Teacher Page
Activity 5a - In Focus

Standards (see Appendix A):

• Science Content Standards: A. Science as Inquiry, E. Science and Technology.
• Mathematics Standards: Representation, Measurement, Connections.
• Technology Tools: Research; Problem Solving and Decision Making.

Unifying Concepts:  Systems; Order and Organization.

Objective:

Students will understand what happens to images as a telescope is focused.  They will find that focusing the telescope allows one to achieve a clearer, brighter images, with better resolution of the celestial targets.

Overview:

This activity builds on students’ understanding of focal point, focal plane, and focal length, introduced in the F Box experiments of Activity 4.  Students experience the increased resolution of targets as the telescope is focused.  Students review how telescopes focus light and the explore the idea that on either side of the focal plane the image will be out of focus.  Students examine a series of images of a star from extremely out of focus to focused.  In a second set of images, the images become progressively closer to being in focus, resulting in finally being able to distinguish two stars as separate from one another.

Through these exercises, students gain experience identifying how images look when they are out of focus.  Seeing two stars where it appeared to be a single object to start, expands the concept of resolution from that of sharpness to the ability to distinguish two objects as separate from one another.  Resolving power of telescopes is more fully developed in Activity 8.

Background:

Many people wear glasses or contact lenses to bring images into focus on their retina; the shape of their eyeballs, being too elongated or too short, does not match where the lens of their eye is able to adjust to focus the image on the retina.  In effect the lenses in glasses or contacts refract the light before it enters the lens of the eye. When one looks through a telescope, one often has to change the focus of the eyepiece to match his or her vision.

Components of telescopes change with temperature.  The temperature of the system changes with the weather conditions.  So, telescopes have to be refocused before every observing session.  Sometimes, the focus needs to be adjusted during observing sessions as well.  When one uses cameras on telescopes to take images, focus adjustments also have to be made to make sure the image is focused on the detector.

When astronomers use a CCD camera to take telescope images, they have to place the CCD exactly at the place where the image is in focus.  This can be accomplished either by adjusting one or more of the components of the telescope’s optics, or by moving the CCD camera forward or backward using the focus knob.

For example, when taking images with the CCD on the Yerkes 24 inch telescope, the CCD is focused either by changing the position of the secondary mirror. This slightly changes the focal length of the telescope.  There is a button on the telescope housing which activates a small motor that moves the secondary mirror backwards or forwards slightly.

Another example is focusing with the Meade LX200 telescopes such as the ones in the Yerkes Rooftop Telescope Observatory.  These types of telescopes adjust focus by movement of the primary mirror forward or back.  This repositioning of the optics also changes the focal length slightly.

The out of focus images produced by many reflecting telescopes appear donut shaped with a dark area in the center.  This central dark region is caused by the shadow of the secondary mirror, which is suspended in the center of the tube near the opening of the telescope.  Out of focus images created by refracting telescopes are just fuzzy, because they include only lenses and do not have secondary mirror obstructions.

Preparation

• Image Sets:  The image sets for this activity show stars that begin out of focus, with successive images gradually bringing the stars into focus.  This is a very time consuming activity in CCD imaging.  It is interesting to see how out of focus images look.  The center of the circles that is dark shows the area of the mirror blocked by the secondary mirror in reflecting telescope systems.
• 5a-focus_star are images of a random star used for focusing the Yerkes 24 inch telescope.
• 5a-gam1andfocus are images of star Gamma 1 in Andromeda, also called Almaak.  The second star of this double is very faint!  When focusing the telescope, we used this star for focus, but we were unaware of the second star.  So, it was quite a surprise to find the second star when the telescope was focused properly.  Try not to give away this surprise so students can experience it for themselves. Students may wish to use a planetarium program or search the library or Internet for more information about Almaak.
• 5a-ngc1514 includes two images, one in focus and one out of focus.  This is a planetary nebula with a very bright central star.  The central star masks the nebula unless one is careful with focusing.  This is one example of how focusing aides in seeing faint nebulosity.
• This is another lesson where working with an amateur astronomer / colleague will be very helpful.  Investigate focus during a star party or during the day using telescopes or binoculars to view land objects.  (Never use a telescope to view the Sun unless you have the proper filters over the telescope itself and understand how to use them.)  Assuming that you have already collaborated with an amateur astronomer when conducting Activity 1, you may wish to invite her or him to bring in a simple telescope for students to use during this activity.
• Colored pencils are helpful but not necessary for completing the sketches.

Time: 50 minutes

Directions

1. It is a valuable experience for students to have a chance to look through a telescope and focus on an object.  If an amateur astronomer is available try having small groups of students take turns at the telescope.
2. If students are not familiar with constructing ray drawings assist them in completing questions 1 and 2 of the student page.  Be sure everyone has a correct understanding of focal length and focal plane before completing the remaining questions.  Using the light ray diagrams, emphasize that it does not matter which side of the focal point the camera was at to start, the image will become more clear as it is moved toward the focal point and less focused if it moved in the wrong direction.
3. Once you have reviewed the locations of the images, allow students to work independently or in pairs or groups to complete their work.
4. When students have explored the images and completed student pages, ask the groups to come to discuss their analyses, ideas, and reach consensus if possible.

Evaluation/Assessment:  In the Apply Your Knowledge section on the Student Page, they are asked to compare and evaluate two images.  Along with other indicators, students should be able to see a nebula in the focused image.  They should be able to correctly answer question 3 by explaining that the CCD in not positioned exactly at the focal plane of the telescope; the CCD could be slightly in front of or in back of the focal plane.