Part 3F: Photometry


How do we measure the light collected from an astronomical object?

This process is called photometry.

We will start this part with a definition and a review.
Chris will help with the definition of photometry in the 1:56 video below.



a) The CCD camera has a grid of light sensitive elements called pixels.

  • Recall, our “hands-on” CCD camera was a 3×3 grid, so it had just 9 pixels.
  • Also recall, the Skynet telescope used in Chile has a CCD camera with over four MILLION pixels.

b) When light falls onto a pixel, the atoms of the sensor within the pixel release electrons. The number of electrons is proportional to the number of incoming photons. We used white map pins or sticky gems to represent the electrons detected in each pixel of our 3×3 grid.

c) At the end of the exposure, the number of electrons in each pixel is recorded as voltage, which is then converted to a number, and finally stored in a data array in the computer. We used the egg crate and glass gems to simulate a data array and how it is stored and read by a computer.

d) The numbers stored in the data array are called “counts”. These numbers are then used to determine the object’s apparent magnitude. (Recall, at the beginning of section 3 we discussed apparent magnitude).

The methods of photometry are used to determine this apparent magnitude.

BUT, there are complications . .

1. Earth’s atmosphere makes objects in space appear to “twinkle”, that is, the light appears to shift ever so slightly.

  • This effect is called “astronomical seeing”.
  • When taking an image for ten seconds to any number of minutes, the twinkling will smear the light into a fuzzy disk. This will cause bad “seeing”.
  • But we can still use the information, we just have to make sure we measure all the light in the fuzzy disk.
  • This will be addressed later.

2. The Earth’s atmosphere also changes from night to night AND from image to image.

3. And, what if the weather changes?

  • Would wind affect anything?
  • What about the phase of the moon?
  • How about the angle the telescope is tilted at, which determines how much of the atmosphere the light has to go through?

4. And then, we know all telescopes and CCD cameras are NOT uniform. This means the light collected depends on the instruments used.


All these effects have been thought about and different methods of photometry have been developed to find the apparent magnitude for different situations.

So let’s define a few of these methods, and then learn how to use the one we need.