Photometry with Iris Photometry with Iris

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1 Author: Daniel Duggan & Sarah Roberts

2 - Faulkes Telescope Project Introduction Photometry is the measurement of the intensity or brightness of an astronomical object, such as a star or galaxy by adding up all of the light from the object. For example, a star looks like a point of light when you look at it just with your eyes but diffraction due to the Earth s atmosphere blurs it out into something that looks like a round blob when you use a telescope to look at it. In order to measure the total light coming from the star, we must add up all of the light from the smeared out star. Photometry is generally used to generate light curves of objects such as variable stars and supernovae, where the interest is the variation of total light energy output by the system over time. It can also be used to discover exo-planets, by measuring the intensity of a stars light over a period of time. Deviations in the light output can indicate objects in orbit around the star. Loading Images Launch Iris When the program opens, you may see two windows appear. The first is the main Iris window, and the second is the Threshold window. Load your image. Go to File>Load If the threshold window does not appear, go to the File Menu and choose Threshold to open it. On the Threshold window, select Auto to scale the brightness and contrast in the image. Page 2 of 5

3 - Faulkes Telescope Project Photometry Settings Make sure that the zoom button is on x1. Next, go to Analysis>Aperture Photometry Choosing Aperture Size Aperture photometry consists of defining an aperture within which the star is centered, and then defining another circle which contains only the background sky. The intensity of light in the 2 circles can then be measured, and the background value subtracted from the star value to obtain just the value of the star s intensity. When carrying out aperture photometry in Iris, it is best to use the 3 circle option. With this method, the star or object whose light you wish to measure should be centered in the first circle, with the sky intensity being calculated from the ring between the 2nd and 3rd circles as shown in grey in the diagram below. The ring in-between is not used in measuring the intensity values - it s there just to ensure that none of the signal from the star is included in the sky background measurement or visa versa. When choosing suitable aperture sizes for objects in Iris, you should try and ensure that the first circle encompasses the whole of the star but not too much of the background. The second and third circles which define the annulus for calculating the sky background should be large enough to contain the background sky, but should ideally not overlap onto nearby stars. However, by selecting the median background option in Iris, any influence of faint stars or cosmic rays in the annulus should be eliminated in the measurement. Page 3 of 5

4 - Faulkes Telescope Project For many objects, the following values should be fine to use, but they should be altered accordingly if there are any objects nearby to the object which you are conducting the photometry on. Circle Number: 3 Median Background - selected Radius 1:8 Radius 2:12 Radius 3:20 Magnitude Constant: 0 Carrying out the photometry Click on the comparison star and record the intensity value. Next, click on the star or object of your investigation and again, make a note of the intensity value. Now, calculate the magnitudes of the comparison star and object. Page 4 of 5

5 - Faulkes Telescope Project Modified Julian Day You will also need to record the Modified Julian Day (MJD) from the FITS Headers of every image. This will be the unit of time for your light curve and is explained more in the spreadsheet instructions. To find the MJD in Iris, go to File>Image Info Exposure Times In order to calculate the magnitude of the stars, the exposure times of the images need to be known. To find the exposure times for each image, in Iris, go to File>Image info and scroll down to the EXPTIME value. Calculating the magnitude of the stars The magnitudes of the stars can be calculated either by using a spreadsheet specifically designed for the project being carried out, or from the equation below. If you would like to use the spreadsheet, it will be available from the relevant project page. For more information on how to use the apparent magnitude formula to calculate magnitudes for each project, go to the relevant project page and download the magnitude document. apparent magnitude, m = -2.5*log (counts/exp time) Page 5 of 5