Extension tube!?

[long_arms]

Hello,

I have currently been using just the stock skywatcher 2x barlow for all my images, but have experimented with inserting the barrel of an eyepiece into it as a small extension tube.

I may be buying a 3x barlow soon, but for the meantime I have some pipe that is ideal for making an extension tube with.

I was just wondering if anyone could help me figure out how long the extension tube would have to be to create a magnification of 4x? Or 3x?

I would love to see the actual math involved.

Many thanks,

Dan

[Cornelius Varley]

Copied from wikipedia

"The amount of magnification is one more than the distance between the Barlow lens and the eyepiece lens, when the distance is measured in units of the focal length of the Barlow lens. A standard Barlow lens is housed in a tube that is one Barlow focal-length long, so that a focusing lens inserted into the end of the tube will be separated from the Barlow lens at the other end by one Barlow focal-length, and hence produce a 2x magnification over and above what the eyepiece would have produced alone. If the length of a standard 2x Barlow lens' tube is doubled, the lenses are separated by 2 Barlow focal lengths and it becomes a 3x Barlow, if the tube length is tripled, the lenses are separated by 3 Barlow focal lengths and it becomes a 4x Barlow, and so on."

and

http://stargazerslounge.com/topic/135637-barlow-and-extension-tube-estimating-effective-magnification/