I am using just four minute exposures for the Vela mosaic as it is so faint.
This is Carina with 1m, 2m and 4m exposures at 1600 ISO for both HA and O3. Usual HDR method after combining exposure stacks HA to red, O3 green to green and O3 blue to blue.
Large image 3MB
http://d1355990.i49.quadrahosting.co.../car_ha+o3.jpg
It shows how faint the Vela SNR is or how bright Carina is.
Bert