Location of Barlow and its affect on magnification

[sojourneyer]

I have read in some places  that you should affix the barlow before the diagonal and other places where you should put the barlow after the diagonal.

What do you do and how does the location affect magnification. ??

[vlaiv]

Magnification of a barlow is affected by distance between its lens and focal plane and depends on focal length of barlow lens itself.

In order to calculate exact magnification - you need to know this focal length of barlow element. Some companies publish this data. For example, Baader VIP barlow has

Note that focal length is negative - this is because barlow is a negative lens (it diverges rays rather than converging them like positive lens).

In any case, magnification given by barlow lens is calculated like:

magnification = 1 - distance / focal_length

(you must use negative focal length in above equation).

For Baader VIP barlow to work as prescribed, distance between barlow element and focal plane must be:

2 = 1 - (X / -65.5) => X/ -65.5 = -1 => X = 65.5mm.

If you change that distance to say 80mm you will actually have:

magnification = 1 - (80 / -65.5) = ~ x2.22

Prescribed magnification is usually achieved when you place focal plane at the "shoulder" of the eyepiece inserted into barlow lens - or when you put focal plane at the end of barlow body:

To answer your question - if you place barlow in front of diagonal you will get quite a bit bigger magnification than prescribed one.

 

[Louis D]

Also, the shorter the Barlow tube (shorter focal length) or the longer the diagonal's optical path length (2" diagonal is longer than 1.25" diagonal), the larger the multiplier effect of putting the Barlow before the diagonal will be.  Looking at the above equation, it should be obvious why these two points are true.

[sojourneyer]

13 minutes ago, vlaiv said:

Magnification of a barlow is affected by distance between its lens and focal plane and depends on focal length of barlow lens itself.

In order to calculate exact magnification - you need to know this focal length of barlow element. Some companies publish this data. For example, Baader VIP barlow has

Note that focal length is negative - this is because barlow is a negative lens (it diverges rays rather than converging them like positive lens).

In any case, magnification given by barlow lens is calculated like:

magnification = 1 - distance / focal_length

(you must use negative focal length in above equation).

For Baader VIP barlow to work as prescribed, distance between barlow element and focal plane must be:

2 = 1 - (X / -65.5) => X/ -65.5 = -1 => X = 65.5mm.

If you change that distance to say 80mm you will actually have:

magnification = 1 - (80 / -65.5) = ~ x2.22

Prescribed magnification is usually achieved when you place focal plane at the "shoulder" of the eyepiece inserted into barlow lens - or when you put focal plane at the end of barlow body:

To answer your question - if you place barlow in front of diagonal you will get quite a bit bigger magnification than prescribed one.

 

thank you.. Very interesting reading