By my understanding, the effective aperture is the diameter of the circle with the same area as your compound lens:
Total Area = pi/4*140^2 + pi/4*180^2 = 15394 + 25447 = 40841 mm^2
A circle with a diameter of 228 mm would have the same area, so your compound lens would have an effective aperture of 228 mm by this method.
However, I am confused by how you would combine the two images into a single image. It must be hard enough to get multiple identical lenses aligned with each other so that the images can be combined, but I am not sure how you would do it with two or more dissimilar lenses without introducing "noise".
You will presumably have a different image scale on each OTA, meaning you will need to enlarge or reduce one of the images to be able to combine into a merged image (which is analogous to increasing or decreasing the focal length of that OTA). I imagine this would introduce all sorts of "dithering" artefacts when you try to combine the scaled and un-scaled images.
Or am I missing something?
