Do you see what I see?
A lesson about astronomical imaging.
The sun through
different filters


This lesson will examine the role that different filters play in viewing an astronomical object. Each student will build their own filter wheel through which they will examine a color image of the Crab Nebula.

After practicing with the filter wheel we will examine various images of the Sun. Each solar image has been obtained with a different solar filter. The student will analyize the strengths and weaknesses of this data collection process.

Scientists often used different filters to help them gather information about a particular object. Often particular information is enhanced with one filter and "hidden" with another. When a scientist is interested in studying a certain aspect of an object he or she might view that object through a filter that enphasized what they are interested in and at the same time keeps the other information from becoming too distracting.

The knowledge gained in this lesson lends well to independent research projects in spectroscopy, medical imaging, or satellite imaging.

Let's get started by examining what different filters can do.



Lesson designed by the YPOP Team

For questions about this lesson, please contact Michelle B. Larson
The Filtered Landscape

The Filtered Landscape

Yellow Filter
Blue Filter
Red Filter
No Filter

The image above had been constructed with very basic colors. Click on the appropriate buttons to investigate what happens to our perception of the image when we cover it with different colored filters.

Yellow Landscape

The Yellow Filtered Landscape

Yellow Filter
Blue Filter
Red Filter
No Filter

A yellow filter is a filter that only allows yellow light to pass through it. When we place a yellow filter over a multicolored image only the yellow light with the same wavelength as our filter reaches our eye. All other wavelengths of light are absorbed by the filter.

Some of the features in the image may be a color that does not contain ANY wavelength the same as our filter. All of the light from these features will be absorbed. They will become invisible to us when we view the image through our filter.

Other portions of the image may be a color that contains SOME of the same wavelength as our filter. In this case we will still be able to see the object. It will be faint because ALL of the light is not transmitted throught the filter.

Blue Landscape

The Blue Filtered Landscape

Yellow Filter
Blue Filter
Red Filter
No Filter

A blue filter is a filter that only allows blue light to pass through it. When we place a blue filter over the multicolored landscape image only the blue light with the same wavelength as the filter reaches our eye. All other wavelengths of light are absorbed by the filter.

We see here that the blue light from the sky is transmitted most effectivly through the blue filter.

Red Landscape

The Red Filtered Landscape

Yellow Filter
Blue Filter
Red Filter
No Filter

A red filter is a filter that only allows red light to pass through it. When we place a red filter over the landscape image only the red light with the same wavelength as the filter reaches our eye. All other wavelengths of light are absorbed by the filter.

We see that the light from the red trees is transmitted most effectively to our eye through the red filter. If you were a scientist interested in studying the type of tree that is represented in red, you might find it useful to view this image with a red filter. By using the correct filter you can study the features that interest you without being distracted by the other items in the image.

A Colorful Landscape

A Cartoon Landscape



We have seen that an image can be composed of many different colors. By placing a filter over the image we can restrict what kind of light makes it to our eye.

The use of filters can be very helpful to scientists. When we use the correct filter we can enhance a particular feature and mute the others. Sometimes this makes the feature we are interested in easier to study.

When you observe an object through a filter you must remember that you are not seeing all the information. When you look at the landscape above with a red filter you are lead to believe that there is only one kind of tree on this hillside. When you use a yellow filter you also think there is only one kind of tree, but it is a differet kind. If you want to understand the entire landscape you must remember to include the information you gather from each filtered image.

Now that you understand a little bit about how filters work, let's make a filter wheel and use it to view the Crab Nebula!

Making a color filter wheel


NOTE: Do not use this filter wheel to look at the sun.
You should never look directly at the sun without protection.
This filter wheel is NOT for viewing the sun.

filterwheel

Materials needed for EACH filter wheel




To begin construction of your color filter wheel cut out a small, wedge shaped, section from one of your paper plates. You want this section to be a little bit less than one third of the paper plate. See the image above.

Once you have cut out your first section, place this paper plate on top of the second plate and trace around the inside of the cutout portion onto the other plate. Now rotate your cut plate about 1/3 of the way around and trace out the section again. Rotate the top plate another 1/3 and trace out the section one more time onto the uncut plate. The uncut plate should now have three, equal sections traced onto it. Make sure there is enough space between each section so the plate will hold together once you cut them out. Cut out each of the sections that you just traced onto the plate.

You should now have two paper plates, one with a single section and another with three cut out sections, as shown. Take the plate with three sections and tape a different color of cellophane onto the back of each opening. Make sure to trim your cellophane so that it does not extend over more than one opening.

Punch a small hole through the center of each plate with your pencil

You should now have two plates that look like the images below.

top and bottom plates


construction


Fasten the two paper plates together using a brass brad. The plate with the cellophane should be on the bottom.





The two plates should be able to rotate easily so that you can look through one color at a time.


We are now ready to use our filter wheel to view the Crab Nebula!


The Crab Nebula

Crab Nebula
This image was obtained by Sven Kohle and Till Credner of Bonn, Germany on October 26, 1995.


Here is a higher resolution image of the Crab Nebula to print on your color printer!



This activity works well in teams of three. EACH PERSON on the team will need the following materials.

Materials



The image you see above is an image of the gas cloud that remains after a star blows up. The material in this gas cloud will eventually be used to form many new stars. This gas cloud is called the Crab Nebula, the star exploded in the year 1054 AD. The explosion was so bright that several Chinese astronomers reported seeing the bright light in the sky during the daytime!

The gas is several different colors because it is at different temperatures. The hottest part is blue in color, the coldest is red.

We are going to look at the Crab Nebula through our color filter wheel. Within your team assign one color from the color wheel to each team member. Someone will be blue, someone will be red and the other person will be yellow.

Hold your filter wheel, with your assigned color showing, over the image of the Crab Nebula. Sketch what you see. Do not remove the filter while you are sketching. Try to make your sketch as accurate as you can in color, size and detail. Your partners should do the same thing with their color wheel turned to the color assigned to them.

Once everyone on your team has finished making a sketch of the Crab Nebula compare your sketches with one another. Are they all the same? Are they different? If they are different, discuss the ways in which they differ.


Viewing the Crab Nebula

Are these images of the same object?

filtered images


When you and your partners looked at the Crab Nebula through the different colors on your filter wheel you should have seen different images. Can you identify the image you saw from the choices above.



Why do you think each person saw a different image depending on which color filter they used?

Use the filter table below to investigate further.

Yellow Filter
Blue Filter
Red Filter
No Filter

The Yellow Filtered Crab

The Yellow Filtered Crab Nebula

Yellow Filter
Blue Filter
Red Filter
No Filter




The image above should look similar to the Crab Nebula when it is viewed through the yellow window on your filter wheel. The yellow cellophane looks yellow in color because it is absorbing all colors except yellow. This piece of cellophane allows the yellow light from the image to pass through it. The bright portions of the red and blue gas in the image also have some yellow light in them so they pass through this window and you are able to see most of the image. Try the blue and red filter buttons to see what happens to the image through those filters.


The blue filtered Crab

The Blue Filtered Crab Nebula

Yellow Filter
Blue Filter
Red Filter
No Filter




When you look at the Crab Nebula through the blue window on your filter wheel it is easy to see that most of the blue light passes through the blue cellophane. The red and other colored light is mostly absorbed. This is why the cellophane looks blue to us. It is absorbing all colors except blue. What do you expect the Crab nebula to look like through the red cellphane?


The red filtered Crab

The Red Filtered Crab Nebula

Yellow Filter
Blue Filter
Red Filter
No Filter




As you might have predicted, when viewed through the red window on your filter wheel the Crab Nebula looks much different than through the blue cellophane. Here we can see all of the cooler, red gas on the outer edges of the nebula which was mostly invisible in the image before.


Why Do Scientists Use Filters?


filtered images




When astronomers observe astronomical objects in the sky they often use different filters to enhance the aspects they are most interested in. At the same time, this mutes other features that might be distracting.

The blue filter on your color wheel for instance is a good choice if you are interested in studying the hot, central portion of the Crab nebula. The red filter is a better choice for enhancing the outer gaseous regions. The yellow filter enhances the filament structure in the center of the nebula.

Filters are a very useful tool for scientists. When taking data from an image one must always be careful and remember that things look different if the image has been filtered. The Crab nebula looks very hot and small through the blue filter and very large and cool through the red. Niether of these views alone give an accurate representation, but by combining the information gained with the different views, we can create an accurate discription of the object.

This method of observing through different filters and then collecting the information into a complete picture is used all time when we look at our nearest star, the Sun. Let's investigate how solar scientists use filters to view the Sun.

Viewing the Sun through different eyes

solar filter wheel



Much like the Earth, the Sun has many different layers that define its structure. Unlike the Earth, the Sun is completely gaseous; there is no solid surface on the Sun. Although the Sun is completely made of gas, the density and temperature of the gas changes drastically as you travel from the outermost regions to the center.

Scientists look at different layers of the Sun by using telescopes with different filters. The filter wheel on board the Yohkoh solar telescope is much like the filter wheel you constructed earlier in this lesson. By rotating one wheel with a window the telescope operators are able to select the type of filter they want to use to view the Sun. These solar filters can sort out many kinds of radiation, not just colored light.

In the image below you can see the filter wheel is located near the back of the soft x-ray telescope on board the Yohkoh satellite.


soft


Our Sun emits radiation in almost every wavelength. In addition to the visible light we can detect with our eyes, the Sun also emits radio waves, infrared and ultraviolet radiation as well as x-rays and gamma rays. Lucky for us we can focus in on a specific layer of the Sun by using a filter to remove all but one type of radiation.

Let's look at the sun as seen through many different filters.



The Filtered Sun

Our Star the Sun

Calcium K
Helium 1
Soft X-ray
No Filter

This is a picture of the Sun's visible layer; the layer you see from the Earth's surface. This layer of the Sun is the furthest we can see by direct means. It is called the photosphere.

This image has recorded all wavelengths of visible light emitted from the Sun. No single wavelength has been filtered out, but a filter was used to reduce the amount of light. The Sun is very bright. When we look at the Sun we must reduce the brightness so we do not damage our eyes or our camera.

What do you see in this image? Click on it to see a larger version. You should be able to find some dark spots, called sunspots.

The Calcium Filtered Sun

The Calcium Filtered Sun

Calcium K
Helium 1
Soft X-ray
No Filter



This image of the Sun was taken through a filter that only transmits light with a wavelength of 393.4 nanometers. The blue light is emmited by once ionized Calcium atoms. This filter shows us a region of the Sun called the chromosphere, it is approximately 2000 km higher than the photosphere.

This image allows us to see some of the magnetic structure on the Sun. The bright sections show where the magnetic field is the strongest.

So you see any correlations between this image and the other solar images?


The He Filtered Sun

The Helium Filtered Sun

Calcium K
Helium 1
Soft X-ray
No Filter




What features do you notice in this picture?

This image was taken through a filter which only allows radiation with a wavelength of 1083 nanometers to pass through. This wavelength corresponds to infrared radiation, or heat. Radiation of this wavelength comes from ionized Helium atoms in the Sun.

Before this radiation leaves the Sun some of it is absorbed by dense gas in the Sun's atmosphere. These dense regions appear dark because there is no radiation of this wavelength left to make it to the camera.

This image provides information about a layer of the Sun above that seen in the Calcium image.


The X-ray Filtered Sun

The X-ray Filtered Sun

Calcium K
Helium 1
Soft X-ray
No Filter




The image you see here was taken through a filter which transmits only low energy X-rays. Low energy x-rays are often referred to as soft X-rays.

X-rays from the Sun are produced by hot gas of more than one million degrees. This gas exists in the highest layer of the Sun called the corona.

Do you notice any correlation between this image and the other solar images?


The Layers of the Sun

The Many Layers of the Sun






These four images are all images of the Sun. Each shows the Sun through a different filter, or no filter at all as in the first image.

What differences and similarities do you notice between the different images? (If you click on each image you will get a larger version.) You might have noticed that the sunspots in the whitelight image are in approximately the same place on the Sun as the intense magnetic regions in the Calcium image. The dense gaseous regions which appear dark in through the Helium filter also occur in this general area as do the bright active regions in the Xray image. Remember that each image is of a different layer in the sun. Some layers are higher up in the Sun's atmosphere than others.

Here again we see how different filters can be helpful to scientists. In addition to those seen here there are many other filters available for solar scientist to help them study the Sun. If you are interested in studying the magnetic field in the Sun you would want to include a Calcium image and perhaps an X-ray image which shows some magnetic loop structure. If you are interested in the way the density of the atmospheric gas varies around the disk of the Sun you would study a Helium image.

The use of filters allows us to study one or two factors at a time without being distracted by too much information. Later, by considering all the information together we can then paint a representative picture of the Sun or whatever object we happen to be studying.


Try some of the following suggestions for further projects related to this lesson





The YPOP Homepage


Lesson designed by the YPOP Team.

For questions about this lesson, please contact Michelle B. Larson
larson@solar.physics.montana.edu


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Association. Copyright 1999-2002.