M13 in Photoshop

Processing A Globular Cluster in Photoshop

Here we see the layers of the finished image. To start, we need to put the RGB and Luminance files on top of each other. We can use the "Load files into stack" script to accomplish this. It can be found in the File menu.

We'll put Luminance on the bottom and RGB above, then create layer groups for each. The layer groups contain adjustments that only apply within that group.

As usual, we'll leave the Lum group in normal blending mode (pass-through) and the put RGB group in color blending mode. We'll add some global adjustments to top things off.

Luminance Layer Group

Lum Layer

This is the luminance image we created in PI. It will provide detail for our imaged.

Smart-sharpen

We can tighten up the stars further with a small amount of the Smart-Sharpen filter.

Levels

Next, we'll reduce the core brightness slightly and darken the background.

Levels Panel

Moving the black triangle under the histogram to the right lowers the black point. Moving the bottom white triangle to the left reduces the highlights.

RGB Layer Group

Smart Sharpen

I usually don't sharpen RGB layers. Since the point of this image is small stars, we'll make an exception.

Adjusting The RGB White Point

It was necessary to adjust the white point to reduce the core's brightness.

Levels

You can see how we adjusted the core using a combination of both white triangles.

Hue/Saturation

Two different Hue/Saturation adjustment layers helped dial in the color. Here you can see the adjustments for the cyans. This layer also contains adjustments for blue, while the other layer adjusts the yellows.

Overall (global) Adjustments

Our image currently looks like this.

Here is the makeup of the global adjustments layer group.

First, we've used Levels to lower the black point and darken the background.

Color Balance Mask

Now we want to adjust the color of the stars further, without changing the background. A copy of luminance can be used as a mask. The adjustments will only appear in the white areas.

Color Balance

Color balance can be a useful way to make final adjustments.

Hue Saturation Mask

Simply inverting the previous mask will allow us to target the background sky without affecting star colors.

Hue/Saturation

We don't want the background to show any color, especially mottled color noise. Let's achieve that by lowering the saturation to -100.

Color Dodge

Let's add a bit of pop by creating a solid-fill adjustment layer and setting it to "color dodge" blending mode.

Color Picker

A dark black works well for this. Clicking on the solid-fill layer icon brings up the Color Picker dialog box. Values of 19 for R, G, and B can give us what we need.

Lighten

Copying the Color Dodge layer and setting the copy to "lighten" blending mode will ensure nothing is completely black, and helps to hide noise in the shadow regions.

The Final Result

Here we can see one 30-second luminance image, and results after stacking and processing 5+ hours using both Pixinsight and Photoshop.

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