Thank you Mike you answered a question I had without have to ask. Now how about this , if a 1.25 Ha filter is mounted at the ENVIS lens and then 3X magnifier mounted. Will this achieve the "the best wide-field views" you mention in post# 54?
p.s. I don't own any light buckets just 4 & 5 inch refractors.
Here's a picture of my device with the magnifier.
The H-a filters will show detuning at the edges when used with the 3x and the filter mounted on the ENVIS.
For example, if I use it on Barnard's Loop and Anglefish nebula, I can get both into the same field of view, but only the parts in the more central part of the field show bright, and as nebula runs out towards the edges of the field, you can see that they dim pretty substantially.. This means that at 3x, I have to pan across to see things at their brightest.. If I center the Anglefish nebula, then the southern part of Barnards loop will be very dim. If I center the southern part of Barnard's Loop, then the Angelfish (which is very dim as compared to Banard's Loop) will be almost impossible to see.
Now for the Anglefish alone, because it fits more or less completely into the center of the field, the detuning does not matter, and I think the 3x gives the best view of this nebula. Likewise, as you pan Barnard's Loop with the 3x, you see structure that is not revealed at 1x, but you can only see this structure when in is in the center 50% of the field.
For the best view then of this particular pairing, I find the binocular at 1x actually provides a better view. Now I can see the entire region and the brightness is very even across the field.
You absolutely get detuning with the 3x ahead of the filter, but remember that the 3x is very fast. I think it is about f/1.4. I have heard different figures though.
Now at f/2.8 with the same filters, detuning is hard to see. Nebula at the center of the field do not dim substantially when drifted to the edge of the field.
There is no doubt that detuning is an issue as speeds increase, but the question is at what focal ratio is it likely to damage the view enough to cause great concern.
As I mentioned in my post to Mike, many of us may never encounter the speeds he can achieve afocally because he has the great fortune of having some super-fast mirrors. Most of us either can' afford or don't want a 20" f/3.3 telescope. For those that can though, I have zero doubt that Mike is correct and that the filter will give the best result at the front of the eyepiece.
For a guy using a 10" SCT, I am not sure that this is the case, but hoping Mike or Glenn can provide insight.