Someone here on CN asked me a question that prompted me to investigate something I have suspected for quite sometime and that is the amount of back focus gain the Baader GPCs provide, may depend on their distance from the focal plane, or, in other words, the optical path length of the viewer/GPC combination.
Basically, I figured that, like any barlow element, the further you place the GPC from the focal plane, the greater its magnification and with the greater magnification, comes a greater amount of back focus. As the viewers I use all have very different optical path lengths, I was curious so I ran some additional back focus measurements using the Baader 1.7X GPC with the viewers you see below with the exception of the Denk II.
And indeed, I was right.
For example, I measured the back focus gain with my Zeiss Sharpest with the Baader Click Lock eyepiece holders (first viewer on the left in the group photo below) with the Baader 1.7X GPC installed facing down into the top of the quick changer (it does not screw into the bottom of the viewer the way it does with the Baader MK V or Zeiss 30mm viewer), and with the element oriented properly. This configuration, with this viewer, gives the greatest optical distance between the GPC element and the focal plane of all of my viewer/GPC combinations. Well, I gained an additional back focus of.....54.5mm.
Now the shortest optical distance between the 1.7X GPC element and the focal plane is with my Baader MK V (first viewer from the right) with the GPC screwed into the bottom of the viewer (again, with the GPC element properly oriented). This configuration gained an additional back focus of 35mm. A very large difference of 19.5mm!
So,as you can see, the back focus gain when using the Baader 1.7X GPC depends very much on the optical path length of the specific viewer and the placement of the 1.7X GPC element (either up inside the viewer, or down inside the top of the quick changer). The longer the combined path length, the greater the gain in back focus...just like a barlow, which is what it is. And each configuration will have a unique magnification associated with it too.
No wonder people have reported different magnification levels for the 1.7X GPC!
Here is the data from left to right in the picture (with the Denk II excluded) with the Baader 1.7X GPC mounted down in the top of the Baader Quick Changer (except as noted):
1. Zeiss "Sharpest" with Baader Click-lock eyepiece collets from Denis Levatic: 137mm viewer optical path length, 54.5mm BF gain
2. Zeiss Sharpest like above but with "blue" eyepiece collets, again from Denis: 127mm viewer optical path length, 48.5mm BF gain
3. Zeiss 30mm with blue collets from Denis: 118mm viewer optical path length, 44.0mm BF gain, 39mm with GPC threaded into the viewer
4. Good old Baader MK V, super charged by Denis for 30mm clear aperture: 112mm viewer optical path length, 40.5mm BF gain, 35mm with GPC threaded into the viewer
I will have to repeat these measurements using the 1.25X Baader GPC.
Stay well and safe.
Edited by Jeff B, 22 March 2020 - 11:54 PM.