[raymo]

The dynamic range of M42 is so great that you cannot do a single image of
it that shows most or all of the nebulosity without blowing out the core,
regardless of what ISO is used. You need to stack a mix of very short subs
that correctly expose the core, and long subs that capture most or all of the
nebulosity, and use layers, masks etc to end up with an image that
preserves the core.
I have attached an image to illustrate my point. I took it years ago, all longish subs so
that most of the nebulosity is shown, but the core is blown out even though
I reduced the highlights in the image to the minimum possible without
stuffing up the image. The core was initially incredibly blown out.
raymo