I understand visually that through a telescope, deep space objects, planets, etc, look much "better" than through a DSLR lens. However, I fail to find a good, technical reason why is this true.
Indeed, telescopes have long focal lengths. But an 150-600mm lens with a 2x teleconverter can reach to 1200mm. On a crop sensor, this can get to 1800mm.
It is my understanding that, when doing astrophotography, the telescope becomes the lens of the camera (and no eyepiece is needed). Then why is a 1600mm telescope better than 150-600mm lens with a teleconverter, for example?