If your tube is long enough, consider making it about F10. Then you won't have to parabolise - just look for straight lines with a ronchi screen and a good edge. Your tester can then be very simple - just a light behind the ronchi screen. No measurements are needed.
I would always make my own tester. There are heaps of plans on the net and they are simple to build. Just purchase a ronchi screen - glass ones are better but the film ones are OK too.
This site may be useful re testing:
http://www.users.bigpond.com/PJIFL/ronchi_index.html
Good Luck with your project and Merry Christmas!
Rod.