6.2. Basic Photometry in AstroImageJ

6.2.1. Initial guess for the aperture size

  1. Click line selection tool
  2. Draw line across one of the stars that is circled in this image (or the image of interest to you:

  1. Select Analyze -> Plot seeing profile

  2. When asked whether to save the aperture radii, make the following changes:

    • Make the gap between the aperture radius and the inner sky radius at least 5 pixels
    • Make the difference between the inner and outer sky radii at least 10 pixels


This is not the best way to set the aperture size or the inner and outer sky radii. We will come back to how to best choose those later.


  1. What is the FWHM of the star you took the seeing profile of?
  2. What does the FWHM mean? Answer both in terms of what the acronym means and an explanation in plain english what it represents.
  3. What aperture size was chosen? How large is it compared to the FWHM (e.g. 2X, or half)?

6.2.2. Setting up aperture photometry

  1. Double click on the photometry tool in the toolbar.

  2. Check that the first window that opens matches this, except for the aperture radius and the inner and outer sky radii, which you should choose based on the FWHM:

  3. When done, click OK, and in the next window that opens check/set several things:

    • Gain for the Apogee Alta U9 is roughly 1.48 (feel free to use the value you got earlier this semester)
    • Read noise is roughly 28 or 29 electrons per pixel (feel free to use the value you got earlier)
    • Dark current is about 0.2 electrons per pixel per second
    • The Apogee Alta U9 saturates around 38000 counts.
    • Change the FITS headers output to list only AIRMASS and EXPOSURE as shown below, and make sure the boxes you have checked include at least all of the ones in the image below.
  4. Click OK

6.2.3. Doing aperture Photometry

  1. Choose one of the stars circled in red below, then, IN IMAGEJ, click on the star.
  2. Photometry information will appear in the measurement window.


  1. Write down, from the measurement table, for the star you chose:

    • the RA and Dec (in decimal hours and decimal degrees)
    • net counts (Source-Sky)
    • source radius
    • minimum and maximum sky radius
    • Sky counts/pixel
    • Source Error
    • Source SNR
  2. What was the net number of photons received from the star in the aperture? Hint: Don’t forget about the gain.

  3. Pay attention to the folks up front for a brief lecture about error and signal-to-noise ratio (SNR). Take useful notes.

  4. Calculate the source error and SNR and compare to the values of each from AstroImageJ

  5. What is the largest source of error?

  6. Of the “noise” terms (dark, sky, read noise), which makes the largest contribution?

  7. How could you reduce that source of noise?