical operator on two function s f and g, producing a third function that is typically viewed as a modified version of one of the original functions....
) over many pixels
Pixel

In digital imaging, a pixel is the smallest item of information in an image. Pixels are normally arranged in a 2-dimensional grid, and are often represented using dots, squares, or rectangles....
 by the point spread function
Point spread function

The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system....
. This broadening is due to the optics in the telescope as well as to astronomical seeing
Astronomical seeing

Astronomical seeing refers to the blurring and scintillation of astronomical objects such as stars caused by turbulence in the Earth's atmosphere....
 (twinkling). When obtaining photometry for a point source (an object with an angular diameter
Angular diameter

The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle. In the vision sciences it is called the visual angle....
 that is much smaller than the resolution
Angular resolution

Angular resolution describes the resolving power of any such as an Optical telescope or radio telescope, a microscope, a camera, or an eye....
 of the telescope) the goal is to add up all the light from the object and subtract off the light due to the sky. The simplest technique, of adding up the pixel counts within a circle centered on the object and subtracting off an average sky count, is known as aperture photometry. When doing photometry in a very crowded field, such as a globular cluster
Globular cluster

A globular cluster is a sphere collection of stars that orbits a Galactic Center as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers....
, where the profiles of stars overlap significantly, one must use de-convolution techniques, such as point spread function fitting, to determine the individual fluxes of the overlapping sources.

Calibrations


After determining the flux of an object in counts, one must calibrate the measurement in some way. Which calibrations are needed depend in part on what type of photometry is being done. One typically speaks of performing differential, relative or absolute photometry. Differential photometry is the measurement of changes in the brightness of an object over time; these measurements are compiled into a light curve
Light curve

In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band....
 of the object. Relative photometry is the measurement of the apparent brightnesses of multiple objects relative to each other. Absolute photometry is the measurement of the apparent brightness of an object on a standard photometric system
Photometric system

In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used....
, these measurements can be compared with other absolute photometric measurements obtained with different telescopes/instruments. In most cases differential photometry can be done with the highest precision
Accuracy and precision

In the fields of science, engineering, industry and statistics, accuracy is the degree of closeness of a Measure d or calculated quantity to its actual Value ....
, while absolute photometry is the most difficult to do with high precision. In general, accurate photometry is more difficult when the apparent brightness of the object is fainter.

To perform differential photometry one must correct measurements for temporal changes in the sensitivity of the instrument as well as changes in the atmospheric extinction
Extinction (astronomy)

Extinction is a term used in astronomy to describe the Absorption and scattering of electromagnetic radiation emitted by astronomical objects by matter between the emitting object and the observation....
 through which the object is observed (when observing from the ground). This is typically done by simultaneously observing a number of comparison stars, which are assumed to be constant, together with the object(s) of interest.

To perform relative photometry one must correct measurements for spatial variations in the sensitivity of the instrument and the atmospheric extinction. This is often in addition to correcting for their temporal variations, particularly when the objects being compared are too far apart on the sky to be observed simultaneously.

To perform absolute photometry one must correct for differences between the effective passband through which an object is observed and the passband used to define the standard photometric system. This is often in addition to the all of the other corrections discussed above. Typically this correction is done by observing the object(s) of interest through multiple filters and also observing a number of photometric standard stars
Photometric standard stars

Photometric standard stars are a series of stars that have had their light output in various passbands of photometric system, measured very carefully....
. If the standard stars cannot be observed simultaneously with the target(s), this correction must be done under photometric conditions, when the sky is cloudless and the extinction is a simple function of the airmass
Airmass

In astronomy, airmass is the optical path length through Earth's atmosphere for light from a celestial source. As it passes through the atmosphere, light is attenuated by scattering and absorption ; the more atmosphere through which it passes, the greater the attenuation....
.

Applications


Photometric measurements can be combined with the inverse-square law
Inverse-square law

In physics, an inverse-square law is any physical law stating that some physical quantity or strength is Inverse ly proportionality to the square of the distance from the source of that physical quantity....
 to determine the luminosity
Luminosity

Luminosity has different meanings in several different fields of science....
 of an object if its distance
Distance

Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria ....
 can be determined, or its distance if its luminosity is known. Other physical properties of an object, such as its temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
 or chemical composition, may be determined via broad or narrow-band spectrophotometry. Typically photometric measurements of multiple objects obtained through two filters are plotted on a color-magnitude diagram, which for stars is the observed version of the Hertzsprung-Russell diagram
Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram shows the relationship between absolute magnitude, luminosity, stellar classification, and effective temperature ofstars....
. Photometry is also used to study the light variations of objects such as variable stars
Variable star

A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth....
, minor planets
Minor planet

An asteroid group or minor planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid....
, active galactic nuclei
Active galactic nucleus

An active galactic nucleus is a compact region at the centre of a galaxy which has a much higher than normal luminosity over some or all of the electromagnetic spectrum ....
 and supernovae
Supernova

A supernova is a Astronomy#Stellar astronomy explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months....
, or to detect transiting extrasolar planets
Methods of detecting extrasolar planets

Any planet is an extremely faint light source compared to its parent star. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out....
. Measurements of these variations can be used, for example, to determine the orbital period
Orbital period

The orbital Periodicity is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars....
 and the radii
RADIUS

Remote Authentication Dial In User Service is a networking protocol that provides centralized access, authorization and accounting management for people or computers to connect and use a network service....
 of the members of an eclipsing binary star
Binary star

A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star or secondary....
 system, the rotation period
Rotation period

The rotation period of an astronomy astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars....
 of a minor planet or a star, or the total energy output of a supernova.

See also

  • Photometric filters
    Photometric system

    In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used....
  • Spectroscopy
    Spectroscopy

    Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g....
  • Radiometry
    Radiometry

    In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry_, which deal with brightness as perceived by the human eye, rather than absolute power....
  • Albedo
    Albedo

    The albedo of an object is the extent to which it diffusely reflects light from the Sun. It is therefore a more specific form of the term reflectivity....
  • Hapke parameters
    Hapke parameters

    The Hapke parameters are a set of parameters for a quasi-experimental model that are commonly used to describe theBidirectional reflectance distribution function of the airless regolith surfaces of bodies in the solar system....
  • Bidirectional reflectance distribution function
    Bidirectional reflectance distribution function

    The bidirectional reflectance distribution function is a 4-dimensional function that defines how light is reflected at an opaque surface. The function takes an incoming light direction, , and outgoing direction, , both defined with respect to the surface normal , and returns the ratio of reflected radiance exiting along to the irradiance...