…continuedFreeware from Sky & Telescope
In the Telescope Techniques department in the January 2002 issue of Sky & Telescope (PDF of article) Alan Adler explains how forcing airflow over the primary mirror of a Newtonian reflector dramtically improves performance. The article features a DOS program called Cool that models the effects of different thermal situations. You can download it here. Be sure to select "Save program to disk" when prompted.