Introduction To Masking For Astrophotography
Chapters 1-4
This is the first four of an eight chapter series of the introduction to masking in Photoshop by Ken Crawford. These tutorials will cover the basics to the advanced methods of creating and refining masks in Photoshop to selectively enhance your images. Learning these masking methods will benefit your image processing for years to come.

Here is the link to Chapters 5-8  
Chapter 1: This tutorial will introduce you to the the concept of a masks with what they are, where they live, and how they work.
Chapter 2: Creating masks from selections and saving them as Alpha channels. Then learn to apply the mask into a layer mask with refinements for total control.
Chapter 3: This tutorial will demonstrate how to blend details with refinements for smooth transitions
Chapter 4: This tutorial will demonstrate blending high dynamic range areas of an image