Scratches and Pinholes Oh My!
You've probably heard that it's better to tolerate some dust on your telescope's mirror rather than risk scratching it by too frequent cleaning. So what does a scratch do? Scratches (sometimes referred-to as "sleeks") may occur when the mirror is cleaned and they actually alter the surface of the mirror by acting as edges from which diffraction occurs. This can then scatter light, reducing contrast. A few sleeks aren't going to make any noticeable difference, but thousands of tiny scratches will have more severe consequences. A mirror does require cleaning now and then, but to ensure its good performance during the life of the coating, careful cleaning is the answer.
A pinhole is a tiny spot that is less reflective than the rest of the mirror. They can often be seen by looking at the back of the mirror where little points of light betray these tiny thin spots. Pinholes don't scatter much light - they just don't reflect as much of it back to the focal plane. There is a small amount of diffraction that results from pinholes, but being a tiny point, it's not nearly as much of a problem as is a scratch (which is a line). At worst, pinholes are a tip-off that the mirror coating is breaking down.
