Photometry (astronomy)

Last updated
Kepler Mission space photometer Kepler Mission Space Photometer smaller.jpg
Kepler Mission space photometer

In astronomy, 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. [1] 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.

Contents

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. [2] A more advanced technique is spectrophotometry that is measured with a spectrophotometer and observes both the amount of radiation and its detailed spectral distribution. [3]

Photometry is also used in the observation of variable 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] or relative photometry by comparing the brightness of the target object to stars with known fixed magnitudes. [6] 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. [7] Precision photoelectric photometers can measure starlight around 0.001 magnitude. [8]

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. [9] 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.

Methods

Eta Carinae light curve in several different passbands Eta Carinae lightcurve at multiple wavelengths (1987 - 2014).png
Eta Carinae light curve in several different passbands

Astronomy was among the earliest applications of photometry. Modern photometers use specialised standard passband filters across the ultraviolet, visible, and infrared wavelengths of the electromagnetic spectrum. [4] 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. [10] Several important systems are regularly used, such as the UBV system [11] (or the extended UBVRI system [12] ), near infrared JHK [13] or the Strömgren uvbyβ system. [10]

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. [4] These have largely been replaced with CCD 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]

Magnitudes and colour indices

Modern photometric methods define magnitudes and colours of astronomical objects using electronic photometers viewed through standard coloured bandpass filters. This differs from other expressions of apparent 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] while photographic 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 star 51 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] For 51 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 in colour–magnitude diagrams, which for stars is the observed version of the Hertzsprung-Russell diagram. Typically photometric measurements of multiple objects obtained through two filters will show, for example in an open cluster, [24] the comparative stellar evolution between the component stars or to determine the cluster's relative age. [25]

Due to the large number of different photometric 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 by Gaia are 'G' [26] (with the blue and red photometric filters, GBP and GRP [27] ) or the Strömgren 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 and interstellar extinction. [28] Strömgren allows calculation of parameters from the b and y filters (colour index of b − y) without the effects of reddening, as the indices m 1 and c 1. [28]

Applications

AERONET photometer AERONET sunphotometer.jpg
AERONET photometer

There are many astronomical applications used with photometric systems. 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 also be determined via broad or narrow-band spectrophotometry.

Photometry is also used to study the light variations of objects such as variable stars, minor planets, active galactic nuclei and supernovae, [7] 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 supernovae. [7]

CCD photometry

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 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. [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 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. [31]

Calibrations

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. [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 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.

Absolute photometry

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.

Relative photometry

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.

Differential photometry

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 a light curve. [5]

Surface photometry

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. [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 square arcsecond, and surface brightness is often expressed in magnitudes per square arcsecond. The diameter of galaxies are often defined by the size of the 25th magnitude isophote in the blue B-band. [33]

Forced photometry

In forced photometry, measurements are conducted at a specified location rather than for a specified object. It is "forced" in the sense that a measurement can be taken even if there is no object visible (in the spectral 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]

Software

A number of free computer programs are available for synthetic aperture photometry and PSF-fitting photometry.

SExtractor [37] and Aperture 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]

Organizations

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):

See also

Related Research Articles

<span class="mw-page-title-main">Apparent magnitude</span> Brightness of a celestial object observed from the Earth

Apparent magnitude is a measure of the brightness of a star or other astronomical object. An object's apparent magnitude depends on its intrinsic luminosity, its distance, and any extinction of the object's light caused by interstellar dust along the line of sight to the observer.

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.

<span class="mw-page-title-main">Observational astronomy</span> Division of astronomy

Observational astronomy is a division of astronomy that is concerned with recording data about the observable universe, in contrast with theoretical astronomy, which is mainly concerned with calculating the measurable implications of physical models. It is the practice and study of observing celestial objects with the use of telescopes and other astronomical instruments.

Joel Stebbins was an American astronomer who pioneered photoelectric photometry in astronomy. He was director of the University of Illinois Observatory from 1903 to 1922 where he performed innovative work with the selenium cell. In 1922 he became director of the Washburn Observatory at the University of Wisconsin–Madison where he remained until 1948. After 1948, Stebbins continued his research at Lick Observatory until his final retirement in 1958.

<i>Gaia</i> (spacecraft) European optical space observatory for astrometry

Gaia is a space observatory of the European Space Agency (ESA), launched in 2013 and expected to operate until 2025. The spacecraft is designed for astrometry: measuring the positions, distances and motions of stars with unprecedented precision, and the positions of exoplanets by measuring attributes about the stars they orbit such as their apparent magnitude and color. The mission aims to construct by far the largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars, among others.

<span class="mw-page-title-main">High Speed Photometer</span> Scientific instrument on the Hubble Space Telescope

The High Speed Photometer (HSP) is a scientific instrument formerly installed on the Hubble Space Telescope. The HSP was designed to measure the brightness and polarity of rapidly varying celestial objects. It could observe in ultraviolet, visible light, and near infrared at a rate of one measurement per 10 microseconds. The design was novel in that despite being able to view through a variety of filters and apertures, it had no moving parts "except for electrons" as principal investigator Prof. Robert Bless was fond of saying. Filter and aperture selection was accomplished using image dissector tubes and the HST pointing system. It was functional from launch in 1990 until it was removed at the end of 1993, and it helped diagnose an issue with the Hubble's primary mirror.

In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on its surface luminosity density, i.e., its luminosity emitted per unit surface area. In visible and infrared astronomy, surface brightness is often quoted on a magnitude scale, in magnitudes per square arcsecond (MPSAS) in a particular filter band or photometric system.

The Astronomical Observatory of Belogradchik or Belogradchik Observatory is an astronomical observatory owned and operated by the Institute of Astronomy of the Bulgarian Academy of Sciences. It is located near the town of Belogradchik in northwestern Bulgaria, at the foot of the Western Balkan Mountains. The other observatory operated by the same institute is the Rozhen Observatory.

<span class="mw-page-title-main">Magnitude (astronomy)</span> Logarithmic measure of the brightness of an astronomical object

In astronomy, magnitude is measure of the brightness of an object, usually in a defined passband. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.

Psi Tauri, which is Latinized from ψ Tauri, is a solitary star in the zodiac constellation of Taurus. It has a yellow-white hue and is visible to the naked eye with an apparent visual magnitude of +5.22. The distance to this system, as determined using an annual parallax shift of 36.2 mas as seen from the Earth, is 90 light years. It is drifting further away with a radial velocity of +9 km/s.

<span class="mw-page-title-main">Iota Trianguli Australis</span> Binary star in the constellation Triangulum Australe

Iota Trianguli Australis is a binary star system in the constellation Triangulum Australe. It is visible to the naked eye with a combined apparent visual magnitude of +5.27. Based upon an annual parallax shift of 25.77 mas as seen from the Earth, it is located around 127 light years from the Sun. The system appears to be moving closer to the Sun with a radial velocity of around −6 km/s.

Photographic magnitude is a measure of the relative brightness of a star or other astronomical object as imaged on a photographic film emulsion with a camera attached to a telescope. An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction of light by interstellar matter existing along the line of sight to the observer.

<span class="mw-page-title-main">SkyMapper</span>

SkyMapper is a fully automated 1.35 m (4.4 ft) wide-angle optical telescope at Siding Spring Observatory in northern New South Wales, Australia. It is one of the telescopes of the Research School of Astronomy and Astrophysics of the Australian National University (ANU). The telescope has a compact modified Cassegrain design with a large 0.69 m secondary mirror, which gives it a very wide field of view: its single, dedicated instrument, a 268-million pixel imaging camera, can photograph 5.7 square degrees of sky. The camera has six light filters which span from ultraviolet to near infrared wavelengths.

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.

<span class="mw-page-title-main">UBV photometric system</span>

The UBV photometric system, also called the Johnson system, is a photometric system usually employed for classifying stars according to their colors. It was the first standardized photometric system. The apparent magnitudes of stars in the system are often used to determine the color indices B−V and U−B, the difference between the B and V magnitudes and the U and B magnitudes respectively.

The Strömgren photometric system, abbreviated also as uvbyβ or simply uvby, and sometimes referred as Strömgren - Crawford photometric system, is a four-colour medium-passband photometric system plus Hβ (H-beta) filters for determining magnitudes and obtaining spectral classification of stars. Its use was pioneered by the Danish astronomer Bengt Strömgren in 1956 and was extended by his colleague the American astronomer David L. Crawford in 1958.

<span class="mw-page-title-main">Astronomical Observatory (University of Illinois Urbana-Champaign)</span> United States historic place

The University of Illinois Astronomical Observatory, located at 901 S. Mathews Avenue in Urbana, Illinois, on the campus of the University of Illinois Urbana–Champaign, was built in 1896, and was designed by Charles A. Gunn. It was listed on the National Register of Historic Places on November 6, 1986, and on December 20, 1989, was designated a National Historic Landmark.

A photometric redshift is an estimate for the recession velocity of an astronomical object such as a galaxy or quasar, made without measuring its spectrum. The technique uses photometry to determine the redshift, and hence, through Hubble's law, the distance, of the observed object.

Gerald Kron was an American astronomer who was one of the pioneers of high-precision photometry with photoelectric instrumentation. He discovered the first starspot and made the first photometric observation of a stellar flare.

References

  1. ↑ Casagrande, Luca; VandenBerg, Don A (2014). "Synthetic stellar photometry - General considerations and new transformations for broad-band systems". Monthly Notices of the Royal Astronomical Society. 444 (1). Oxford University Press: 392–419. arXiv: 1407.6095 . Bibcode:2014MNRAS.444..392C. doi: 10.1093/mnras/stu1476 .
  2. ↑ Brian D. Warner (20 June 2016). A Practical Guide to Lightcurve Photometry and Analysis. Springer. ISBN   978-3-319-32750-1.
  3. ↑ C.R. Kitchin (1 January 1995). Optical Astronomical Spectroscopy. CRC Press. pp. 212–. ISBN   978-1-4200-5069-1.
  4. 1 2 3 4 5 Miles, R. (2007). "A light history of photometry: from Hipparchus to the Hubble Space Telescope". Journal of the British Astronomical Association. 117: 178–186. Bibcode:2007JBAA..117..172M.
  5. 1 2 3 Kern, J.~R.; Bookmyer, B.~B. (1986). "Differential photometry of HDE 310376, a rapid variable star". Publications of the Astronomical Society of the Pacific. 98: 1336–1341. Bibcode:1986PASP...98.1336K. doi: 10.1086/131940 .
  6. 1 2 Husárik, M. (2012). "Relative photometry of the possible main-belt comet (596) Scheila after an outburst". Contributions of the Astronomical Observatory Skalnaté Pleso. 42 (1): 15–21. Bibcode:2012CoSka..42...15H.
  7. 1 2 3 4 North, G.; James, N. (21 August 2014). Observing Variable Stars, Novae and Supernovae. Cambridge University Press. ISBN   978-1-107-63612-5.
  8. ↑ "Overview: Photoelectric photometer". Oxford University Press. Retrieved 20 May 2019.
  9. 1 2 Palei, A.B. (August 1968). "Integrating Photometers". Soviet Astronomy. 12: 164. Bibcode:1968SvA....12..164P.
  10. 1 2 3 4 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.
  11. ↑ 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.
  12. ↑ 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 .
  13. ↑ Hewett, P.C.; Warren, S.J.; Leggett, S.K.; Hodgkin, S.T. (2006). "The UKIRT Infrared Deep Sky Survey ZY JHK photometric system: passbands and synthetic colours". Monthly Notices of the Royal Astronomical Society. 367 (2): 454–468. arXiv: astro-ph/0601592 . Bibcode:2006MNRAS.367..454H. doi: 10.1111/j.1365-2966.2005.09969.x .
  14. ↑ CSIRO Astronomy and Space Science (2015). "Photoelectric Astronomy". CSIRO : Australian Telescope National Facility. Retrieved 21 May 2019.
  15. ↑ Walker, E.W. "CCD Photometry". British Astronomical Association. Retrieved 21 May 2019.
  16. 1 2 MacRobert, A. (1 August 2006). "The Stellar Magnitude System". Sky and Telescope. Retrieved 21 May 2019.
  17. 1 2 Norton, A.P. (1989). Norton's 2000.0 : Star Atlas and Reference Handbook . Longmore Scientific. p.  133. ISBN   0-582-03163-X.
  18. ↑ Cayrel de Strobel, G. (1996). "Stars resembling the Sun". Astronomy and Astrophysics Review. 7 (3): 243–288. Bibcode:1996A&ARv...7..243C. doi:10.1007/s001590050006. S2CID   189937884.
  19. 1 2 "51 Peg". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 22 May 2019.
  20. 1 2 CSIRO Astronomy and Space Science (2002). "The Colour of Stars". CSIRO : Australian Telescope National Facility. Retrieved 21 May 2019.
  21. ↑ Keenan, R.C.; McNeil, P.C. (1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars". The Astrophysical Journal Supplement Series. 71: 245–266. Bibcode:1989ApJS...71..245K. doi:10.1086/191373. S2CID   123149047.
  22. ↑ Luciuk, M. "Astronomical Magnitudes" (PDF). p. 2. Retrieved 22 May 2019.
  23. ↑ Mittag, M.; Schröder, K.-P.; Hempelmann, A.; González-Pérez, J.N.; Schmitt, J.H.M.M. (2016). "Chromospheric activity and evolutionary age of the Sun and four solar twins". Astronomy & Astrophysics. 591: A89. arXiv: 1607.01279 . Bibcode:2016A&A...591A..89M. doi:10.1051/0004-6361/201527542. S2CID   54765864.
  24. ↑ Littlefair, S. (2015). "PHY217 Observational Techniques for Astronomers : P05: Absolute Photometry". University of Sheffield : Department of Physics and Astronomy. Archived from the original on 13 September 2019. Retrieved 24 May 2019.
  25. ↑ James, A. (19 April 2017). "Open Star Clusters : 8 of 10 : Evolution of Open Star Clusters". Southern Astronomical Delights. Retrieved 20 May 2019.
  26. ↑ Jordi, C.; Gebran, M.; Carrasco, J.~M.; de Bruijne, J.; Voss, H.; Fabricius, C.; Knude, J.; Vallenari, A.; Kohley, R.; More, A. (2010). "Gaia broad band photometry". Astronomy and Astrophysics. 523: A48. arXiv: 1008.0815 . Bibcode:2010A&A...523A..48J. doi:10.1051/0004-6361/201015441. S2CID   34033669.
  27. ↑ "Expected Nominal Mission Science Performance". GAIA :European Space Agency. 16 March 2019. Retrieved 23 May 2019.
  28. 1 2 Paunzen, 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. S2CID   73623700.
  29. 1 2 Mighell, K.J. (1999). "Algorithms for CCD Stellar Photometry". ASP Conference Series . 172: 317–328. Bibcode:1999ASPC..172..317M.
  30. ↑ Laher, R.R.; et al. (2012). "Aperture Photometry Tool" (PDF). Publications of the Astronomical Society of the Pacific . 124 (917): 737–763. Bibcode:2012PASP..124..737L. doi:10.1086/666883. S2CID   21572643.
  31. 1 2 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 .
  32. ↑ Gerald R. Hubbell (9 November 2012). Scientific Astrophotography: How Amateurs Can Generate and Use Professional Imaging Data. Springer Science & Business Media. ISBN   978-1-4614-5173-0.
  33. ↑ Sparke, L. S.; Gallagher, J. S. III (2000). Galaxies in the Universe: An Introduction. Cambridge University Press. ISBN   978-0-521-59740-1. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  34. ↑ "PS1 Forced photometry of sources - PS1 Public Archive - STScI Outerspace".
  35. ↑ Makrygianni, L.; Mullaney, J.; Dhillon, V.; et al. (2021). "Processing GOTO survey data with the Rubin Observatory LSST Science Pipelines II: Forced Photometry and lightcurves". Publications of the Astronomical Society of Australia. 38. arXiv: 2105.05128 . Bibcode:2021PASA...38...25M. doi:10.1017/pasa.2021.19.
  36. ↑ Burenin, R. A. (2022). "Forced Photometry for Pan-STARRS1 Objects Based on WISE Data". Astronomy Letters. 48 (3): 153–162. Bibcode:2022AstL...48..153B. doi:10.1134/S1063773722030021. S2CID   253022975.
  37. ↑ "SExtractor – Astromatic.net". www.astromatic.net.
  38. ↑ "Aperture Photometry Tool: Home". www.aperturephotometry.org.
  39. ↑ "aavso.org". www.aavso.org.
  40. ↑ "Exoplanet - Amateur Detection". astronomyonline.org.
  41. ↑ "CBA @ cbastro.org - Center for Backyard Astrophysics". www.cbastro.org.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.