Saturday, May 17, 2014

A different approach to process luminance images in Pixinsight

Recently, the Pixinsight team released a new tutorial how to do Deconvolution and Noise Reduction with some of the new tools in Pixinsight. I tried this out on data that I took of M106.

After using CanonBandingRemoval and neutralizing the background, this is how it looked like:

The first step in the tutorial is to create a PSF. It encourages to a) focus on small stars, and b) on stars around the galaxies in the image. Here is my selection:

Created a star mask:

Luckily, my star mask didn't pick up any of the non-star elements, so I don't have to process it further.

And with the PSF and the star mask we can finally use Deconvolution:
Clearly, lots of detail recovered in the galaxy. But if you look closely, there is some dark halo around the stars. It's easier to in darker regions:

I played with all the deringing parameters and the one that is most effective here was the "Global Dark" parameter. Setting it to 0.009 (it was 0.005), I get this result:

Now, the halo is too bright. Setting it to 0.007:

Almost gone. 0.006:

Perfect - now, there is no halo around the stars at all!

Next is noise reduction. In the past, I used a copy of the image or RangeSelection to protect the larger structure. But because the images isn't stretched yet, we can use a "Linear Mask" in the MultiscaleLinearTransformation tool. With the preview functionality, it is pretty easy to create it using the "Amplification" slider (one thing to remember is to create a duplicate of the image as this won't create a new image, but modify the existing one!) I want to get just enough to protect the 3 smaller, fainter galaxies below M106. This one looks good:

It's worth checking the created mask with an autostretch - it protects much more then you would think from this image:
I found that I had to play with the strength of the mask a lot by applying the noise reduction (next step) and trying to make the background as little blotchy as possible.

Next we apply the noise reduction with MultiscaleLinearTransformation:

Nice noise reduction without harming the galaxy details. Next, we stretch the image. I now always do a strong first stretch until the background is just getting a little bright. Then I adjust the black point just so that I don't clip any pixels (or at least only a very few). Then I do a second, much smaller stretch and adjust the black point again.

Looks good. Finally, we are doing a dynamic range compression using HDRMultiscaleTransform:

This is how the luminance image now looks:

I'm very happy with it - lots of detail in the galaxy, I preserved the faint outer layers. And the other galaxies below also have some structure.