Hidden Valley Observatory (HVO) is located at 4715 Hidden Valley Road, Rapid City, South Dakota. The observatory sits on the south side of Hidden Valley Rd and the driveway to the observatory is marked by a post with a white address number sign of 4715. It is ~100 yards on the gravel driveway to the observatory. The nights of public workshops a larger sign will be present at the gravel entrance.
For a view of the property with parking and other amenities noted, see this page