QUESTION:
You're going to be taking photographs of Comet Hyakutake this week using
Hubble. How difficult a task is it to photograph something that close
travelling that fast?

ANSWER from Ed Colbert on March 23, 1996:
Using previous information about the location of the comet and computer 
programs that are used for predicting orbits, one can predict the exact 
location of the comet in the sky as a function of time and tell the Hubble 
where to point as a function of time.  This is actually what is done to
take pictures of objects moving in the solar system.  Then, in the electronic 
image, or "photograph", the comet is "stationary" and the stars are blurred.

ADDITIONAL ANSWER from Anuradha Koraktar on March 26, 1996:
Suppose you were taking a picture of a parked car; it would be clear and 
well defined.  But if you were photographing a car that was moving very 
fast (compared to the speed of your film) the image would be blurred. 

The same phenomenon happens when you take images with the Hubble telescope 
of moving targets and stationary targets.  Taking images of the comet is 
not as easy as taking images of a galaxy for instance.  This is because 
the galaxy hardly moves during the exposure, but the comet moves during 
an exposure and leaves streaks on the images if the telescope is not moved 
along with the comet. To eliminate the streaking of the comet in an image, 
the Hubble Space Telescope works in the gyro mode and adjusts its pointing 
according to the speed of the comets motion. This excercise is acheived by 
a process called "guide star hand-over". Many short exposure images are 
taken and then added together to get a higher signal image that can be 
analysed.