What is Photometry: Definition and 29 Discussions

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. 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.
The methods used to perform photometry depend on the wavelength regime 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. A more advanced technique is spectrophotometry that is measured with a spectrophotometer and observes both the amount of radiation and its detailed spectral distribution.Photometry is also used in the observation of variable stars, by various techniques such as, differential photometry that simultaneously measures the brightness of a target object and nearby stars in the starfield or relative photometry by comparing the brightness of the target object to stars with known fixed magnitudes. 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. Precision photoelectric photometers can measure starlight around 0.001 magnitude.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. 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.

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  1. F

    A Coefficient correlation between 2 cosmological probes

    Hello, I have the demonstration below. A population represents the spectroscopic proble and B the photometric probe. I would like to know if, from the equation (13), the correlation coeffcient is closed to 0 or to 1 since I don't know if ##\mathcal{N}_{\ell}^{A}## Poisson noise of spectroscopic...
  2. yucheng

    I Stromgren photometric data in Vizier

    I would like to try the photometric transform given in my book, for instance ##V = y - 0.12[(b-y) - 0.55]^2##, however, the two catalogs I've consulted, Paunzen, 2015 and Hauck, 1997 only provides the indices. Do these catalogs provide the uvby magnitudes as well, just hidden somewhere? Are...
  3. C

    Calculating Magnitude Difference: Is it Right?

    I have found the difference in the magnitude from the counts to be -.073 using -2.5log(11347/10606) but I m unsure if this is right or how to calculate the uncertainty
  4. L

    Extinction correction (photometry)

    For example suppose the uncorrected extinction instrumental magnitude is ##v^A_V = 9.00##, will the corrected extinction instrumental magnitude always be greater than ##9##?
  5. F

    A Cross correlations with 2 probes: Approximation of a 2D + 3D synthesis

    I am interested, in the context of my work, in the cross correlations between a spectroscopic probe (which gives a 3D distribution of galaxies with redshifts, which is also called spectroscopic Galaxy clustering, GCsp) and a photometric probe (which gives an angular distribution, that is to say...
  6. K

    Color Index and Intrinsic Magnitude of a Star

    I first determined ##E_{U - B}## by using the second equation listed above: $$U - B = (U - B)_0 + E_{U - B}$$ $$8.15 - 8.50 = -0.45 + E_{U - B}$$ Then since the ratio to the B-V color excess is known, I solved for that and obtained ##E_{B - V} = 0.14##. Using the ratio of total to selective...
  7. A

    I Calculate Retinal irradiance - from photometric units

    I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system. Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a monochromatic one (using the 460 nm peak). From the datasheet, the LED intensity range goes from 6...
  8. L

    I Transit Photometry: Questions about Detecting Exoplanets

    Hello everyone, As my first post in this forum, i want to ask two questions, mainly out of personal interest, concerning the possibilities of transit photometry in detecting exoplanets: For larger planets or planets orbiting farther away from the host star, the measured drop of intensity...
  9. R

    Mathematics necessary for Radiometry, Photometry, Thermodynamics

    What are the math concepts I have to learn for Radiometry, Photometry and Thermodynamics (all Calculus-based) as applied in building science (engineering, architecture, etc.). I'm almost done with Multivariable Calculus and I'm aware that MV Calculus is necessary, but what specific calculus...
  10. Q

    A Contrast-Calculation between two planes

    Hi there, I am new to the Forum! I have a little bit of a tricky Problem. I am trying to calculate the Contrast between two planes as seen here: So I started by cutting the Problem in two Parts. Part 1, The Monitor: I know the Luminance of the Monitor X, and the reflectivity of the...
  11. R

    Learning Radiometry, Photometry, Optics, Thermodynamics, ....

    Learning Radiometry, Photometry, Optics, Thermodynamics, Fluid Mechanics(Statics & Dynamics) with great resources. I'm looking for resources for understanding the topics above. I prefer video tutorials as opposed to books but I couldn't find any video resources on Photometry and Radiometry (I...
  12. TheSodesa

    Magnitude of a component of a triple star

    Homework Statement The apparent total magnitude of a triple star is ##m_0 = 0,0##. The apparent magnitudes of two of its components are ##m_1 = 1,0## and ##m_2 = 2,0##. What is the apparent magnitude of the third component? Answer: 0,9 Homework Equations Since according to Norman Pogson...
  13. S

    Stargazing Can you me with this Hershel telescope beam?

    I came to know that the PACS instrument of HERSHEL has a resolution of 5" at a scan speed of 20''/sec whereas it's 10'' at 60''/sec. The resolution has almost doubled with the scan speed. Can you please tell me why it's so?
  14. mishima

    A Will this setup work for backyard photometry?

    I've been looking for a way to do photometry cheaply (variables and standards). I have no interest in eyepiece observations of the night sky or astrophotography. I am currently considering purchasing this...
  15. S

    Astronomical source extraction in non uniform background

    I have been working with some Herschel images and I am finding it difficult to extract the astronomical sources from the image. The image background is extremely non uniform so i am not able to differentiate between the background and the source. I used some tasks in HIPE like source extractor...
  16. mishima

    Is It Possible to Do Photometry Without Using a CCD?

    CCDs are very cheap, but I was wondering if there was an even cheaper alternative to CCDs if the goal is differential photometry on magnitude 5 and under stars. The data only needs enough precision to prove/demonstrate the concept, and to develop a good sense of the workflow in doing...
  17. mishima

    Practice Images for CCD Photometry?

    Hi, it seems astronomical FITS images are abundant on the internet. However, I am looking for FITS images of bias/flat and standard stars along with observations to practice CCD photometry and having a harder time. I would like to be able to take a set of images and obtain the transformation...
  18. seo_andromeda

    Transforming FERMI SMARTS data from magnitude to flux

    I'm using data from the FERMI SMARTS website, which gives the emissions of blazers. The data has been calibrated using differential photometry (dividing by the average magnitude of a number of reference stars.) "The raw photometry of comparison stars in the field of the blazar was calibrated...
  19. A

    Photometric and Radiometric measurement concepts

    Hello, I am trying to understand the concepts of photometric and radiometric measurements in an electronic sensor. Various photo sensors based on CCD (Charged Coupled Devices) or CMOS photo diode array technology are available in the market for measurements in Visible Spectrum. Now I want to...
  20. J

    What Does a 1cm Detector Record with a 100W Non-Lambertian Light Source?

    Homework Statement If the light source in figure 12.1 (see figure below) of the Warren Smith text is replaced by a 100W light bulb, and the lens aperture is 20cm, what would the 1cm detector at point E record? Assume all other values are the same, and the light bulb is not Lambertian...
  21. M

    Astrophysics Photometry Exercise Coursework

    hi guys I'm a 2nd year undergrad doing astrophysics, and I was set this piece of coursework a couple of weeks ago, and it's due in a few weeks, so I'm getting started. Problem is, I don't know where to go with it. We've basically been given a load of data of measured fluxes in the U, B and V...
  22. L

    Photometry: Determining a binary star system.

    Homework Statement By performing CCD photometry on a pair of nearby stars A and B we obtain their relative magnitudes in the V filter and their colors: Star A: mV = 8.70 , (B − V )= 1.30 Star B: mV = 11.90 , (B − V )= 1.81 Star A is known to be a of a main sequence K0V type, while no other...
  23. P

    Basics of Photometry and Magnitudes

    http://www.exoplanet.eu/catalog-all.php?&mode=0&more=yes". On one of the this it has for the for each exoplanet, in the list it gives the information about the parent star's properties especially its phototemetry. Specifically it lists star's the V-order, I-order, H-order, J-order, and K-order...
  24. maverick280857

    Simple photometry question with something going wrong?

    [EDIT] To be deleted. [EDIT] To be deleted.
  25. R

    Photometry of SDSS Data: A How-To Guide

    Does anyone have any idea how to get the photometry of SDSS data?
  26. S

    Photometry- zero points of telescopes.

    Homework Statement I need to calculate the zero point of the telescope and camera i used. Homework Equations The Attempt at a Solution I have count rates of various points on the screen in a 19x19 pixel box of a standard star using the two filters V and R. I've used exposure...
  27. tony873004

    BVRIJHK Photometry: What Do the Letters Stand For?

    What do the letters stand for? Blue, Violet, Red, Indego? Does anyone have a link explaining the process? Google only seems to give articles describing results from using the technique without explaining what the technique is.
  28. G

    Photometry / radiation question

    Hey guys (and girls) I've got a rather annoying question. If you were given the radiant power (45 mW) and luminous flux (22 l), and then was told it was spread uniformly spread over a hemisphere. (a) how do find the luminous intensity??, (b) the illuminance 1.8m away, and (c) illumance 5.0...