Astrophysics (Index)About

photometry

(studying starlight using optical filters)

In astronomy, the term photometry is used for the study of the brightness (magnitude) of sources (such as stars or other bodies), optionally using optical filters to measure magnitudes over specific spectral ranges, e.g., to determine color indices. The result of such observation may be termed the source's photometry.

This is as opposed to a more complete characterization of the body's light via spectroscopy, which requires a more specialized instrument. Photometry has the advantage of requiring less light (apparent magnitude), thus can be done at a greater distance and can be used with many more stars. It also is generally "multi-object", making it also much more efficient to survey many stars.

The filters' passbands have been standardized (photometric systems) to allow comparison and general use of photometric data.

Filters with different passband-widths (bandwidths) are used, according to need and light-availability, yielding some levels of spectral detail, but short of using full spectroscopy. For example, direct imaging of extra-solar planets offers limited light, making photometry more practical than spectroscopy. Terms used for some subtypes of photometry:

As photometry developed in the 20th century, electronic sensors were developed that are capable of capturing and counting a significant percentage of photons (quantum efficiency), first the photomultiplier tube (PMT) (a type of vacuum tube), and more recently, photosensitive solid state devices, often incorporated in CCDs. These practices are termed photoelectric photometry (PEP) and CCD photometry, but the former term is sometimes used to mean both. These terms are now rarely used because electronic sensors are ubiquitous in research astronomy.

Carrying out consistent, reproducible photometric measurements is a challenge, particularly from the ground, given the effects of Earth atmosphere (airmass). Variable measurements are in some sense not reproducible given constant variation, except that two different observers observing at the same time would aim to produce the same measurements. The methods to accomplish such consistency and accuracy are non-trivial. The aims of the techniques fall under two classes: attempts to measure on some absolute scale, and attempts to measure the relative magnitudes of a target star compared one or more others. The terminology used for these is not universally consistent.

Absolute photometry is clearly an attempt to get a photometric measurement on an absolute scale. Some writers reserve the term for direct comparison with lab sources of electromagnetic radiation, but others also include indirect comparison, comparing the target star with some star whose magnitude is very well established (a standard star aka standard source aka photometric standard star, whose magnitude may have been determined through direct comparison as above). The term relative photometry is sometimes use to distinguish the two above methods, to refer to the latter.

Differential photometry is the comparison of the photometry of a target source with one or more other sources. Comparisons of a variable star with a star that has very little variation which is in the same field, i.e., in the same portion of the celestial sphere, is sometimes better than attempts to translate the variable star's magnitude to an absolute scale; the accuracy of a single comparison can be better than the resulting accuracy from a chain of comparisons. The term relative photometry is sometimes used to mean such differential photometry.


(science,astronomy)
Further reading:
https://en.wikipedia.org/wiki/Photometry_(astronomy)
https://en.wikipedia.org/wiki/Photometric-standard_star
http://www.spaennare.se/PUBL/lic2.pdf
https://www.schoolsobservatory.org/learn/teach/techniques/photometry
https://www.gaia.ac.uk/sites/default/files/resources/Photometry_in_Astronomy.pdf
https://lco.global/spacebook/telescopes/what-is-photometry/
https://www.britannica.com/science/photometry-astronomy

Referenced by pages:
All Sky Automated Survey (ASAS)
All-Sky Compiled Catalogue (ASCC)
APASS
aperture photometry
astronomical catalog
Balmer-break galaxy (BBG)
band shifting
BATC
Bright Star Catalog (HR)
California-Kepler Survey (CKS)
Carlsberg Meridian Catalogue (CMC)
Carnegie Supernova Project (CSP)
Catalog of Azzopardi & Vigneau (AzV)
Cerro Tololo Inter-American Observatory (CTIO)
CFHTLS
CHEOPS
color index
cosmic dust
DAOPHOT
Dark Energy Survey (DES)
DECaLS
Deep Lens Survey (DLS)
Deep Multicolor Survey (DMS)
dropout
EPOXI
ESO Nearby Abell Cluster Survey (ENACS)
exoplanet eclipse light curve
Extreme Ultraviolet Explorer (EUVE)
extremely red object (ERO)
first galaxies
forward model
Gaia
galaxy age determination
galaxy SED
GPX
Guide Star Catalog (GSC)
Hawaii K-band Galaxy Survey
Herschel Space Observatory
IPHAS
IRAS
IRTF
jansky (Jy)
Jeans anisotropic modeling (JAM)
JHK photometric system (JHK)
kinemetry
line blanketing
Local Group Galaxy Survey (LGGS)
Lyman break (LB)
Magellanic Clouds Photometric Survey (MCPS)
Munich Near-Infrared Cluster Survey (MUNICS)
NOAO Deep Wide-field Survey (NDWFS)
Outer Solar System Origins Survey (OSSOS)
passband
photometer
photometric redshift (photo-z)
photometric system
photomultiplier tube (PMT)
PLATO
Qatar Exoplanet Survey (QES)
Rubin Observatory (VRO)
Second Byurakan Survey (SBS)
SHARDS
signal-to-noise ratio (SNR)
SIMSTACK
sky subtraction
SkyMapper Southern Survey (SMSS)
Sloan Digital Sky Survey (SDSS)
spectroscopic parallax
spectroscopy
stellar parameter determination
stellar temperature determination
Strömgren photometric system
Supernova Cosmology Project (SCP)
supernova light curve (SN light curve)
Swope Supernova Survey (SSS)
synthetic photometry
Sys-Rem
T4 Automated Photometric Telescope (T4 APT)
telluric star
templates
Type Ia supernova
UBV photometric system
ugriz photometric system
USNO Twin Astrograph
VIMOS Public Extragalactic Redshift Survey (VIPERS)
VLT Survey Telescope (VST)
Wide Field Infrared Explorer (WIRE)

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