QUESTION:
How many different photos must be taken and composed until the image is
complete?  How do you process it?

Answer from Anuradha Koratkar on April 15, 1996:
The WFPC2  images are composed from multiple observations of the target in
different wavelength bands. For example, the target is observed in the red,
blue and green colors and then all these observations are merged to get a
"visible light" image. To get the pretty pictures that you see normally we
co-add images from at least 3 different wavebands.

Things that we do during processing is to correct for any instrumental
anomalies and background radiation. This process is "cleaning" the data.
We also have to keep track of how the position of the target might have
changed (this is specially true of planets and other solar system
objects), so that the superimposition of the various observations is as
perfect as possible.
The co-adding of data is done only after the images have been  "cleaned".