question on summing images

Hope this isnt a dumb question. When summing together multiple images, brighter portions (ie galaxy cores) reach saturation and eventually subtle brightness gradients become "washed out". Is there any way to avoid this ? I need to sum all of the images to bump up the S/N of the dimmer parts.
Thanks,
Dan

You need to sum the images into a 32 or 64 bit buffer then
scale the sum back to 16 bit to avoid the washed out image.

Some programs allow this others don't.

Mike