Technically, you don't need a telescope for astrophotography, but for planetary work, it works a lot better.
For wide field, you're usually better off with a camera lens; a good prime will have better resolution than your sensor, approaching what even very slow films can manage. For high magnification, however, the reduced element count in a telescope (especially in a reflector, which also eliminates some aberrations that are very hard to remove from glass lenses) improves image quality.
Your zoom plus converter probably has sixteen to twenty elements total, each of which has surface errors, scatter, and reflections that degrade the image. A 100 mm f/12 refractor (what a photographer would call a 1200 mm lens) will have two or three elements in the form of a cemented achromat or apochromat.
Add to that the fact that many telescopes include some means of tracking the sky (nothing has yet beat a clock-driven equatorial mount, correctly aligned, for this) and for a camera, that's another add-on -- and you're probably ahead buying a decent telescope, with tracking mount, and a conversion to mount your camera on the focuser, compared to trying to get good images with a zoom.
A compromise would be to replace your zoom with a long prime -- 400mm aren't hard to come by, and play much nicer with teleconverters than zooms do. You'll still need a tracking mount, of course.