Processing CCD Images


Making magazine-quality astronomical CCD images is easy, right? Right. You just have to have nearly perfect polar alignment, good quality optics, a very stable and well-tracking mount, good optics, a night of good seeing and transparency, no light pollution...well, if you've tried your hand at astronomical CCD imaging, you know that it's a very challenging pursuit, and there are lots of things that can go wrong. However, even after you've conqured many of the hardware and optical problems, and gathered a bunch of data in the form of image files on your computer's hard drive, there is still a lot of work to do! How you process that data to produce a final image can make the difference between a mediocre image and one that others will marvel over. For example, a few years ago I made what I thought was a very good image of M16, the Eagle Nebula. I did the best that I knew how to do at the time when processing the raw data, and the image on the left below was what I got.
Recently during a spell of cloudy weather with no new images to process, I opened up the old M16 raw data files, and took another pass at processing them using what I'd learned about working with images in the intervening years.The result is the image below on the right. Honestly, I was amazed at the difference in the two images. I don't think anybody would argue about the second image being better -- and both were made from the same exact data, and for the most part processed using the same software! Clearly, I'd acquired some new skills over the years, and learned more about dealing with images that allowed me to make a much better looking image the second time around!



This web image processing tutorial is an attempt to put down some of those lessons that I've learned over the past few years into a format that will help others learn some new tricks. I've organized the information into the chapters listed below -- starting with the basics, moving on to intermediate levels, and finishing off with some more advanced techniques. Of course, feel free to skip parts that you already know -- but if you're here looking to improve your image processing skills, I'd suggest you at least browse the basic sections to see if there's anything you've missed, and to get a feel for my own methods of preparing the data for the more advanced sections.

This tutorial is by no means all-encompassing -- I'm still learning myself every time I sit down to work on an image, and I suspect that will be the case for as long as I do astronomical imaging! However, as I started off doing images myself a few years ago, and was looking for help learning how to process my images, I found bits and pieces of information scattered over various sources (web pages, books, talks with friends, etc.). I could really have used a web resource that took a tutorial approach to the subject, and attempted to gather a lot of the available information into one place! My hope is that this tutorial will fill such a need, and will be useful to others at various levels of skill and experience in imaging. I would greatly value feedback, comments, criticisms, and corrections at the e-mail address at the bottom of the page. And if you have tips or tricks that I don't have listed here, I'd be very happy to include them in the tutorial, with my very great thanks.

So, on to the tutorial! Chapters that are available are listed below. I hope you enjoy it and find it useful.

Tutorial Introduction
Chapter 1: Gathering and Preparing the Image Data
Chapter 2: Calibration -- Darks and Flats
Chapter 3: Aligning and Stacking Images
Chapter 4: Maximizing Your Data -- Histograms, Stretching, Contrast, Brightness, Gamma
Chapter 5: Dealing with Imperfections and Artifacts
Chapter 6: Basics of Color Images
Chapter 7: Advanced Color Image Processing
Chapter 8: Advanced Techniques -- Masking and other tricks
Chapter 9: Summary and Final Thoughts











All text and images Copyright (c) 2003, Paul LeFevre
Mail me with comments & criticisms!