Barlow Configurations

[jermng]

Hi,
I've read in a few places that one can have different ways of using a barlow lens for different magnifications. e.g. before the diagonal, after the diagonal, with an extension tube etc.
Also read that the GSO barlow in particular can have the lens unscrewed and do things like screw directly below the EP and things like that.
Can someone explain the different configurations and what they achieve? The only way I know how to use a 2x barlow now is after the diagonal and get a 2x for my EP... as advertised .. =)

Jeremy

[weixing]

Hi,
Below taken from one of the website in the internet on how to calculate Barlow magnification:
F = focal length of objective or primary
f = focal length of Barlow [1]
J = joint focal length (effective focal length)
d = separation of Barlow and original focal plane (objective focal plane)
x = separation of barlow and new focal plane (eyepiece focal plane)
M = amplification of Barlow
J = (F×f)/(f-d) ...(1) (combined lens formula)
M = J/F ...(2) (by definition)
= f/(f-d)
The separation of the Barlow and the new focal plane can be calculated from M and f:
x = f×(M-1) ...(3)
...from which we get :
M = 1 + (x/f)

Basically, for a 2x barlow: 2x when barlow inserted after diagonal and 3x when barlow inserted before diagonal.

Have a nice day.

[jermng]

Cool! But brother ... I only understand the last line ... HAHA ... Got simpler way of explaining or not? I one night out from camp dun wanna waste brain cells ah .. HAHA ...

[weixing]

Hi,
In simple form, the magnification is determine by the ratio of the focal length and the distance between the barlow and the eyepiece. In short, the greater the distance between the barlow and eyepiece, the higher the magnification and via visa.

Have a nice day.