Hi Rex,
attached is an attempt at colour wiping your original one.
I've also attached just the gradient that startools detected, imagine this as being overlayed over the data.
that gradient is what has been subtracted from the original.
i'm no startools expert but this is my understanding.
this is an attempt at blinking the corrected one vs. uncorrected. blinking through two or more images is a good way to narrow down what you're final version would be
https://www.dropbox.com/s/yw85vr0qn1..._200k-anim.gif
couple of tips. again, no expert here..
sometimes when you look at an image for too long, you sort of get used to it and you won't be able to pick up that its actually got a color tinge.
if you're using photoshop or gimp and if you want to know if your white balance is correct, you can always use the eyedropper info tool to see what the RGB components of a white star are. just hover it over a white star. that will give you an idea if the white stars actually have a colour tinge. we use this in pre-press where monitors aren't calibrated but the RGB values give us an idea of what the colour is actually like and how it'll turn out when you print it as opposed to seeing it in an uncalibrated monitor.
There are some good images that have been taken from dark sites and that gives you a good reference to see where the nebulosity actually ends in the different areas in your image.
Your F4 actually has captured quite a bit of the nebulosity which is missing in most other images especially at 2 o'clock, so that's good.
i've also attached a crop of a HaLRGB image of the same area. now this will have a lot more detail because he's added Ha details into a colour image but is a good reference to identify where the nebulosity ends and starts in the image. always a good idea to check out the best image you can find and tease out as much as you can from yours. aim is not to see which is better or worse, but to see what you can get out of the image.
this was the source
http://www.nightsky.at/Photo/Neb/M16_HaRGB.jpg
what was the altitude of this when you took it?
I'd suggest avoiding low altitude targets. I once went to a dark site about 160k's away and tried M31 which was about 10deg above horizon. my 7 min subs with a qhy8 turned out brown. so I slewed to a target about 60 deg up, and the background came black. that's when I realized that there was a small town due north of that site and the brown hue was LP glow.
again, this is not a big issue and can easily be removed, but worth remembering.
As mentioned, do experiment with different length exposures and try blinking them to see which is the sweet spot. I'd do this before starting your exposure run as that way you'd get the best use of the clear skies rather than reviewing it at the end.
Cheers
Alistair
