We went in late afternoon and took a couple of the tours led by docents. Interesting, but lots of screaming kids on the tours...Did the 3D Mars exhibit which was pretty cool and watched the Mars Lecture/slide show. Then when it got dark we were able to look through the monster telescopes and view the moon and Jupiter. That was worth the trip by itself. My only complaint is that the Rotunda Museum is not open to just go in a browse. There has to be a tour going on or staff inside. We had time to kill waiting for sunset and wanted to go in and look around, but could not. The volunteers and staff were so good. It was very obvious that they enjoy what they do.

We spent an afternoon going through the Mars exhibit (amazing 3D images, wear the glasses furnished) and taking the Mars tour with a wonderful docent. There's another tour that immediately follows about Pluto. The observatory is open many evenings for viewing, though hours are reduced starting November, and we picked an off day. The exhibits and historic buildings are fascinating, especially the building housing the Clark telescope, which features amazing wood construction and an old, Jules Verne-like telescope. Best idea is to come for the tours in the afternoon on a day they have evening hours, then return for the evening viewing.

Staff and volunteers were lovely. Film was good, and we got to see the nights sky thru mega telescopes used by the pro's.
Need to have more info about the constellations in their exhibit area.

Our friends told us to just go at night, but we are glad we called before we went. They told us that our admission would cover anytime during one day, so we went for the tours in the afternoon and came back for star viewing at night. The movie & the tours were were a bit long and boring for our kids (ages 5, 7, 10), but the adults thought they were very interesting and informative. Be sure you go on a clear night or they won't have nighttime viewing. And go early to avoid waiting in a long line for telescope viewing.