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Old 05-12-2017, 02:26 PM
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PRejto (Peter)
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Star Substitution Question

I could use a bit of advice please.

For the first time I've imaged a dim galaxy from a dark site. I failed to take into account that in my quest to capture the galaxy I've totally blown out all the brighter stars. When I stretch the galaxy is good but of course the stars are blown and too large. I tried masking the stretch but the result isn't great and there is no way they would have any colour. I think the only way to improve the image will be to replace the stars from a series of much shorter subs. I think if I try this after the images are stretched there will be a size disparity and I'm likely to get an unnatural looking result. Is it somehow possible to do the swap in the linear state perhaps using PixelMath and a star mask. I'm really new to the program so I could use some advice, or a completely different approach to solving the problem. I'm more familiar with CCDStack. Would substituting values likely lead to a good result?

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Old 05-12-2017, 03:30 PM
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RickS (Rick)
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Have you tried MaskedStretch? (as opposed to stretching with a mask.) You could blend the stars from a MaskedStretch with a HT stretched galaxy using a mask. It requires some mucking around but it is certainly possible.

Another option is to just reduce the bloated stars after the stretch. I have done this by creating masks for the larger stars/halos then using a little HT and/or Curves reduction. Smaller stars can be reduced with MT and a star mask.

If you push hard enough you might be able to find some colour in those blown out stars. A star mask and big saturation boost is worth a try. Blurring the chrominance of the stars a little with one of the wavelet tools (I typically use MMT) will also help colour up the cores. The new arcsinhstretch tool does a great job of keeping star colour too.

If you do decide to go the route of collecting some short subs then you can incorporate them using HDRComposition.

Hope that gives you a few ideas...

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Old 05-12-2017, 09:36 PM
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Hi Rick,

Thanks very much for your reply!

Yes, I have been trying MaskedStretch and it seems no matter what I do I get dark rings. I've tried all kinds of settings, even stretching in 3-4 goes such as setting .04 in target background. The artifacts are always there. MS does work fairly well on my luminance.

I'll experiment with your other suggestions!


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