How do telescopes "zoom" and change angle of view? Images from the Hubble Space Telescope have widely different angles of view. Is this only the result of composing multiple images, or can the telescope itself change its "zooming" optically, mechanically? How does the ability to change the angle of view differ between the major scientific telescopes of today and the near future, space and ground based?

And what about cameras on interplanetary probes? Can for example New Horizons optically change its zooming as it passes Pluto?
Telescopes tend to have a fixed focal length. What changes is the size of the sensor in the instrument used. If a small sensor is used, then a smaller section of the field of view is exposed, resulting in a narrower field of view being imaged than the equipment is capable of. If a larger sensor is used, more of the field of view of the telescope is utilised. 

Additional optics in the light path that are associated with the specific instrument being used on the telescope will affect the field of view also. 

Telescopes do not offer the capability of 'zooming' as you understand it from your consumer digital camera.