Quasar Quest

Same Object, Different Results?

Because the result from the Brightness Tool is the number of counts from an object in a particular image Data File, it can be different for other observations of the same object. There are many things that can cause these differences. These include:

• Different exposure times: If the exposure time is longer, you will get more counts in your object.
• Different telescopes: Different telescopes and cameras will collect different amounts of light (with larger telescopes collecting more).
• Changes in the weather: As the weather changes, even very slightly, the sky can become more or less transparent. If it is less transparent, the number of counts you get for your object will get smaller, if it gets more transparent, you will get more counts.

As you can see, it can get quite complicated! Fortunately, because measuring brightness is very important, astronomers have worked out ways of fixing all these problems. We will be using a technique called Relative Brightness Calibration.

Relative Brightness

If you measure the brightness of two objects in your observations - the object you are interested in and a star that you do not expect to change - you can compare them to see if your object has changed.

To see how this works, look at the table of results below:

If you take the brightness of the two objects in each observation and divide one by the other you get this:

From this you can see that the object has not changed in brightness between the first two observations, but in Observation 3, it has got a bit brighter.

This is called making Relative Brightness Measurements and is very good way of studying the way things like quasars change.