From my limited experience photographing telescopes themselves, you need a very wide-angle lens. This is because telescopes are usually large, especially if it's an older observatory with refracting telescopes (lenses instead of mirrors), in which case the telescopes will be very long. And, you won't be able to get far away from them while in the observatory (the domes are expensive so they're as small as possible) for a standard lens to work.
Otherwise, you will need to resign yourself to doing stitching of several images into a panorama. Or just not getting the entire telescope. If the latter is what you plan, then you could try to get people standing next to the telescope to get scale, and then if the telescope extends outside the shot, peoples' reactions would be closer to, "Wow, that telescope is really big," as opposed to, "Why'd they only photograph part of the 'scope?"
Otherwise, telescopes can be kinda boring. One of my favorite shots was not of an actual telescope, but of telescope domes silhouetted against a sunset, which might be a shot you could try (see my image below as an example).
Good luck.