# The Pre-Flight Briefing

## Activity 2C: The KAO Telescope

### Objective

To understand basic principles of the operation of the KAO telescope and to construct a simplified model of it.

Ask students to describe the largest telescope they have ever seen--in person or in movies. Most will draw a large refracting telescope (using lenses) with a long thin tube. A few may describe a reflecting telescope, with an aperture and a mirror at the bottom. It's this latter kind of telescope, a reflecting one, which flies on the KAO. On-line from last year's FOSTER project you'll find some easy-to-understand Questions and Answers that explain how bouncing light between mirrors inside the telescope tube gives the KAO's telescope a power much greater than would otherwise be possible inside a telescope body small enough to fit inside the airplane's fuselage.

Explain that the KAO's telescope is the heart of the Observatory, and that the whole purpose of the KAO is to carry this telescope into the stratosphere and give it a relatively smooth and vibration-free look at the universe.

Materials

• activity page 2C, page 37
• 35 mm film canister
• 2 pushpins
• a small piece of clay
• 2 popsicle sticks
• 1 thumb tack
• a dime (or penny!)

Procedure Distribute the Activity sheets. Read the section which describes the telescope. Then work with students to help them make the calculations about light gathering power. (Pi is the same for both, but the KAO telescope's mirror is 144 times wider than the human eye. Therefore the KAO gathers 144 multiplied by 144 times more light or 20,736 times more light.) The KAO's planned successor, SOFIA, will have a telescope of 97.5" compared to 36", that is, some 2.7 times bigger than the KAO's telescope. Accordingly its light gathering power will be 7.29 times greater. For another excellent hands-on activity about light gathering power, see Space Based Astronomy, page 56)

Read the model building activity with students. Distribute materials and complete each step. Push in the pins for younger students.

The finished telescope will balance pretty well with the thumb tack resting on a finger. For even better results use a dime or penny, which will allow the model to stay in place as the student's finger rotates left or right. If everything's working, the telescope should continue to point in more or less the same direction. There are some important engineering differences in how the actual KAO telescope is mounted, as will be explained in the videos. But astronomer Al Harper, Principal Investigator for the LFS flights, thinks this model provides a fundamental insight into how to keep a telescope on target, while flying through turbulence at 3/4 the speed of sound! Students can experiment with even larger versions of the same basic design, using soft drink or coffee cans, and scaling up dimensions and materials. They might even try an open truss design like the photograph.

Interdisciplinary Connection

Our civilization is filled with examples of things that are designed to smooth out vibrations, just as with the KAO telescope. Ask students to think about cars. With good shock absorbers, even when the road is bumpy, a car still has a pretty smooth ride. Encourage students to investigate what other parts of a car keep bumps in the road from reaching passengers. How are the principles involved in automobile shocks the same as, or different from, those used in the KAO telescope? Hint: Tires, shocks and the KAO telescope all use air under pressure.

KAO Connection

Read the "KAO Backgrounders" on-line to find out more about how the telescope is mounted. This article can be shared with high school physics classes.

### The KAO Telescope

Telescopes are designed to gather more light than the human eye. The KAO telescope has an opening (aperture) of 36 inches. The human eye has an opening of 0.25 inch. How many times wider is the KAO telescope opening?

The light gathering power of a telescope depends on the area of the opening. The area of a circle equals the radius multiplied by itself (squared) and then multiplied by pi (3.1416). Calculate the light gathering area of the KAO telescope and your eye. How many times more light can the KAO telescope gather than your eye?

Sofia, the Stratospheric Observatory for Infrared Astronomy is slated to replace the KAO in the future. It will carry a 2.5 meter (97.5 inch) telescope mounted in a Boeing 747 aircraft. How much more light gathering area will SOFIA's telescope have?

The KAO's telescope is installed in the cabin, in front of the aircraft's left wing. Its 36 inch wide primary mirror gathers light and reflects it to a second mirror. This mirror focuses the beam onto a third flat mirror which reflects the focused beam into the airplane's pressurized cabin through an air bearing. The astronomer mounts a detector at the end of the bearing and operates the telescope from inside the cabin. The telescope is isolated from aircraft vibrations and stabilized by gyroscopes and a video tracking system.

### Making a KAO Telescope Model

To make a model of the KAO telescope, you need a 35mm film canister, two pushpins, 2 popsicle sticks, a thumb tack, masking tape, a little bit of modeling clay and a dime or penny.

a. Cover the inside bottom of the canister with clay. Round it like the curved surface of a spoon. The clay will make the canister heavier on the bottom just like the telescope.

b. With sharp scissors or a craft knife, cut one popsicle stick in half.

c. Carefully insert a pushpin through the curved end of each half stick.

d. Continue pushing the pins through the canister at a point just above the clay. The pins should be on opposite sides of the canister. Once both pins are in place, the canister should swing freely up or down--just as the telescope does when the astronomer moves it to find a new object.

e. With scissors, score the second popsicle stick in two places, 1 inch from either end. Then bend the stick to form a "U" shape, as shown. Be careful not to break the stick.

f. Tape the ends of the stick to the half-sticks to form a supporting yoke for the telescope.

g. Push the thumb tack through the middle of the yoke top--pointing inward toward the canister.

h. Gently balance the tack on the coin resting on your finger. Notice that you can move your finger around and the telescope stays in place. The KAO telescope is also balanced on air-bearings that allow the plane to vibrate without disturbing the telescope.

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