Human HET: CHALLENGE 3 MIRROR ALIGNMENT

By Brad Armosky and Mary Kay Hemenway

INTRODUCTION inside CCAS.A control computer instructs each mirror to tip,

Following Challenges 1 and 2, in which all the mirrors were tilt, and/or piston into position until the reflected beam, at the
set in the array, the next step is to align them so that they act center of curvature, is within a target area the size of George
as a team of mirrors focusing light to the same place. This Washington’s eye on a U.S. quarter. Once all the reflected
alignment takes advantage of the geometry of a circle: at any beams lie within the target area, the mirror array is aligned and
point on the circle, the tangent line is perpendicular to the ready for astronomical observations.
radius. In this case, the tangent line is the small flat mirror, and The students will move their hand-held mirrors in just the
the radius is a light beam shining from the center of curvature. same manner.Their hands and wrists act like the actuators on
A mirror is aligned once it reflects the beam back to the cen- the back of each mirror, while their arms and bodies act like
ter, or the source of the beam. the truss. In fact, with their free arm, they may interlock with
their neighbor, to form a more rigid model truss.
While students hold their mirrors, their arms and hands
mirror might jiggle, or slightly wander about. In a few minutes, the
segments model mirror will significantly drift out of alignment. Similarly,
over the course of a few hours, the HET’s mirrors slowly drift
out of alignment.Temperature changes in the telescope struc-
ture warp the truss and change the directions at which the
mirrors point. During astronomical observing, the tempera-
radius ture changes because the dome and telescope are open to the
arc sky and night air. HET staff has several solutions for this prob-
center of arc lem that maximize the time the mirrors remain aligned. First,
curvature the dome is cooled during the day to the temperature expect-
ed for nighttime observations. Second, several fans circulate air
beneath the mirrors to keep the temperatures constant
throughout the structure. Third, HET staff has programmed
the control computer to adjust the mirrors at a given tem-
perature. The computer can tip, tilt, and piston mirrors to
compensate for the anticipated truss deformation.All three of
tangent these methods extend usable observing time and increase the
lines observing efficiency of the telescope.
Each mirror segment on the HET primary mirror array can
move in three ways: Goal
Tip Tilt Piston
Devise a method to align the mirrors of the model HET mir-
ror array to the center of curvature.

Materials
1. One flashlight per class.
2. One manila folder per class.
3. One small flat mirror per member of the array.
Tip — top or bottom edge moves toward or away from the center of cur- 4. Arc and CCAS location that are marked on the floor from Challenge 2.
vature. 5. Graph paper (one piece for each alignment trial) and pencil.
Tilt — left or right edge moves toward or away from the center of cur-
vature. Texas Essential Knowledge and Skills
Piston — mirror moves toward and away from the center of curvature.
SCIENCE TEKS PROCESS SKILLS
The HET aligns all of its mirrors in just a few minutes.A laser • plan and implement investigative procedures (6.2, 7.2, 8.2, IPC, Physics,
at CCAS (center of curvature alignment system) is fired at all Astronomy)
the mirrors; the mirrors reflect the light back to a detector • relationships between science and technology (8.5)
© 2000 The University of Texas at Austin McDonald Observatory
SCIENCE TEKS CONCEPTS Tips
• Systems (6.5, 7.5) This challenge follows directly where Challenge 2 left off.
MATH TEKS After completing Challenge 2, the model is set up for running
• Geometry and spatial reasoning (6.6, 6.7, 7.8) a solution for Challenge 3.
• Underlying processes and mathematical tools (6.11, 7.13, 8.14) This is an excellent application of the concepts of precision
and accuracy. The precision is the size of the target space in
Preparation which all the reflected beams must fit in order for the array to
The students’ model mirror array should be on the arc and be considered aligned. The accuracy is the distance between
ready for alignment, after completing Challenge 2.The student the reflected beam and the center of the target space.You may
at the center of curvature acting as CCAS will need a flash- wish to have a contest to see how small an area each group
light and reflective card. This may work best if the card hangs can achieve.
on a string around the student’s shoulders. Explain the prob-
lem to the students. Extension
Set up an experiment to determine the average duration
Problem that the mirror array remains aligned.
Tell the students to align the mirrors using “tip, tilt, piston”
while they are all held at the same distance from CCAS.When Tips
the CCAS student shines a flashlight at the mirrors (individu-
ally), each student should try to position the reflection as close Establish the alignment precision — how far to allow the
as possible to the center of the reflective card. Let the stu- mirrors to drift before declaring them out of alignment. Ask
dents decide the measure of success (i.e., how big is the tar- the members of the mirror array to close their eyes during
get area.).They may wish to record the size of the desired tar- this experiment. With their eyes open, the students can
get by drawing a circle on the graph paper and comparing it “cheat” — each segment can adjust tip, tilt, and piston while
with the actual result when the light beams are reflected receiving visual feedback from CCAS to stay on target. With
there. their eyes closed, that feedback loop is closed, just like with
the real HET. Some new telescopes operate as if the students’
eyes were open using a technology called adaptive optics. In
Solution this case, the mirror array is always in alignment.
Elect a student to stand at the center of curvature to act as Ask for suggestions for methods to extend the “align time.”
CCAS. The student should systematically aim the flashlight
beam at each mirror in succession. The student holding the
mirror should move the mirror until the reflected light hits the
target area of alignment. CCAS and the student holding the
mirror may talk to each other to establish alignment just as
the mechanisms and computers do for HET.

© 2000 The University of Texas at Austin McDonald Observatory