I'm considering that very barlow, Jeff. Thanks.
Thanks, Darryl. I've looked at those graphs in the past. I had to put pencil to paper and really look where the lenses were positioned for the entire length of the image train.
To maximize the 2.5x PM, I have 35mm between chip and TOP surface, BUT the bottom surface of the PM has 90mm between it and the rear cell opening...all filled with optical back, filter wheel and adapters, making the total distance from rear cell opening to chip 170mm. That 90mm distance is the problem, and needs to be shorter?
On the other hand, inserting the Big Barlow into the optical back, I start with the barlow lens 7mm from the rear cell opening. But, between there I have 150mm to go from its top surface to the chip, filled by the barlow neck, adapters and the filter wheel. But, this barlow will magnify images as this distance increases and I have already in effect turned it into a 3x barlow?
So, if I'm looking at this correctly, unless I shorten both front and back distances of the PM barlow, all I can do is add some distance with extension tubes and the Big Barlow configuration and start shopping for another barlow of the 3x kind.
Thanks, guys. I think it is starting to make sense.