Inastronomy,photometry, fromGreekphoto- ("light") and-metry ("measure"), is a technique used inastronomy that is concerned withmeasuring theflux orintensity of light radiated byastronomical objects.[1] This light is measured through atelescope using aphotometer, often made using electronic devices such as aCCD photometer or a photoelectric photometer that converts light into an electric current by thephotoelectric effect. When calibrated againststandard stars (or other light sources) of known intensity and colour, photometers can measure the brightness orapparent 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 photometricoptical bandpass filters, and then capturing and recording the light energy with a photosensitive instrument. Standard sets ofpassbands (called aphotometric system) are defined to allow accurate comparison of observations.[2] A more advanced technique isspectrophotometry that is measured with aspectrophotometer and observes both the amount of radiation and its detailedspectral distribution.[3]
Photometry is also used in the observation ofvariable stars,[4] by various techniques such as,differential photometry that simultaneously measures the brightness of a target object and nearby stars in the starfield[5] orrelative photometry by comparing the brightness of the target object to stars with known fixed magnitudes.[6] Using multiple bandpass filters with relative photometry is termedabsolute photometry. A plot of magnitude against time produces alight curve, yielding considerable information about the physical process causing the brightness changes.[7] Precision photoelectric photometers can measure starlight around 0.001 magnitude.[8]
The technique ofsurface photometry can also be used with extended objects likeplanets,comets,nebulae orgalaxies that measures the apparent magnitude in terms of magnitudes per square arcsecond.[9] Knowing the area of the object and the average intensity of light across the astronomical object determines thesurface 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 orluminosity per unit surface area.
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Astronomy was among the earliest applications of photometry. Modernphotometers use specialised standardpassband filters across theultraviolet,visible, andinfrared wavelengths of theelectromagnetic spectrum.[4] Any adopted set of filters with knownlight transmission properties is called aphotometric system, and allows the establishment of particular properties about stars and other types of astronomical objects.[10] Several important systems are regularly used, such as theUBV system[11] (or the extended UBVRI system[12]),near infrared JHK[13] or theStrömgrenuvbyβ system.[10]
Historically, photometry in the near-infrared through short-wavelengthultra-violet was done with a photoelectric photometer, an instrument that measured the light intensity of a single object by directing its light onto a photosensitive cell like aphotomultiplier tube.[4] These have largely been replaced withCCD cameras that can simultaneously image multiple objects, although photoelectric photometers are still used in special situations,[14] such as where fine time resolution is required.[15]
Modern photometric methods define magnitudes and colours of astronomical objects using electronic photometers viewed through standard coloured bandpass filters. This differs from other expressions ofapparent visual magnitude[7] observed by the human eye or obtained by photography:[4] that usually appear in older astronomical texts and catalogues.
Magnitudes measured by photometers in some commonplace photometric systems (UBV, UBVRI or JHK) are expressed with a capital letter, such as "V" (mV) or "B" (mB). Other magnitudes estimated by the human eye are expressed using lower case letters, such as "v", "b" or "p", etc.[16] E.g. Visual magnitudes as mv,[17] whilephotographic magnitudes are mph / mp or photovisual magnitudes mp or mpv.[17][4] Hence, a 6th magnitude star might be stated as 6.0V, 6.0B, 6.0v or 6.0p. Because starlight is measured over a different range of wavelengths across the electromagnetic spectrum and are affected by different instrumental photometric sensitivities to light, they are not necessarily equivalent in numerical value.[16] For example, apparent magnitude in the UBV system for the solar-like star51 Pegasi[18] is 5.46V, 6.16B or 6.39U,[19] corresponding to magnitudes observed through each of the visual 'V', blue 'B' or ultraviolet 'U' filters.
Magnitude differences between filters indicate colour differences and are related to temperature.[20] Using B and V filters in the UBV system produces the B–V colour index.[20] For51 Pegasi, the B–V = 6.16 – 5.46 = +0.70, suggesting a yellow coloured star that agrees with its G2IV spectral type.[21][19] Knowing the B–V results determines the star's surface temperature,[22] finding an effective surface temperature of 5768±8 K.[23]
Another important application of colour indices is graphically plotting star's apparent magnitude against the B–V colour index. This forms the important relationships found between sets of stars incolour–magnitude diagrams, which for stars is the observed version of theHertzsprung-Russell diagram. Typically photometric measurements of multiple objects obtained through two filters will show, for example in anopen cluster,[24] the comparativestellar evolution between the component stars or to determine the cluster's relative age.[25]
Due to the large number of differentphotometric systems adopted by astronomers, there are many expressions of magnitudes and their indices.[10] Each of these newer photometric systems, excluding UBV, UBVRI or JHK systems, assigns an upper or lower case letter to the filter used. For example, magnitudes used byGaia are 'G'[26] (with the blue and red photometric filters, GBP and GRP[27]) or theStrömgren photometric system having lower case letters of 'u', 'v', 'b', 'y', and two narrow and wide 'β' (Hydrogen-beta) filters.[10] Some photometric systems also have certain advantages. For example,Strömgren photometry can be used to measure the effects of reddening andinterstellar extinction.[28] Strömgren allows calculation of parameters from theb andy filters (colour index ofb − y) without the effects of reddening, as the indices m 1 and c 1.[28]
There are many astronomical applications used with photometric systems. Photometric measurements can be combined with theinverse-square law to determine theluminosity of an object if itsdistance can be determined, or its distance if its luminosity is known. Other physical properties of an object, such as itstemperature or chemical composition, may also be determined via broad or narrow-band spectrophotometry.
Photometry is also used to study the light variations of objects such asvariable stars,minor planets,active galactic nuclei andsupernovae,[7] or to detecttransiting extrasolar planets. Measurements of these variations can be used, for example, to determine theorbital period and theradii of the members of aneclipsing binary star system, therotation period of a minor planet or a star, or the total energy output of supernovae.[7]
A CCD (charge-coupled device) camera is essentially a grid of photometers, simultaneously measuring and recording the photons coming from all the sources in the field of view. Because each CCD image records the photometry of multiple objects at once, various forms of photometric extraction can be performed on the recorded data; typically relative, absolute, and differential. All three will require the extraction of the raw imagemagnitude of the target object, and a known comparison object. The observed signal from an object will typically cover manypixels according to thepoint spread function (PSF) of the system. This broadening is due to both the optics in the telescope and theastronomical seeing. When obtaining photometry from apoint source, the flux is measured by summing all the light recorded from the object and subtracting the light due to the sky.[29] The simplest technique, known as aperture photometry, consists of summing the pixel counts within an aperture centered on the object and subtracting the product of the nearby average sky count per pixel and the number of pixels within the aperture.[29][30] This will result in the raw flux value of the target object. When doing photometry in a very crowded field, such as aglobular cluster, where the profiles of stars overlap significantly, one must use de-blending techniques, such as PSF fitting to determine the individual flux values of the overlapping sources.[31]
After determining the flux of an object in counts, the flux is normally converted intoinstrumental magnitude. Then, the measurement is calibrated in some way. Which calibrations are used will depend in part on what type of photometry is being done. Typically, observations are processed for relative or differential photometry.[32]Relative photometry is the measurement of the apparent brightness of multiple objects relative to each other. Absolute photometry is the measurement of the apparent brightness of an object on astandard photometric system; these measurements can be compared with other absolute photometric measurements obtained with different telescopes or instruments. Differential photometry is the measurement of the difference in brightness of two objects. In most cases, differential photometry can be done with the highestprecision, while absolute photometry is the most difficult to do with high precision. Also, accurate photometry is usually more difficult when theapparent brightness of the object is fainter.
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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 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 ofphotometric 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 theairmass.
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To perform relative photometry, one compares the instrument magnitude of the object to a known comparison object, and then corrects the 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.[6] When doing the calibration from an image that contains both the target and comparison objects in close proximity, and using a photometric filter that matches the catalog magnitude of the comparison object most of the measurement variations decrease to null.
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Differential photometry is the simplest of the calibrations and most useful for time series observations.[5] When using CCD photometry, both the target and comparison objects are observed at the same time, with the same filters, using the same instrument, and viewed through the same optical path. Most of the observational variables drop out and the differential magnitude is simply the difference between the instrument magnitude of the target object and the comparison object (∆Mag = C Mag – T Mag). This is very useful when plotting the change in magnitude over time of a target object, and is usually compiled into alight curve.[5]
For spatially extended objects such asgalaxies, it is often of interest to measure the spatial distribution of brightness within the galaxy rather than simply measuring the galaxy's total brightness. An object'ssurface brightness is its brightness per unitsolid angle as seen in projection on the sky, and measurement of surface brightness is known as surface photometry.[9] A common application would be measurement of a galaxy's surface brightness profile, meaning its surface brightness as a function of distance from the galaxy's center. For small solid angles, a useful unit of solid angle is the squarearcsecond, and surface brightness is often expressed in magnitudes per square arcsecond. The diameter of galaxies are often defined by the size of the 25th magnitudeisophote in the blue B-band.[33]
Inforced photometry, measurements are conducted at a specifiedlocation rather than for a specifiedobject. It is "forced" in the sense that a measurement can be taken even if there is no object visible (in thespectral band of interest) in the location being observed. Forced photometry allows extracting a magnitude, or an upper limit for the magnitude, at a chosen sky location.[34][35][36]
A number of free computer programs are available for synthetic aperture photometry and PSF-fitting photometry.
SExtractor[37] andAperture Photometry Tool[38] are popular examples for aperture photometry. The former is geared towards reduction of large scale galaxy-survey data, and the latter has a graphical user interface (GUI) suitable for studying individual images.DAOPHOT is recognized as the best software for PSF-fitting photometry.[31]
There are a number of organizations, from professional to amateur, that gather and share photometric data and make it available on-line. Some sites gather the data primarily as a resource for other researchers (ex. AAVSO) and some solicit contributions of data for their own research (ex. CBA):
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