[Rigel003 (Graeme)]

Extending my first attempts in DSLR photography and tried taking a set of flats for the first time. White T shirt over the end of the telescope. I ended up taking them the next day with the same optical train and temperatures may have been different. In my processed shots the dark smudgy spots (presumably dust on the sensor) have come out as bright coloured spots.

Can anyone give me advice about exposure length for these? Also are they meant to be white or gray? Mine looked pale pinky orange on the laptop screen and the dust marks and vignetting were only just noticeable.

Processed horsead photo attached. Spurious light blue splodge at bottom left, also blue streaks at top left and is that amp glow in the bottom right? Is flat fielding supposed to take that out too?

Grateful for any help.