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The following section describes the advantages of shooting digital camera images in RAW format and gives a step-by-step example of using RAW settings to get the most out of an astronomical image.


Why Shoot in RAW?

Digital SLR cameras can typically record images in two formats:  JPEG and RAW.  JPEG is a compressed image format.  This gives each image a much smaller file size than a RAW file, but it compromises the quality of the image.  The difference in quality is subtle as long as the highest-quality JPEG setting is used.  So why bother shooting in RAW format?  The main advantage of RAW is that the camera does not permanently apply any settings to the image.  When a camera shoots a JPEG image, it sets the white balance, exposure, brightness and contrast, sharpness, etc.  After the picture has been taken, none of these settings can be changed.

Using RAW format, these settings can be changed after the fact, allowing the most detail to be extracted from the image.  By opening a file in the Camera RAW plug-in for Photoshop, settings can be adjusted before the image is opened in Photoshop.  This places the best possible unprocessed image into Photoshop for further enhancement.


Camera RAW Settings

When an image is opened in Camera RAW, the image is displayed within a separate window.  Specifications of the image appear below the image, while image settings such as white balance and contrast appear to the right of the image.  The most important tool in the Camera RAW window is the histogram.  The histogram consists of three curves (red, green, and blue) in the upper right corner of the Camera RAW window.  The histogram tells the values of the pixels in the image in each color channel.  Almost all of the adjustments that will be made in Camera RAW will be done using the histogram as a guide.

Above:  The Camera RAW window in Photoshop.  Note the histogram in the upper right corner; this is the most important thing to watch when adjusting RAW files.  Horsehead Nebula image by James McGaha.

In the above example, note the default settings of some of the image parameters.  Temperature = 3800, Tint = -50, Shadows = 5.  These are the three settings that are most often adjusted in Camera RAW, but of course the others can be used if necessary.


Setting the White Balance

Setting the white balance of the image is critical and should always be done first.  This will give a good base image to work from for other adjustments.

It is easy to adjust the white balance to the correct settings by examining the histogram.  Note in the above image, the default settings create a histogram with three separate peaks.  This results in a pink sky background in the image.  To create a neutral sky background, the three peaks of the histogram must coincide.  Begin by adjusting the Temperature slider.  This will usually bring two of the histogram peaks together.  In this example, dropping the Temperature to 2800 moves the red and green peaks together.

Above:  Adjusting Temperature in Camera RAW.  Value was changed from default of 3300 to 2800.


Next, adjust the Tint slider.  This will bring the third peak into registration.  In this example, changing the Tint value to -38 brings all three histogram peaks together.  This results in a neutral sky background color.

Above:  Adjusting Tint in Camera RAW.  Value was changed from default of -50 to -38.


Expanding the Histogram

Notice in this image that the histogram is shifted to the right.  In other words, there is a lot of empty space in the histogram window to the left of the main histogram curve.  This indicates that most of the pixels in the image and relatively bright.  In an astronomical image, most of the pixels are the sky background.  In this image, the exposure was deep enough to register the background sky glow, even from a dark location.  This is ideal as it indicates that all possible information was captured.

It is possible to simply adjust the Levels in Photoshop to darken the background.  However, it is better to make as much adjustment as possible in Camera RAW, leaving as little as possible for Photoshop.  Recall that an image opened in Photoshop contains 256 levels of brightness.  The histogram values range from 0-255.  If the histogram is left at the Camera RAW default and the image opened in Photoshop, the left toe of the histogram (the darkest part of the image) has a value of 107 in this example.  This means the black Levels slider will be brought up to 107 to darken the background.  The result is that the remaining values in the image (108-255) are expanded into the 256 possible levels.  43% of the histogram (0-107) is thrown away.

However, if the histogram is expanded in Camera RAW, less Levels adjustment needs to be made in Photoshop.  The best way to accomplish this is by increasing the Shadows setting in Camera RAW.  This has the same basic effect as moving the black slider in Levels.  The histogram curve will expand as the Shadows value is increased.  The important thing is to avoid clipping the histogram.  Ignore the appearance of the image for now; just watch the histogram.  Increase the Shadows value but do not cut off the left side of the histogram.  If you do, information will be lost.  In this example, the Shadows value was increased to 76.

Above:  Adjusting Shadows in Camera RAW.  Value was changed from default of 5 to 76.


Adjust Sharpness

Most cameras apply a default sharpening to an image.  This one major drawback to using JPEG images.  For regular photography, a slight sharpening of the image usually looks great.  However, for astrophotography, in-camera sharpening results in blocky star images.  It is best to eliminate the in-camera sharpening and apply any sharpening in Photoshop after other image adjustments have been made.

Select the Detail tab in Camera RAW.  The top slider is Sharpness.  The default for the camera used in this example is 25.  Adjust the slider to 0 to eliminate any sharpening.

Above:  Adjusting Sharpness in Camera RAW.  Value was changed from default of 25 to 0.


The Final Image

After making the Camera RAW adjustments, click OK and the image will open in Photoshop.  The standard aesthetic image adjustments can now be made.  Below is the final image from the RAW file above.  The image is a single 15-minute exposure using a Canon EOS 20Da and Takahashi Epsilon 180.

Above:  Final processed image of the Horsehead Nebula.  Image by James McGaha.

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