Processing The Witch in Photoshop Part 1

This workflow is courtesy of Craig Stocks at Utah Desert Remote Observatory

To be clear, I didn't invent this way of using Photoshop for astro images. I found this workflow on the YouTube channel of UTDR, presented by Craig Stocks. It's a fantastic way to process images that I haven't seen anywhere else. I've used Pixinsight to complete these steps in the past, but Craig's method is much easier. When it comes to color, sharpening, and blending, Photoshop can't be beat.

You could use another photo editor, like GIMP or Affinity Photo, to achieve the same result. I'm sticking to Photoshop because was very familiar with it even before I started astrophotography.

Load master files into a stack:

First, I use the Load Files Into Stack script to load all of our masters as separate layers in the same file.

Re-Arrange the Layers:

The layers will probably be in a random order so I move them into this order:

Layer Groups and Adjustment Layers

Next, I put each master (except Red) into its own layer group to keep things neat. Then, I add some adjustment layers to each layer group. The main adjustment layers we'll use are:

Levels

We'll use Levels to adjust how much each layer contributes to the overall image and to tweak brightness and contrast.

Hue/Saturation

As you might expect, Hue/Saturation will let us adjust colors, either globally or by a specific color.

Curves

Curves is a more advanced form of Levels. We'll use it to tweak overall contrast.

Layer Groups and Blending Modes

Here you can see all of our layers and layer groups so far. It looks complicated, but it's really not. From top to bottom, we have:

Stars
Halpha
Color (our RGB image)
Detail (our Lum image).

Each group has a Levels adjustment layer, and the groups that contain color have a Hue/Saturation adjustment layer.

Blending Modes allow us to change how a layer or layer group interacts with the layer(s) below it. Here are the modes I'll use:

Normal- a layer set to normal will block anything underneath.
Color- color layers contribute color but don't affect brightness.
Screen- screen mode only allows the parts of a layer to be visible if they are brighter than the pixels underneath.
Subtract- subtracts a layer from the one underneath.
Pass-through- the default mode for a layer group. Acts like "normal" mode does for a single layer.

I put our layer groups into the following modes:

Stars-Screen
Halpha-Screen
RGB Color- Color
Luminosity- leave it in Pass-Through

All of the layers themselves will be in Normal, except for RGB red, which is in Subtract blending mode.

Now our image looks like this.

Reflections from Rigel.

After cleaning.

Cleaning Up The Layers

Unfortunately, all of our layers contain reflections from the nearby star Rigel. These streak across the image and look terrible. Astrophotography purists should look away now as I use the Remove tool to get rid of them.

Sharpening and Local Contrast

Next, I usually add some sharpening and try to enhance small-scale contrast using either Smart Sharpen, Unsharp Mask, or the High-Pass filter. This image doesn't call for much, so I only use a bit of High-Pass sharpening. I put the High-Pass layer in "Overlay" mode and use a mask to limit its effects to the witch's face.

All the other layers are turned off so I can see the results of the High-Pass filter. The RGB and Halpha layers don't need sharpening because they'll only contribute color information.

The mask used to limit the High-Pass sharpening to the witch's face.

Darkening shadows of the Lum layer.

Increasing color layer contribution.

How much should each layer contribute?

Next, I'll adjust the brightness of the image by using Levels in the Lum group. I bump the black point up to 15, darkening the shadows. I don't want this image to be too contrasty, so I leave it there.

I add a bit more color by brightening the mid-tones of the color layer to 1.30. I tweak the color of the RGB layer using 2 Hue/Saturation layers with masks.

Our image so far, with the Halpha and Star groups turned off.

Next we'll finish the image in Photoshop Processing, Part 2

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