Ray Menke is building a Helios in Texas (see blog) and had some questions on the focal drift and mirror calibrating process that are not well explained in the existing material so I wrote back a fairly long diatribe about the issues, and in case others have the same questions I’m posting the response here.
I also drew up an image explaining focal "cross over" which happens with the spiders: the light from one corner will cross the reflection and hit the other corner but the focal point size stays exactly the same. What does change however is that the internal stresses in the mirror increase leading to breakage. I think most of the mirror breaking is caused by this sort of thing as shown bellow.

Techniques


