Have I changed the focal length ?

[ZOG]

Last week, when I collimated the scope pre Kielder, I tightened the primary collimation knobs all the way in, wound them out 2 full turns, and collimated from that position.

When I got to Kielder none of my EP's would come to focus without using an extension tube. Have I inadvertantly changed the focal lengh of the scope by moving the primary up the tube ? It seems like a small movement but the scope acts like the primary has been moved up the tube.

[GazOC]

Yes, that does sound like whats happened Zog.

[Merlin66]

Certainly sounds like the main mirror is further into the tube than previously. What length spacers did you have to use at the eyepiece?

That would give you an indication of the movement needed to get the mirror back to it's original position.

I'm assuming nothing else has changed.

BTW - Which telescope?

[jgs001]

Isn't the focal length fixed by the shape of the mirror ? Moving the mirror would move the focus point at the same focal length ?

[GazOC]

Isn't the focal length fixed by the shape of the mirror ? Moving the mirror would move the focus point at the same focal length ?

Yep, the focal length will be same (I didn't make that clear , sorry) but now the mirror has been moved up the tube the point of focus know falls at a point beyond where it was before and extension tubes are needed.

[beamish]

Gaz is right , the FL is the same, just the image plane has been moved out.

[ZOG]

The scope is a 12" SW dob and I just used a SW barlow with the lens removed as an extension tube.

sometime this week I'll re collimate but this time I'll wind the knobs fully in then back out for 3 or 4 turns and see what happens

Thanks for the help.

Bill