Thank you guys for your kind words...
Yes, this lens is pretty good, but not perfect - without external aperture (f/3.5 ~ f/4) there is too much of CA (I haven't tried narrow band yet).
But for the price I paid for it (and with help from Stefan),
it is OK.
Also, I am using 680nM
UVIR cut off filter (otherwise some stars are bloated (due to IR).