Roland Christen wrote:
From a paper on MTF I read last year (cannot find it right now)I believe that the correct curves would show that the obstructed scope begins at a lower value at the lowest frequency at the left, and continues to a higher frquency level at the right hand part of the chart.
Surely not the paper you read, but there is an article in Sky and Telescope that gets at some of the same issues that you discuss. Note that in Rickey's article, the MTF curve very definitely is normalized to 100 percent at zero spatial frequency; it is not scaled downward to account for the lower light throughput. That is precisely the motivation for the "system characteristic function"--a singularly uninformative name, incidentally, but I admit I don't have a better one of the top of my head.
Anyway, for those who are interested, the article is "Secrets of Telescope Resolution," by Daniel Rickey, in the June* 2006 issue of Sky and Telescope.
*I corrected this; it used to say January 2006, but it's really the June 2006 issue.
