Obviously there is some difference between the two and as like alot of the words in the english language they have different meanings. Like with a cassergrain type of telescope the secondary is not a diagonal so therefore it has the meaning secondary (second mirror). With a typical newtonain the secondary is actually diagonal, so we have used the term diagonal for the secondary. I think that because the newtonian design was the first one to use the terms, most newtonian users use the term diagonal, although I use secondary.
Geoff: isnt that 90 degree mirror actually a 45 degree mirror? it's angle is at 45 degrees exactly the same as a secondary in a newtonain. Look at a periscope, same system.
boy if that didn't confuse me nothing will
