Photometry, from Greek photo- ("light") and -metry ("measure"), is an instrumental 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, and are commonly expressed as photometric magnitudes.
The methods used to perform photometry depend on the wavelength regime under study. At its most basic, photometry is mainly 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.
Photometry is also used in the observation of variable stars because it produces more sensitive and accurate magnitude estimates compared to the human eye. 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. When photometry is spread across a range of wavelengths within the electromagnetic spectrum it is called spectrophotometry and measured with a spectrophotometer : measuring both the amount of radiation and its spectral distribution.
Photometers employ the use of specialised standard passband filters across the ultraviolet, visible, and infrared wavelengths of the electromagnetic spectrum. Any adopted set of filters with known light transmission properties is called a photometric system, and allows the establishment of particular properties about stars and other types of astronomical objects. Several important systems are regularly used, such as the UBV system (or the extended UBVRI system) or the Strömgren uvbyβ system.
Historically, photometry in the near-infrared through short-wavelength ultra-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 a photomultiplier tube. These have largely been replaced with CCD cameras that can simultaneously image multiple objects, although photoelectric photometers are still used in special situations, such as where fine time resolution is required.
Photometric magnitudes and indiciesEdit
Photometric magnitudes are the brightness of astronomical objects as measured by instrumental photometric methods. This differs from other expressions of apparent visual magnitude observed by the human eye that usually appear in older astronomical texts and catalogues. Photometric magnitudes are expressed with a capital letter e.g. 'V', "B', etc, while visual (or photographic magnitudes) are expressed in lower case letters. e.g. "v", "b" or "p", etc. Hence, a 6th magnitude star may be expressed as 6.0V, 6.0B, 6.0v or 6.0p. Because they are measured across different range of wavelengths across the electromagnetic spectrum and different instrumental photometric sensitivities to light, they are not necessarily equivalent in numerical value. For example, apparent magnitude in the UBV system could be for a 10th magnitude star as either as 10.0V, 10.5B or 9.6U, corresponding to magnitudes observed through each of the visual 'V', blue 'B' or ultraviolet 'U' filters.
Magnitude differences between the B and V filters, or index (B–V), indicate colour, and in the example above, suggest a yellow coloured star. There are many applications used with such photometric systems. e.g. A plot of the apparent magnitude against the (B–V) star colours forms the important relation of colour–magnitude diagram, showing comparative stellar evolution between open clusters and determination of their relative ages. Due to the large number of different photometric systems adopted by astronomers, there are many expressions of photometric magnitudes and their indices. Each system assigns an upper or lower case letter to the filter used. e.g. Photometric magnitudes in the near infrared are J, H and K or in the Strömgren photometric system have the letters of u, v, b, y, and a narrow and wide β filter. (This can sometimes be confusing as 'v' could mean a simple visual magnitude or 'v' or as a Strömgren filter centred on 411 nm.)
Some uses of photometric systems also have certain advantages. Strömgren photometry can be used to measure the effects of reddening and interstellar extinction. Strömgren allows calculation of parameters from the b and y filters (b − y) without the effects of reddening, as the indices m 1 and c 1.
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A CCD 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 image magnitude of the target object, and a known comparison object. The observed signal from an object will typically cover many pixels according to the point spread function (PSF) of the system. This broadening is due to both the optics in the telescope and the astronomical seeing. When obtaining photometry from a point source, the flux is measured by summing all the light recorded from the object and subtracting the light due to the sky. 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. This will result in the raw flux value of the target object. When doing photometry in a very crowded field, such as a globular 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.
After determining the flux of an object in counts, the flux is normally converted into instrumental 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. 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 a standard 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 highest precision, while absolute photometry is the most difficult to do with high precision. Also, accurate photometry is usually more difficult when the apparent brightness of the object is fainter.
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 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 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. 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.
Differential photometry is the simplest of the calibrations and most useful for time series observations. 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 a light curve.
For spatially extended objects such as galaxies, 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's surface brightness is its brightness per unit solid angle as seen in projection on the sky, and measurement of surface brightness is known as surface photometry. 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 square arcsecond, and surface brightness is often expressed in magnitudes per square arcsecond.
Photometric measurements can be combined with the inverse-square law 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 stars, minor planets, active galactic nuclei and supernovae, 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.
A number of free computer programs are available for synthetic aperture photometry and PSF-fitting photometry.
SExtractor and Aperture Photometry Tool 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.
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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- Warner, Brian (2006). A Practical Guide to Lightcurve Photometry and Analysis, Springer, ISBN 0-3872-9365-5
- "Overview: Photoelectric photometer". Oxford University Press. Retrieved 20 May 2019.
- Bessell, M.S. (September 2005). "Standard Photometric Systems" (PDF). Annual Review of Astronomy and Astrophysics. 43 (1): 293–336. Bibcode:2005ARA&A..43..293B. doi:10.1146/annurev.astro.41.082801.100251. ISSN 0066-4146.
- Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas". The Astrophysical Journal. 117 (3): 313–352. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
- Landolt, A.U. (1 July 1992). "UBVRI photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator". The Astronomical Journal. 104: 340–371. Bibcode:1992AJ....104..340L. doi:10.1086/116242.
- James. A. (19 April 2017). "Open Star Clusters : 8 of 10 : Evolution of Open Star Clusters". Southern Astronomical Delights. Retrieved 20 May 2019.
- Paunxen, E. (2015). "A new catalogue of Strömgren-Crawford uvbyβ photometry". Astronomy and Astrophysics. 580: A23. arXiv:1506.04568. Bibcode:2015A&A...580A..23P. doi:10.1051/0004-6361/201526413.
- Mighell, K.J. (1999). "Algorithms for CCD Stellar Photometry". ASP Conference Series. 172: 317–328.
- Laher, Russ R.; et al. (2012). "Aperture Photometry Tool". Publications of the Astronomical Society of the Pacific. 124 (917): 737–763. Bibcode:2012PASP..124..737L. doi:10.1086/666883.
- Stetson, P.B. (1987). "DAOPHOT: A Computer Program for Crowded-Field Stellar Photometry". Publications of the Astronomical Society of the Pacific. 99: 191–222. Bibcode:1987PASP...99..191S. doi:10.1086/131977.
- Hubbell, Gerald R. (2013). Scientific Astrophotography: How Amateurs Can Generate and Use Professional Imaging Data: 264-266. Springer, ISBN 978-1-4614-5173-0
- North, Gerald, (2004). Observing Variable Stars, Novae and Supernovae, Cambridge, ISBN 0-521-82047-2
- "SExtractor – Astromatic.net". www.astromatic.net.
- "Aperture Photometry Tool: Home". www.aperturephotometry.org.
- "aavso.org". www.aavso.org.
- "Exoplanet - Amateur Detection". astronomyonline.org.
- "CBA @ cbastro.org - Center for Backyard Astrophysics". www.cbastro.org.