Questions about a Barlow (Meade Telenegative 2x)

I am using a Meade 2x Telenegative APO (Meades name for 2x barlow) with a 6 inch F8 Achromat. I notice greater magnification if I put the barlow into the focuser, then the diagonal, then the eyepiece, as opposed to diagonal, barlow, eyepiece. What is the explanation as to why, & any approximation as to the new multiplication?

Also, I seem to notice less false color with this arrangement, than with a shorter focus eyepiece (& no barlow) providing approx. equal magnification. Does the barlow effect of "doubling" the focal length have an added benefit of reducing chromatic aberrations (ie a 6 inch F 16 would have less color than an F 8)?

A Barlow has a focal length, and because the lens has negative power, the focal length is a negative number. The magnification factor given by a Barlow depends on the distance from the Barlow lenses to the field stop of the eyepiece. The magnification factor is 1-D/L where L is the focal length of the Barlow and D is the distance from the Barlow lenses to the field stop of the eyepiece. This actually varies a little from eyepiece to eyepiece because there is no standard to which manufacturers must conform, but somehow the Barlow manufacturer decided on the dimensions of the Barlow assembly to hold the eyepiece a certain distance away. If you increase that distance with an extension tube, and a diagonal mirror is just a fancy extension tube for present purposes, the magnification factor will be increased (remember L is a negative number, so 1-D/L is the same as 1 plus something positive). Usually a 2x Barlow is about 3x when used ahead of the diagonal, but it depends on the Barlow and somewhat on the diagonal because what matters is how much extra spacing there is compared to the focal length of the Barlow.

The easiest analogy I can think of is a slide projector. When you look through a negative lens it makes things look farther away, right? Well, when you insert the Barlow into the light path of a telescope it is analogous to moving a slide projector farther from the screen (of course you must refocus). Increasing the distance between the negative lens and the eyepiece field stop (which is analogous to the projector screen) and refocusing has the same effect.


Like any refractive device, a Barlow can potentially affect chromatic aberration, but usually it only adds to the problem. Cancelling or significantly reducing the problem so near to the focal plane is far beyond a simple Barlow's capabilities. But if you notice it less, then that's a good thing.
--
Mike