From the Northwestern web site:

The Friday night observation tours at the Dearborn Observatory are open to the public, and there are two hours open for visiting:

Spring/Summer Hours:

• 9:00 - 10:00 pm: Reservation hour only.
  Your group will have a tour of the Observatory, and be able to look through the telescope.

• 10:00 - 11:00 pm: Walk-ins welcome.
  We try to allow everyone at least one opportunity to look through the telescope, but space is limited and there is no guarantee that everyone will be able to look through the telescope if we are very crowded.

There is no fee for the Friday night observation hour, however if you make a reservation for a group of 10 or more we require a refundable deposit which will be returned to you on the night of your visit. If you would like to make a reservation or need information about viewing, please call Gretchen Burnett at 847-467-3798.

Please be advised that the dome does not have heat or air conditioning so please dress appropriately. Friday night tours are held "rain or shine," although of course cloud cover affects viewing. Tours are hosted by Ph.D. students and/or undergraduate students majoring in Physics & Astronomy. Visitors are encouraged to ask questions.

Unfortunately, the Dearborn Observatory is NOT ADA-accessible. Several staircases must be climbed in order to reach the telescope.