Frank, maybe you can answer this question for me.
The argument for using the filter in front of the lens is that the light from the sky is parallel so you don't get a bandpass shift with inteference filters, but after the lens, it is a cone and you do get a shift.
But if you try to use in interference filter in front of a wide-angle lens, it is unusable because of the radial bandpass shift off-axis. But it's in front of the lens. Why doesn't it work if the light from the sky is parallel? And, for whatever reason this is, why doesn't the same thing apply to a narrowband Ha filter in front of the lens?
However, if you try to use an interference filter in between the same wide-angle f/2 lens and the sensor, it works fine. I have done both.
What is different conceptually here between the wide lens and the 105mm?
If the argument is the light from the sky is parallel, the interference filter should work in front of a wide-angle lens too.
The interference filters were broad band LPR filters. Is it the narrowness of the bandpass with a narrowband filter that is the reason?
When I see how far the rear optical element is from the sensor with the 105mm, I have to think it's mostly parallel light coming out of the back of the lens at that point.
What would be interesting would be to use a 3nm Ha filter in front of the lens and then compare it when used in between the lens and sensor with the same step-down ring so the focal ratio is the same and the only variable is filter location.