‘I’m getting enfeebled in my old age,’ claims 76-year-old Professor Roger Griffin, who is struggling to climb Utah’s Rainbow Bridge trail in BBC4’s affectionate and informative documentary about four outstanding British astronomers who celebrate 50 years of their work and camaraderie with a US road trip through California, Arizona, New Mexico and Utah, taking in observatories on the way. It’s an uplifting document of endeavour, knowledge and friendship.