I just read Dr. Plait's reminiscences of Johnny Carson on the home page.
If I remember correctly, Johnny was also a bit of an amateur astronomer -- at least to the extent of owning an early orange-tube Celestron (an 8? a 14?) telescope in the early 1970s. Short of a Questar, that was pretty hot stuff in the commercial market back then.
I too remember Dr. Carl Sagan's appearances on the show. It might be worthwhile to see those again, if they could somehow be made available. Turns out there were a total of four appearances.