Photometry (astronomy)
Encyclopedia
Photometry is a technique of astronomy
concerned with measuring
the flux
, or intensity of an astronomical object
's electromagnetic radiation
. Usually, photometry refers to measurement over large wavelength
bands of radiation
; when not only the amount of radiation but also its spectral distribution are measured the term spectrophotometry
is used.
in a telescope
, perhaps passing it through specialized optical filter
s, and then capturing and recording the light energy with a photosensitive instrument. The set of passband
s (filters) is called a photometric system
.
Historically, photometry in the near infrared
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
cameras which can simultaneously image multiple objects, though photoelectric photometers are still used in special situations, such as where high time resolution is required.
of a star) from the raw CCD image. The observed signal from an object will typically be smeared (convolved
) over many pixel
s by the point spread function
. This broadening is due to the optics in the telescope as well as to astronomical seeing
(twinkling). When obtaining photometry for a point source (an object with an angular diameter
that is much smaller than the resolution
of the telescope) the goal is to add up all the light from the object and subtract the light due to the sky. The simplest technique, adding up the pixel counts within a circle centered on the object and subtracting of an average sky count, is known as aperture photometry. When doing photometry in a very crowded field, such as a globular cluster
, 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.
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
, 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
, 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
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
. 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
.
to determine the luminosity
of an object if its distance
can be determined, or its distance if its luminosity is known. Other physical properties of an object, such as its 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. Photometry is also used to study the light variations of objects such as variable star
s, minor planet
s, active galactic nuclei
and supernova
e, or to detect transiting extrasolar planets
. Measurements of these variations can be used, for example, to determine the orbital period
and the radii
of the members of an eclipsing binary star
system, the rotation period
of a minor planet or a star, or the total energy output of a supernova.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
concerned with measuring
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
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 flow per unit area, where flow is the movement of some quantity per time...
, or intensity of an astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
's electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. Usually, photometry refers to measurement over large wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
bands of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
; when not only the amount of radiation but also its spectral distribution are measured the term spectrophotometry
Spectrophotometry
In chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength...
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 radiationRadiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
in a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
, perhaps passing it through specialized optical filter
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
s, and then capturing and recording the light energy with a photosensitive instrument. The set of passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
s (filters) is called 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. For each photometric system a set of primary standard stars is provided.The first known...
.
Historically, photometry in the near infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
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 magnitudeApparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the 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 operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...
) over many pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s 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. The PSF in many contexts can be thought of as the extended blob...
. 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 twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
(twinkling). When obtaining photometry for a point source (an object with an angular diameter
Angular diameter
The angular diameter or apparent size 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. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
that is much smaller than the resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
of the telescope) the goal is to add up all the light from the object and subtract the light due to the sky. The simplest technique, adding up the pixel counts within a circle centered on the object and subtracting of 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 spherical collection of stars that orbits a galactic core 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. The name of this category of star cluster is...
, 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 curveLight 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. For each photometric system a set of primary standard stars is provided.The first known...
, 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, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
, 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 by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...
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, air mass 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. Consequently, celestial...
.
Applications
Photometric measurements can be combined with the inverse-square lawInverse-square law
In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity....
to determine the luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
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, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical distance...
can be determined, or its distance if its luminosity is known. Other physical properties of an object, such as its temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
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. Photometry is also used to study the light variations of objects such as variable star
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...
s, minor planet
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...
s, active galactic nuclei
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
and supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. 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...
e, 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 period 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.There are several kinds of...
and the radii
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
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, comes, or secondary...
system, the rotation period
Rotation period
The rotation period of an 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 filtersPhotometric systemIn 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. For each photometric system a set of primary standard stars is provided.The first known...
- SpectroscopySpectroscopySpectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
- RadiometryRadiometryIn optics, radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye...
- AlbedoAlbedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
- Hapke parameters
- Bidirectional reflectance distribution functionBidirectional reflectance distribution functionThe bidirectional reflectance distribution function is a four-dimensional function that defines how light is reflected at an opaque surface...