I have been considering picking up a achromatic refractor for narrowband imaging. CA should not be an issue when shooting narrowband, so even the f5 achros would work fine. Looking at this one, which I could probably pickup from Telesckop-Express pretty cheaply:
http://www.bresser.de/en/Astronomy/B...e-Hexafoc.html
There is also a four element 152mm made by Bresser that looks good.
Obviously even parafocal filters would need refocusing on an achro, and i suppose a flattener would be required, but as a lightvweight alternative to my heavy Mak-Newt it might be useful.
Any opinions, or experiences, shooting narrowband with acrho? What do you think?