At right is a screen shot of Deep Sky Stacker (DSS) software with the file list for the Rosette Nebula image. There are a total of 60 x 21MB raw files being stacked in this particular example. It gets the computer thinking. The list includes lights, darks, flats, and bias frames, though I only had 10 good light frames at the time of creating this example. More would have been better. DSS starts with a long list of 16bit camera raw files and outputs a calibrated 32bit TIF file that allows for a tremendous amount of stretching. Fortunately, DSS automates most of the process and does the calculations in the background.
The Basic panel includes most of the general color and tonal adjustments. The first step in stretching the image data is to crank the Clarity, Vibrance, and Saturation sliders to 100%. We may need to reduce these later. This amplifies the faint color and structure in the image and allows you to assess the correct color balance. To correct the color, we adjust the White Balance sliders until the background of space is neutral, because we know that space has no color cast.
Working down the Develop panels in Lightroom, we next make more accurate tonal adjustments using the Tone Curve. For many people, this is the most vexing adjustment in any image editing program. Fortunately, Lightroom makes it relatively easy to adjust curves by allowing you to simply click on a tone within the image and drag it up or down to brighten or darken that tone. In the Tone Curve image you can see which parts of the image I selected and how I dragged the curve up or down to emphasize the subject. I kept the blacks of space near zero, while raising the value of the nebula and brighter areas around the stars. Again, some experimentation is in order.