As we know, one of the benefits of a triplet lens is that it brings the three primary colors to a single focus while a doublet lens can only bring two of the primary colors to a common focus. The third forms a kind of blur circle around the other two.
However, if I am doing narrow band imaging with something like a 5nm band pass Ha filter, I am only bringing a single "color" to a focus. So, it would seem that it would be possible to do quality narrow band imaging with a good quality ED doublet lens. A 6" ED doublet refractor is certainly a lot more affordable than a 6" ED triplet!
Is there any reason to believe an Ha image from a doublet would be of lower quality than an Ha image taken with a triplet?