Activity 1: Make a Digital Camera

Transcription

1 Hubble Sight/Insight Color The Universe Student's Guide Activity 1: Make a Digital Camera Astronomers love photons! Photons are the messengers of the cosmos carrying detailed information about our amazing universe. In 1990, NASA launched the Hubble Space Telescope (HST), an unprecedented satellite created entirely to collect photons. But a telescope can only gather the photons; it takes a camera to capture them. For over 100 years, the only way to catch a photon was with photographic film, but film typically only records one out of every ten photons that strike the surface. By the time HST was launched, digital cameras were in wide-spread use in astronomy. Digital cameras contain a charge coupled device (CCD) that is capable of capturing nine out of every 10 incoming photons. NASA engineers at the Jet Propulsion Lab were tasked with the challenge of creating a digital camera suitable for the incredible space-based telescope. The result was the Wide-Field Planetary Camera 2 (WFPC2, pronounced wiff-pick two ), an impressive digital camera that served HST and astronomers for over 16 years. Digital cameras have since become a ubiquitous technology even our phones have digital cameras in them! Surprisingly, all digital cameras from a webcam to WFPC2 work on the same principles. In this activity, we will create a simple model of a digital camera. We will start by modeling how a CCD chip captures a photon of light. First, we have to build a single pixel CCD detector. Part 1: Capturing Photons Step 1: Build a single pixel digital camera model Take a two-foot length of clear plastic tape and cut it into five 4 pieces. Overlap the pieces together with the sticky-side up, making a small square of tape (see image at top right). Make the square of tape much less sticky by attaching and removing it from clothing or carpet several times. Place the square of tape, sticky side down, onto a pile of paperclips. Gather as many paperclips as possible onto the tape. Place the paperclip-covered tape onto the top of a paper/plastic cup, with the paperclips dangling toward the inside of cup (image at bottom right). You have completed your single pixel digital camera. A CCD chip works by a principle known as the photoelectric effect. When a photon of light strikes a material, it can be absorbed causing the release of an electron. This effect allows us to make electricity from light and is the basis of solar panels. Normally, electrons are set free by light and can wander where they please, but in a CCD chip, the electrons are trapped into electric potential wells called pixels. These pixels act like tiny buckets collecting all of the electrons that have been set free by the incoming photons. 1

2 Step 2: Take a picture We will simulate a photon striking your detector by dropping a pencil onto the top of your cup from a height of 2-3 feet above. The pencil should knock loose some paperclips, simulating the release of electrons when an incoming photon strikes the CCD detector. Students should take turns dropping the pencil, trying to dislodge a paperclip from the tape. Do electrons ever fall into the cup without a photon hitting the top? Do you think this ever happens in real CCD cameras? As you consider the model CCD chip that you have made, complete the table below: Model CCD Pixel Real CCD Pixel Paperclip Photon Electric Potential Well Part 2: Capturing an image In order to capture a meaningful picture, the camera must have more than just one pixel. With many pixels, we can compare how many photons strike one part of the CCD versus another. This will allow us to identify bright and dark spots. When a camera gets millions of pixels, it is able to map out exactly what an object looks like. Instead of trying to make a million pixels, we will start with just nine. As our analogy gets more sophisticated, we need to simplify it in other ways. For this portion of our model, we will remove the tape from the cups and act as though each pixel is catching the photon. In reality, though, we know that the camera is capturing one electron for each photon that strikes the CCD. Remove the tape from the top of your cup. Make sure the cup is empty. Place nine plastic cups into a tightly packed 3x3 square. We will simulate light coming from a distant celestial wonder by pouring paperclips through a paper filter located on the last page of this activity. This filter will allow more paperclips to flow through larger holes (simulating bright spots in the object) and less paperclips to flow through smaller holes (simulating dark spots). To create your filter, cut out the hatched areas in the centers of the circles on the last page. Notice that the larger cut-outs align with the light spots and smaller cut-outs with the dark spots. When your filter is ready, hold it approximately 1 foot above your nine-pixel digital camera, and slowly pour about 25 paperclips onto the surface of the paper. Gently shake the paper until all the paperclips have fallen through an opening. As paperclips fall into the cups below, you are capturing an image of the celestial wonder. Did all of your paperclip photons land in the cup they were supposed to (that is, the cup directly beneath the opening they fell through)? Real photons coming from space can be jostled off course as they pass through the telescope's optics and ultimately end up in the wrong pixel. What problems do you think this could cause for astronomers? 2

3 Part 3: Reading out the camera Now that we have captured our first photograph with our model nine-pixel camera, we need to count how many electrons are in each pixel. The process of counting all of the electrons in a CCD is known as reading out and requires advanced electronics to achieve in real cameras. A typical digital camera has millions of pixels, and the camera cannot possibly read them all at once. In fact, most digital cameras can only read one pixel at a time. Moreover, the camera can only really count the electrons in one of the pixels the readout pixel! The sophisticated electronics attached to the back of a CCD chip pass the electrons from one pixel to another like an electric bucket brigade. Even though we could easily count how many paperclip electrons there are in each cup, we will simulate reading out our nine pixel camera by following the same method that real digital cameras use. Step 1: Pass, Read, Repeat Assign each cup an (x,y) coordinate like the image at the right. Choose the corner cup (1,1) to be your readout pixel. Count the number of paperclips in the readout pixel. Record this number in the table under (1,1). Pour the contents of the cup (1,2) into cup (1,1). Then pour the contents of cup (1,3) into (1,2). Again, count the number of paperclips in your readout pixel. Record this number in the table under (1,2), since this was the number of paperclips originally in cup (1,2). Now pour the contents of the cup (1,2) into cup (1,1). Once again, count the number of paperclips in your readout pixel. Record this number in the table under (1,3), since this was the number of paperclips originally in cup (1,3). Pixel Location (1,1) (1,2) (1,3) (2,1) (2,2) (2,3) (3,1) (3,2) (3,3) Paperclip Count Now shift all of the paperclips from row 2 straight up into row 1. Cup (2,1) is poured into cup (1,1) and so on. Likewise, shift all of the paperclips in row 3 straight up into row 2. Repeat the process of reading out row 1 just as before except record your numbers in the table for (2,1), (2,2), and (2,3). Again shift all of the paperclips from row 2 straight up into row 1. Read out row 1 once more except record your numbers in the table for (3,1), (3,2), and (3,3). Wow, that's a lot of work! Now imagine doing that for a million pixels. Digital cameras can do this whole readout process in less than a second. Remember this the next time you get impatient waiting for your digital camera to display the picture you just took. 3

4 Step 2: Creating a picture from numbers Finally, we will create a picture of the celestial wonder from our nine-pixel camera. All digital pictures start as just a table of numbers like the one you recorded in the previous step. Your computer turns this table of numbers into an image. We will do the same process by hand for our nine-pixel image. Below you will see nine circles, each representing one pixel from your camera. For each of the pixels below, look up the corresponding number from the table. If a pixel had 0-3 paperclips in it, shade in the circle completely black. If a pixel had 4-7 paperclips in it, shade it in dark gray. If a pixel had 8-11 paperclips in it, shade it in light gray. If a pixel had 11 or more paperclips in it, leave the circle white. Congratulations, you have created your own digital image by hand! How does your picture compare to the bright and dark spots of the celestial wonder? Part 4: How do we capture color? The image you created consists of four shades of gray (black, dark gray, light gray, and white). So how do digital cameras capture color? In short, color is usually captured by having three neighboring pixels collect the red, green, and blue light separately. These three pixels are then combined as one to give the true color at that location. But astronomers want as much detail in their images as possible, so instead of wasting two-thirds of the pixels to get color information, they use cameras that only take black and white pictures. That's right, all astronomical images are black and white! To create the amazing color pictures that we are used to seeing from HST, astronomers place colored filters in front of the camera and capture separate (black and white) images of the colored light passing through the filter. These images can then be combined on a computer to create a true color image. We will perform this process for ourselves in Activity 6, the culmination of the Color the Universe project. 4

5 The Celestial Wonder 5