QUESTION:
How does the Hubble "turn" to focus--like does it have a rocket
booster of some sort to turn it to the right direction?

ANSWER from Tony Cruz on April 4, 1996:
Your question on how we move the telescope is a very good one.

Two of the toughest pointing control requirements are to move the 25,000
lb (11,600 kg) telescope 90 degrees in less than 18 mins and place a
target in a scientific instrument aperture as narrow as 0.1 arc sec.
Pretty slow!  This is like watching the minute hand of the school wall
clock.  The guidance system on the telescope is so accurate and stable
that it can pinpoint its target within 0.007 arc secs and can stay there
for more than 10 hours.  This accuracy is equivalent to hitting a dime
with a laser beam from 400 miles.  We measure everything in arc secs.
One arc sec is 1/3600 of a degree.

We don't use rockets to move the telescope because a control system using
rocket firings may not be the most efficient, safest, or cheapest design
when you look at all the mission requirements.  The design would have to
factor in many things such as: the spacecraft's orbital parameters
(inclination, altitude, etc.), spacecraft size and weight, how fast the
spacecraft turns, aerodynamic drag and gravity forces, etc.  Managing the
spacecraft's total system momentum so the telescope doesn't tumble out of
control becomes very important.  The biggest problem with rocket firings
is the high risk of contaminating the telescope surfaces including all
the science instruments and mirrors.   Sooner or later HST would also run
out of fuel.  It would be too dangerous to have astronauts replace tanks
that are pressurized and highly explosive.   If there is a need to place
HST in a higher orbit, the space shuttle will be used to reboost the
telescope.  In fact, it is pretty likely that this task will be performed
in the 1999 servicing mission (two service missions away).

Engineers designed HST's pointing control system using attitude actuators
and attitude sensors which are cheaper and replaceable.  The primary
actuator hardware used to change the telescope's attitude are four
reaction wheel assemblies.  The reaction wheels provide most of the
momentum needed to slew the telescope.  Each of these wheels can be
visualized as spinning wheels or flywheels.  Each reaction wheel assembly
is 25 inches in diameter, 20 inches high and weighs 105 lbs.  Short
pulses of electricity spin these wheels in one direction.   Changing the
wheel speeds cause the telescope to rotate in the opposite direction.
This conserves the system's total angular momentum.  The telescope can
also operate with just 3 wheels.  Each wheel can spin up to 3000
revolutions per minute.  Tachometers are used onboard to measure wheel
speeds and are used in the telescope's computer to control the vehicle.
Along with the reaction wheels are three important primary sensors that
measure changes or errors in the telescope's attitude.  These consist of
gyroscopes, star trackers, and fine guidance sensors.  The gyroscopes
create a reference system for the telescope because there is no up or
down in space.  The gyroscopes measure changes and sense when the
telescope has reached its programmed position.  Star trackers are
typically used following a large telescope slew to reduce any attitude
error to less than 90 arc sec.  During science observation periods, the
fine guidance sensors are used to control the attitude to the .007 arc
sec accuracy requirement.

I hope that I haven't confused you too much.  I used terms like angular
momentum, flywheels, and gyroscopes.  Your school or public library
should have textbooks that might help you better understand their
principles.

I didn't cover a lot of the other secondary hardware we use to control
the vehicle such as magnetometers, magnetic torquer bars, and sun sensors.

Thanks for the question!
Tony Cruz