How do telescopes "zoom" and change angle of view?
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.