Peter, Yes! a camera, lens and goto mount will let you get a ton of great targets photographed. The Sigma 150-500mm lens is an awesome lens but at 500mm the weight of the glass will wind it back in to 150mm once you point it upwards. I made a plastic collar to go on mine and hold it out at 500mm.
You need to get away from this mindset of pressing a button and taking a "photo". Astrophotography is about gathering a LOT of photons and combining them to reveal fainter structures that you cant see through a viewfinder and reduce noise.
My photo of the
Pinwheel galaxy was taken with a Nikon D800e and Sigma 150-500mm just mounted on a GOTO mount. No telescopes involved. I took around 500 photos to combine and process, the galaxy wasn't visible in the JPegs from the camera (I take JPEG+RAW always) as it was still too faint but the GOTO mount had put it dead center for me. OK its an underwhelming photo but its what you can expect with processing.