The optics of barlow lenses

[Ags]

I have never owned a barlow lens but I understand they alter the eye relief of an eyepiece, and consequently can also vignette the AFOV. How does this work? Naively, I would expect the barlow to simply stretch the light cone, making a F5 scope into an F10 scope (for a 2x barlow)*, but if I actually put the eyepiece in a real F10 FL scope obviously the eye relief and AFOV are the same as in the real F5 scope. So what does a barlow actually do?

[Peter Drew]

Most Barlows are at least 22mm clear aperture so don't vignet the incoming light path, however large long focal length eyepieces with greater field stops probably would be vignetted and would benefit from a 2" fit Barlow. Such eyepieces are seldom barlowed anyway. The magnification of a telescope is essentially the ratio of the angles subtended by the focal length of the objective and that of the eyepiece. A 2x Barlow will halve the angle of the objective and therefore double the magnification. 

[Louis D]

It has to do with the barlow causing the normally converging rays from the objective to now be diverging.  A second group of lenses are used in telecentric designs (e.g., TV Powermate) after the diverging lenses to bring the rays back to where they were prior to this diverging group.  I don't know if this way of thinking about it is correct, but I visualize the outermost diverging rays slamming into the field stop of the eyepiece instead of passing through it because they're diverging.  This diverging also pushes the exit pupil outward because the eyepiece was expecting to coverge a converging rather than diverging light bundle.

[Sporadic Dobstronomer]

Here is my 2p worth:?

One way of looking at it is that if you take out the eyepiece and look into the focuser without the Barlow, the objective is quite a long way away.  When you insert the eyepiece it, being a lens, creates an image of the objective.  That image is the exit pupil.  The exit pupil is quite close to the eyepiece.

If you now take the eyepiece out, put the Barlow in and peer in again, the objective looks much nearer (and much smaller).  If you put the eyepiece back in, the image of the objective it is then further away than before.  In other words, the eye relief increases -sometimes by quite a lot.

What I think happens with modern eyepieces is that they have light paths that have been carefully designed around an intended location for the exit pupil.  If you move the exit pupil further away by sticking a Barlow in the path, then the light ends up not going where the designer intended so you can get vignetting.

The TeleVue Powermate is essentially a Galilean telescope shoved backwards into the focuser so the objective still looks a long way away and does not cause the same effect.

[25585]

Are there any optics the same as Tele Vue's discontinued Panoptic Interface for adapting their 35mm & 41mm Panoptic eps to their Big Barlow?

I do not want a Powermate as wish to have Big B just as that, but able to take the two big eps. 

[Louis D]

12 hours ago, 25585 said:

Are there any optics the same as Tele Vue's discontinued Panoptic Interface for adapting their 35mm & 41mm Panoptic eps to their Big Barlow?

I do not want a Powermate as wish to have Big B just as that, but able to take the two big eps. 

As far as I know, the TV PBI is unique.  They sometimes come up on the used market.  I picked up mine when they were being cleared out at discontinuation.