If you're desperate there's a road, but come on...
Summary: Mt. Hopkins is home to the Fred Lawrence Whipple Observatory, an astronomical observatory owned and operated by the Smithsonian Astrophysical Observatory (SAO) and is their largest field installation outside of their main site in Cambridge, MA. It is also one of the dominant summits of the Santa Ritas; the towering peak west of Josephine Saddle and Mt. Wrightson, seen easily from all of Tucson. I've always wondered exactly how to hike up to it, a man well-versed in the Santa Ritas showed me the way.
Hike: There are numerous ways to make this hike. This route is a lasso loop out of the Madera Canyon trailhead. From the ever present Roundup trailhead at the top of Madera Canyon head off from the top end along the Old Baldy trail. Within a quarter mile the Old Baldy trail breaks off left at a signed intersection. Continue straight ahead on up the Vault Mine trail, which has been written up before on this site. The trail first climbs up the east side of Madera Creek for a ways, then breaks away left through some wonderful forested areas. Look and listen for the elegant trogon through here, it seems to be a bit of a hotspot. After a total of 0.6 miles from the Baldy intersection an old metal sign on your right announces the intersction for the Carrie Nation trail straight ahead and the Vault Mine trail which breaks off to the right. Stay straight on the Carrie Nation trail which continues up canyon for another 0.6 miles to the old remants of the Carrie Nation Mine at 6480 feet. There is a nice official "End of Trail" sign here. Look to your right to pick up the very clear trail continuing up and away from the sign. This Carrie Nation "route" is just about as easy to follow for another 0.6 miles and 700 feet up to the Agua Caliente Trail. There are a couple of side trails that break right off the route to ignore. The route pretty much follows the drainage up initially with 2 distinct paths breaking away right. The third trail breaking off right is the one you want, otherwise you continue up the drainage as it becomes steeper. This third right takes you steeply up a ridge cominf down from the Agua Caliente trail, your destination. If you see any side paths breaking off once you hit the ridge, just keep going up. There are several small stands of impressive young pines through here, a wonderful stretch of trail. Soon thereafter, you hit the Agua Caliente trail running left-right at an unsigned intersection.
Make a right onto the Agua Caliente trail and head west for roughly 0.2 miles until an unsigned path breaks off to your left and up. This is the use path to the road leading up to Hopkins. It's not an official trail or on any maps, but it's a clear path nonetheless. Follow it up the eastern ridge coming down Hopkins for about 0.6 miles and 640 feet until it dead ends at a small railing and the road leading up to Mt. Hopkins, right at some sattelites you can periodically see through the trees on the way up. Once you hit the road, just hike that bad boy roughly 1.5 miles and aboyt 650 feet to the summit of Mount Hopkins at 8550 feet. The views from the top are sensational. Mexico, Tucson, the towering Sky Islands of Southeast Arizona, Nogales and Baboquivari among others.
Head back down the uppermost steep stretch of road, past the Wind Sock hill on your right. Immediately thereafter, the road breaks hard left. There is a railing here, and the nothingness beyond is your route back down (UNLESS you choose to head back down to the Agua Caliente Trail via the route you took up - a perfectly acceptable and easier route finding approach). IF you want to continue down via a bushwhack down the north slops of Mt. Hopkins, then this is your route. Drop down to the right of the rail and down the drainage. Eventually you will notice a ridge to your right (there is another one to your left you can follow for ways, called "Wrong Ridge" by the locals who know, just don't follow it for too long). The breakout point of this steep descending bushwhack is the Agua Caliente Saddle. You will drop steeply north through the pines for 1 mile and almost 1200 feet. Through the trees you can periodically see Pete Mountain, keep angling for that. There is a route and occasional cairns. Hug the ridge if all else fails.
When you reach the signed Agua Caliente Saddle you are home free. Take a right on the Agua Caliente trail for a quick 0.25 miles to the signed intersection for the Vault Mine trail. Make a left and descend at an almost unbelievable rate of 1360 feet over the next 0.9 miles back to the intersection with the Carrie Nation trail, make a left and strut the last 0.9 miles back to the trailhead and your car.
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.