[graham.hobart (Graham stevens)]

Hi all, I took a bunch of narrow band Helix recently and calibrated them- it left a black speckled appearance to the middle of the neb so this weekend I reshot calibs and it did the same again.
I was using darks taken directly after imaging x 50 at -30'c 300sc FLI mono 8300, flats with a light box aiming between 22-30000 I" values in maxim dl and bias frames , about 50 flats and 100 bias per filter. I am at work but will post a pic later but wondered if anyone else has had this issue and can point me in the right direction. Stacked in maxil dl 5 using either SD mask or sigma reject - both ended up looking the same.
cheers
graham