So if you placed an appropriate spacer between the gpc and the binoviewer, would you be able to achieve the stated magnifications with a MBII?
It is a mixed bag and it is overly simplistic to say that it would work fine.
Here is the reason. Like a Barlow, the GPC power will increase if you move it further from the BV.
The negative about this is that the GPC differs from a Barlow in a major characteristic. The GPC is designed to induce under-correted spherochromatism into the light path.
Imagine passing light through a prism. When you do this, you get a rainbow. This is essentially what happens in the prisms of the binoviewer and this is why you get color error when you use a prism binoviewer in a fast Apo or refactor. Now because the distance that light travels inside the binoviewer, that "rainbow" never gets so big as to actually see it is a rainbow. Mostly you see color fringing. If you took the prism out though, and let the light shine through and projected it on to a wall, because it has to travel more distance to diverge out into a fan, you see the full rainbow. It is the distance that makes this happen.
The GPC was designed so that to function at its best, it is to be placed very near the front prism of the binoviewer. This position allows the rays to expand out just enough so that when they pass through the binoviewer, those different colors will be offset by the positive over-correction of the colors inside of the binoviewer. In other words, the rays are fanned out when they enter, and they are re-converged inside of the binoviewer, so they come out color free.
Think about it this way. As the rays leave the GPC, if you increase the distance between the GPC and the front prism, the result is that the rays from the GPC fan out more before they reach the prism, and cannot be fully re-combined on their trip through the prism. This is not the exact mechanism, but I think it is useful to explain why placing of the GPC is important.
This means that for the best result, the GPC should be placed just before the BV, as specified by Baader. If you are using the micro-bayonet, then it should go inside the Bayonet. If you are not using the micro-bayonet, it should go into the top of the mirror diagonal. If you are using a 1.25" T2 prism diagonal, the GPC should go in front of the diagonal, because the first prism now is the prism in the diagonal.
To be fair, if you make some space, the color correction may not suffer enough to easily see, but if you want the best possible correction, it should be used with no spacing between the binoviewer, and if using a T2 prism, it should be placed between the nose and the prism housing.
This is why I recommend the T2 mirror for people using fast highly corrected Apo telescopes that wish to maintain the wideset field possible. If you need the GPC, then putting it in front of the prism diagonal is the best place. Remember, the light cone will always be slowed by the GPC, so this by itself helps lower the color error because the color leaves the first prism with much better correction and a slower focal ratio.
Bottom line, you can raise the power by moving the GPC away from the binoviewer, but it lowers the correction and is not recommended unless you are moving it to the front of the prism diagonal.
Newtonians typically don't need a GPC because in most cases, they need a powerful Barlow, and this simply slows the light cone down so that the Binoviewer introduces less color. Once the system gets slower than about f/9, color error from the prisms is not typically enough to see, but I still recommend a GPC or Barlow when using an SCT for planetary observing. Every tiny bit helps.
Edited by Eddgie, 26 March 2025 - 07:35 AM.
