Barlow Mag Factors

[red eye]

Good evening Stargazers,

I have a televue 3X barlow. The barlow lens can be unscrewed from the hollow barlow tube, allowing you to screw this lens directly on any 1.25 eyepiece. Does anyone know what the magnification factor would be in this mode. 

Also, is the lens identical in both the 3X as well as the 2X units, the only difference being the length of the tube assembly ?

Thanks,

David

[ronin]

You cannot tell, it all depends on where the eyepiece focal plane is and where the barlow lens is positioned in relation to this.

It therefore means that one more or one less thread along the eyepiece tube will make a large difference. Owing to the fairly significant change in position I would say that relocating it to the eyepiece would never give an image.

It will be on a 1.25" fitting most likely owing to the availability of holders that size and that the machinery required is already set for that diameter and thread. Why change it all for no reason?

Again the lens could be identical in the 2x and 3x with the location being different. At a guess I would say the lens would be closer in towards the scope objective in the 3x then the 2x. However this difference may be only 0.5mm. Small changes can make big differences in optics.

To TV the question is: Is it cheaper to make the same mechanical body and use a different lens, or is it cheaper to make a different body and use the same lens.

I would have expected the same body and a different lens, that way the lens can be tailered to reduce other aberrations at 3x that are are not there at 2x.

The lens may be +ve or -ve depending on the chosen design, usually -ve these days.