QUESTION:
How does the telescope keep a steady camera image during flight?

ANSWER from Rick Doll on October 17, 1995>
Initially the telescope is put in a "free floating" condition.  The
telescope floats on about 3 millimeters of air.  It is also balanced
at a point between the telescope mirror assembly (all of the pieces
in side the telescope cavity) and the bell housing including the
infrared detector assemblies of the scientists (pieces inside of the
fuselage of the airplane).  When the telescope is in this state you
can move the telescope with one finger, and it weighs about 5,000
pounds.

Next the Telescope Operator puts the telescope under gyro control.
Gyro control means the telescope's compensation system now receives
inputs from a three-axes gyro mounted on the telescope.  The three 
axes are Elevation, Azimuth and Line Of Sight (LOS).   The three-axes
gyro (dependent upon it's precession from the telescope) commands
the compensation system to keep the telescope in the same position
as it was in when it was put under gyro control, unless commanded to by
the telescope operator, tracker, scientist or as (demonstrated on "Night
Flight to the Stars") people on the ground.  The Telescope Operator will
point the telescope in the approximate location of the stellar object
that will be used tracking.  Next the Telescope Operator enables the
compensation system to receive inputs from the tracker and the Tracker
Operator's joystick.

Prior to each flight the Tracker Operator receives, from the scientists,
a list of objects that are going to "viewed".  Since most of the 
astronomical positions of the infrared objects the scientists view are
invisible in the optical portion of the spectrum, the Tracker Operator
must apply the IR positions to the charts of the visible objects (mostly
stars) so that, in flight, the Tracker Operator may point the telescope 
at the IR position using the offset visible stars.  After the Telescope
Operator has enable the tracker and Tracker Operator's joystick the
Tracker Operator will move the telescope using the joystick to the
preselected visible star.  The Tracker Operator then will enable tracker
mode on the tracker.  Then the Tracker Operator will use a tweeker box,
part of the tracker to offset the visible star so the scientist can
view the IR source.

The telescope has an Oscillating Secondary Mirror (OSM), a mirror that
moves back and forth allowing the scientist to look at blank sky half of
the time and the other half looking at the IR source.  The reason for
this is so the scientist can add the inverse of the blank sky reading
to the IR source reading and the sum of these two equals just the IR
source.  When the visible star is viewed in the focal plane you can see
two images of the star.  The Tracker Operator will see to "beam"
definition of the star, one left the other right.

It is at this time that the Scientist or ground person can move the
telescope by sending a nod command to the tracker.  The tracker in turn
tells the compensation system to move to the next beam and the three-
axis gyro keeps the telescope stationary in relation to the axis of the
earth.

As you can see the three-axes gyro is what keeps the telescope steady,
but it takes a human to determine the stellar object and track star.
Also a human is who determines which beam to be used.

I hope this answered your question.  You can use a spinning top to see
how precession works.  How does precession help you ride a bicycle or
motorcycle.


Rick Doll
Telescope Operator
Kuiper Airborne Observatory