Photometry (astronomy)
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.
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.
A plot of magnitude against time produces a light curve, yielding considerable information about the physical process causing the brightness changes.
Modern photometers use specialised standard passband filters across the ultraviolet, visible, and infrared wavelengths of the electromagnetic spectrum.
[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.
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).
[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.
[25] Due to the large number of different photometric systems adopted by astronomers, there are many expressions of magnitudes and their indices.
Other physical properties of an object, such as its temperature or chemical composition, may also be determined via broad or narrow-band spectrophotometry.
[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.
The observed signal from an object will typically cover many pixels according to the point spread function (PSF) of the system.
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.
[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.
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.
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.
[31] There are a number of organizations, from professional to amateur, that gather and share photometric data and make it available on-line.
