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Astronomy with an online telescope
Astronomy with an online telescope

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2.4 Aperture photometry

This short video and the following activity explain how to carry out aperture photometry on your own images.

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Activity 2 Aperture photometry

Timing: Allow approximately 40 minutes

Open the photometry tool and upload images

  1. To use the photometry tool, go to the Astronomy with an online telescope page in the OpenScience Laboratory [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] :

    Follow the link FITS photometry. This takes you to the photometry tool. The page has some brief instructions on how to use the tool.

  2. Upload the FITS images of your variable star as shown in the video. Check that the information in the FITS file data window is correct (date and time and name of variable star).

Check exposure

  1. The first step is to check the exposure of your images. For the photometry to work correctly it is important that you take measurements from an image that is correctly exposed – neither overexposed nor underexposed. For your first set of images you should have bracketed exposures of 15, 30, 60 and 120 seconds and you should check each of these in turn.
  2. Set the aperture and ring sizes as shown in the video (Inner ring 10, Gap size 10, Annulus width 10). Adjust these as necessary depending on the size of the stars in your image.
  3. For each image place the aperture over the star that you have identified as your target star and note the maximum pixel value (labelled "Target max"). Repeat for the reference star. An image is correctly exposed if this maximum value is between 10 000 and 50 000 for both stars. If you have more than one image in which both stars are in this range, select the image with the longer exposure.
  4. When you have found which of your images has the correct exposure, make a note of the exposure time (this is shown in the FITS file data window). You can use this same value for the exposure when requesting further images of the same target.

Make the measurement

  1. You are now ready to do the aperture photometry itself. Open the image that you have selected as having the correct exposure.
  2. Place the aperture over the target star and note the Intensity value (labelled "Target flux"). This is a measure of the overall brightness of the star, made by adding up the brightness of all the pixels in the central aperture and subtracting the background from the annulus.
  3. Place the aperture over the reference star and note the Intensity (Target flux) value.
  4. You now have all of the information that you need to plot your observation on the light curve. Click Plot data to send your data to the variable star plotter. This will give you a choice to view the light curve straight away by clicking View plot, or you can simply click OK and view the light curve later from the Astronomy with an online telescope page in the OpenScience Laboratory.