After taking some deep sky testing shots with the DBK, I found the UV/IR signal is ruining the image, making red and blue halo to nearly all stars.
So, I placed a small piece of UV/IR blocking filter to the DBK in front of its CCD, this filter is removed from my Casio QV2800, it matches perfectly.
I've tested it with day time object and it can reach infinity focus, and white balance is correct, too. Originally, I fear that by adding a UV/IR filter on the CCD, some C mount lenses might no be able to reach focus at infinity, luckily, it still can. That UV/IR filter is more than 1mm thick, so the optical path length should be more than 1mm which could give problem to C mount lenses.
Hopefully the sky would be good enough tonight for me to test again.