Don McCarthy's Questions of the Week

1. The focal ratio of the 21" Ray White Reflector is not published on Steward's Web site. How could you measure it?

2. The 61" telescope has a focal-ratio of f/13.5 and the ratio for 90" telescope is f/9. How do their plate scales compare?

3. Both the 61" and 90" telescopes also can be used at f/45 but neither telescope was built with this capability. Can you think of any reasons for wanting that focal ratio?