Originally Posted by Satchmo
If you have a large enough secondary you do not need to offset but you may waste a bit of light through having a slightly larger than necessary secondary.
This is a common misconception. If you do not offset the secondary mirror away from the focuser, then following proper collimation steps will automatically tilt the primary mirror forward towards the focuser to compensate. The end result will be the same. That is, a scope with a minimal sized secondary mirror that is mounted without an offset will be collimated as well as another scope with a secondary mirror mounted with an offset.
See attached illustration. Left illustration is what typically comes to mind but the one on the right is less intuitive. It shows how the primary mirror is tilted towards the focuser. The end result is about the same -- assuming no vignetting is introduced. Typically, the tilt is in around 0.5 degrees.
Interestingly, in the article referred to by Merlin
http://www.lcas-astronomy.org/articl...ory=telescopes
Bryan Greer was quoted stating:
“When you get to the final collimation step of tilt adjusting the primary, you are pointing the primary back at the optical center of the secondary. Once again, the light cone will be intercepted perfectly symmetrically. The only consequence of not offsetting is that the primary mirror is no longer pointed exactly down the middle of the tube.”
Bryan who is knowledgeable about scopes is 100% accurate. However, the author of the article, Jack Kramer, who quoted Bryan did not understand what Bryan meant. Jack goes on to reinforce the common misconception by stating:
“One exception is when the size of the secondary is at the absolute minimum, in which case a non-offset secondary would not intercept the entire cone of light coming from the primary mirror.”
As others have stated, the away-from-focuser offset will bring the optical and mechanical axes closer. That helps DSC and tracking unless the DSC computer has the intelligence to compensate for this error.
To make a long story short, collimate per proper steps and do NOT worry about the secondary mirror offset and do NOT worry about how the secondary mirror is mounted.
Jason