# DSI Observatory Wiki

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photometry

'Photometry' is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation.<ref name=sterken_manfroid1992/> When photometry is performed over broad wavelength bands of radiation, where not only the amount of radiation but also its spectral distribution is measured, the term 'spectrophotometry' is used.

##### Etymology

The word “photometry” derives from the Greek phoz ($\phi \omega \zeta$; genitive photos ($\phi \omega \tau \omicron \varsigma$)) for light and metron ($\mu \acute{\epsilon} \tau \rho \omicron \nu$) for measure.

##### Methods

The methods used to perform photometry depend on the wavelength regime under study. At its most basic, photometry is conducted by gathering photon radiation (a.k.a. light) in a telescope, sometimes passing it through specialized optical filters (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 facilitate accurate comparison of observations.<ref>Warner, Brian (2006). A Practical Guide to Lightcurve Photometry and Analysis, Springer, ISBN 0-3872-9365-5</ref>

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 onto a photosensitive cell. 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.

#### CCD photometry

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, apparent, 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 be smeared (convolved) over many picture elements pixels according to the point spread function of the system. This broadening is due to the optics in the telescope as well as astronomical seeing (twinkling).When obtaining photometry from an object that's a point source (an object with an angular diameter that is much smaller than the angular resolution of the telescope), the flux is determined by adding up all the light recorded from the object and subtract the light due to the sky. The simplest technique, known as (synthetic) aperture photometry.,<ref name=mighell1999/> consists of adding up the pixel counts within a circle centered on the object (the aperture) and subtracting the quotient of the per-pixel average of nearby sky count divided by the number of pixels within the circle. 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 point spread function (PSF) fitting,<ref name=stetson1987/> to determine the individual flux values of the overlapping sources.

A number of computer programs are available for synthetic aperture photometry and PSF-fitting photometry, in some cases at no cost. Aperture Photometry Tool is an example of a recently developed program,<ref name=laher2012/> which has a graphical user interface, can form an elliptical aperture, and has a number of powerful analysis tools. It can be downloaded free of charge at [http://www.aperturephotometry.org aperturephotometry.org].

#### Calibrations

After determining the flux of an object in counts, the flux is normally converted into instrumental magnitude. Next, the measurement must be 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 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 curve 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 or 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 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.

##### Applications

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,<ref>North, Gerald, (2004). Observing Variable Stars, Novae and Supernovae, Cambridge, ISBN 0-521-82047-2</ref> 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.