This diagram demonstrates why blocking part of the primary mirror makes the resulting image dimmer, but does not block the field of view. All parts of the primary mirror contribute to the complete image. In a sense, one can think of the primary mirror as lots of little mirrors, with each sending a complete image to the detector or your eye. The secondary mirror blocks the middle of the primary mirror from receiving light, so this is a good place to put the hole for the light to travel to your eye or the detector.