Hidden Valley Observatory (HVO) is located at 4715 Hidden Valley Rd, Rapid City, South Dakota (this is not a mailing address). 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
Note: There is no mail service to this address. Contact is through https://sdbhas.org/contact-us/