I found a site that has a very complete discussion and some surprising information, namely, at high frequencies 30% obstruction is better than zero.
The MTF curve below (see What is a MTF curve ?) shows that the contrast is modified in a complex manner. Indeed it decreases in comparison with the unobstructed instrument, but only in low and medium frequencies (left part of the curve). On the other hand, in the high frequencies, the contrast is not decreased, it is even slightly increased.
http://www.seds.org/~spider/scopes/obstruct.html
Stu
The MTF curve below (see What is a MTF curve ?) shows that the contrast is modified in a complex manner. Indeed it decreases in comparison with the unobstructed instrument, but only in low and medium frequencies (left part of the curve). On the other hand, in the high frequencies, the contrast is not decreased, it is even slightly increased.
http://www.seds.org/~spider/scopes/obstruct.html
Stu
