Atik 490ex mono, which is very similar to the QHY in size. You will love shooting mono, I just did my first narrowband last month and it was fantastic. I still need to figure out how to get the focus perfect, though. With so little light coming through on the narrowband filters, it is a bit harder to get the focus exact. One of these days I will get an autofocus motor.
I don't image at -20C in the summer though, its too damn warm out here and the cooler cant get it that low without killing the battery. But even at 0C I have never had a stuck pixel.