SDSS Filters

A new principal stands at the entrance to school on the first day of the new year. The students are very enthusiastic and can’t wait to get in. The principal opens the doors expecting to greet the students, but they all come flooding in so fast that she cannot tell one grade level from another (I told you they were enthusiastic). Well, that will not do, so the next day she asks the teachers to sort the students by class so that when she opens the doors they enter according to grade level. How nice. She says “hello” to everyone and is able to focus on one group at a time, getting to know the size and make-up of each class.
And there you have it – filters. Light (the entire student body) comes to us as a mixture of colors (students). Unless we sort the light by some characteristic that all the colors share (class), it will be very difficult to get to know the exact make-up of the light. In the analogy of the new principal and the enthusiastic students, the teachers did the sorting. Teachers were acting as the filters. Filters “say” to light, “You get to pass through the door now; the rest of you wait.”
In the case of the SDSS, the filters are special pieces of glass that are in front of the recording surface of the SDSS camera. There are five filters for the SDSS camera. Every star, galaxy, and quasar imaged in the SDSS sends to us its own unique combination of colors. Filters allow astronomers to study how much light of each color went into what we see. Astronomers have discovered ways to use this information to learn about structure, changes over time, and even distance.
Here is a picture of engineers placing a protective window in front of the filters. The engineers are dressed as though they were ready for surgery. Like surgeons, they know that the quality of the operation depends upon steady hands and a clean environment. Dust and fingerprints can negatively affect the quality of the images taken by the camera.

Engineers placint a protective window in front of the filters.

The next picture is a diagram of the placement of the filters in front of the SDSS camera surface. Each row is labeled with a letter indicating a particular filter. Notice that the SDSS system uses all the filters at the same time! Following our student analogy, all the grade levels (colors) get to enter the school (camera) at the same time but through different doors (rows of filters).

Placement of filters Diagram

Each of the SDSS filters is associated with a particular color of light.
  • u’ – ultraviolet light
  • g’ – blue and green visible light
  • r’ – yellow and red visible light
  • i’ – near-infrared light
  • z’ – near-infrared light
If you are new to the topic of light, you are right to be thinking, “I don’t get it! Green and red are colors, but what are ultraviolet and infrared doing on the list?” Back to our analogy.
Remember, teachers (filters) sorted the students (light) according to grade level (color). Students’ ages are what place them in a particular grade level. In the case of light, the characteristic which all light shares that allows it to be sorted is its wavelength.
It was discovered over 150 years ago that the light we see is part of a much larger continuous spectrum called electromagnetic (EM) radiation. Scientists discovered that EM radiation travels as waves of energy. The electromagnetic spectrum, which includes the rainbow of colors we see, ranges from very short wavelengths to very long. The transition from one wavelength to another is gradual and continuous in the same way that students are born continuously throughout the year. We choose a cut-off date that places students in one grade level or another. Cut-off points for one kind of EM radiation (UV, blue, red, infrared) are chosen along the spectrum. Preflight Training – Light is Electromagnetic Radiation teaches more about this topic.
Filters do their job by allowing electromagnetic radiation of only a certain wavelength to pass through. They act as a window onto a small portion of the continuous spectrum. Imagine that our new principal decides to get to know what is going on at each grade level by having lunch with students. She can’t have lunch with all the students, so she chooses to invite only students born in December. It could be any month or different months for different grade levels. What matters is that she has reduced the number of students she has to fit into her office. The graph to the right shows generally where the window for each of the SDSS filters lies in the range of wavelengths that make up the EM spectrum. You see that three of the filters are completely beyond the visible light spectrum. The u- (or ultraviolet filter) is just beyond the blue end of the spectrum. The i- and z-filters are just beyond the red. Even though our eyes cannot see these portions of the spectrum, the are very important in astronomy.
On the X-axis of the graph, you see that wavelengths are measured in units called angstroms. One angstrom is equal to one ten-billionth of a meter. The largest number on the graph below is still far too small to be seen with a normal microscope.
Once you understand the job that filters do, it is easy to see how they provide a sample of the continuous spectrum of light that is sent to us from objects in the universe. The SDSS camera measures the amount of light passing through each filter. It is this information, the intensity of light transmitted at different locations along the electromagnetic spectrum, that is such a powerful tool in astronomy.
Graph of filters