Managing Large Star Halos?

[tmarkuk]

On Sunday night I took advantage of the local streetlights being broken to take a few frames of the horsehead and flame nebula (attached), and did a quick run through the processing in Pixinsight last night to see what I got.

As you can see from the image, what I have is a not bad picture hiding behind a very large coma / bloom effect from Alnitak. Now I'm fairly sure this was caused by generally unfavourable seeing, though you can also see a small reflection effect to the left of the star caused by internal reflections.

How would you go about reducing or masking out this large effect from the image? In this case I desaturated the blue and green channels to reduce it, but I was still limited on what I could pull out of the data by it.

I have thought of trying to create a mask for it based on intensity, or by using waveletransform, but I figured I would throw it open to the floor for ideas.

Tim.

[ollypenrice]

You can try taking some short exposures and layer masking them in. However, this did not work on Alnitak for me. What I did was a stretch of the standard data in which I lifted the faint stuff as much as I could and kept the bright stuff as unstretched as possible. When Alnitak was showing as a double star I stopped the stretch and pasted it onto the normal image. I put a feathered selection around Alnitak and selected Inverse to remove all but the 'short' Alnitak. Then I played with levels, curves, colour balance and opacity till Alnitak looked a bit better. Flatten and repeat. It took many iterations with the feathered selection getting smaller and smaller in diameter. I was not quite able to get Alnitak to show as a clean double but the result is here:

Widefield images including mosaics and overlays. - ollypenrice's Photos

Tom O Donoghue got it clearly split but he doesn't have it on his site at the moment so I can't link to it.

In general I work on a halo using a big round feathered lasso then Curves. I pin the curve at the level of the background region then drag it down just above the pin. It may affect colour balance so give it a tweak. I never try to do these thngs all at once. I make successive iterations.

Olly