Bino viewer increases magnification but by how much?

[inspectapart]

Hi, I am using my William Optics binviewer in my 3.5 Questar and I can see that there is quite an increase in magnification compared to the same single 20mm ep directly to the scope. I presume this is due to the extra distance the light travels to reach focus in the BV, therefore increasing the focal length of the scope. I just wondered by roughly how much the increase in magnification is?

Any ideas?

Bob.

[Peter Drew]

Probably around 30% extra. As you advance the primary mirror to provide the extra back focus through the binoviewer, the secondary "spot" acts as a Barlow lens.

[inspectapart]

Thanks Peter. I`ll try to do a comparison using different eps to compare the power between them.

and the BV. When the sky clears in Shrops that is.

Bob.