Harquahala Smithsonian Observatory was open from 1920 to 1925. This was never a telescope site. Scientist lived atop the mountain and collected data to assist in forecasting weather. The site contained a theodolite for measuring solar activity among many other interesting devices. After five years of use the operation was moved to California. The summit lies just outside the 1990 established Harquahala Mountains Wilderness. It is possible to follow a 10.5 mile 4X4 road to the summit. Options to connect down the east side into West Fork Sunset Canyon look interesting on map but who knows.
Now read on, Ksorensen has contributed a mini summary.
The first two miles are a steady but relatively easy climb; the last three miles are pretty tough, though the trail is wide and easy to follow the entire way. The hike is quiet and desolate except for at the very summit, where flocks of Canadian snow birds drive up in SUVs and Quads to sip Milwaukee's Beast and enjoy the view. Fortunately the winter visitor approach is from the south side of the mountains and one will not encounter them until at the very peak. This is a difficult but rewarding hike...even my dogs were pretty worn out. We ran into a Western Patchnose Snake but saw no other wildlife.
WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.