A probably stupid collimation question!

[MarkMayf]

I have been reading up on how to collimate my first telescope, a Heritage 130P flextube, fow when the inevitable happens and it drifts out. The process seems simple, I have a cheshire and I am sorting out a collimation cap.

The only question I have is, how do I know if the secondary mirror should be offset or not? Or if the mirror is offset on the spider, which in this case is a single arm?

Or, easier, does anyone have a correct measurement for the length of the arm. Tube wall to bolt centre? (That's cheating a bit!)

Thank You.

Mark

[Freff]

I believe the offset will happen automatically when collimated correctly. Don't try to introduce the offset during collomating, it's a anomaly of fast scopes.

I could be way off but that's my understanding. HTH.

[AlBoning]

Centering the secondary mirror under the focuser (in the view of a sight tube) automatically offsets the mirror in the direction of primary mirror. The question arises if the secondary should also be offset away from the focuser. There's no harm in doing so but there's no harm in not doing so either. It helps to understand when one knows that the optical center of the secondary is not the geometric center. This is a natural consequence of reflecting a conical rather than cylindrical light beam. Imagine the secondary mirror as the hypotenuse of a 45° right triangle. Each side represents one of the two offsets mentioned above. The quick and dirty formula for calculating offset is: offset = minor axis/(4*focal ratio). To calculate the distance from the geometric center to the optical center one multiplies this by the square root of two. The consequences of not offsetting away from the focuser are very minor, first the angle of reflection is something other than 90° which is of no consequence, and second there is cone error associated with rotating the tube, this may become important if one is doing photography but otherwise it is of no consequence either. Offset grows in direct proportion to secondary size and inversely proportional to focal ratio. Fast scopes with large secondaries have an offset of several millimeters, slow scopes with small secondaries almost none.