Focal Pointe Observatory
Astrophotography by Bob Franke

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Non-Linear Stretching With Curves

First a quick note about my units of measure.  Even though I process images with 16-bit data, I have PhotoShop set to display pixel values from zero to 255.

Non linear stretching is the work horse for producing "pretty pictures" with astrophotography.  This process completely destroys the data for any scientific use and is frowned upon by some purists.  However, this is the only way to simultaneously show the faint and bright detail. Here's how to do it.

The 4-11 curve, Fig. 1, is very aggressive.  I usually apply this successively to the image until the darkest parts get to a brightness above 30. Then I adjust the black point, with the levels function, shown in Fig. 2. This is done by moving the far left arrow or entering a number in left input box.  Set the black point so the darkest area of the image is at about 25 to 30.  It is tempting to go darker, but you will loose faint detail.  After this point, always reset the black point after each non linear stretch.

Eventually you will start saturating the brighter stars.  It's OK to have a few, but this generally needs to be controlled.  Holding back on the brightness is the key to getting good star colors.  So now you will want to use a more gentle two point curve.  This way you can continue to brighten the faint detail, but not the brighter stars.  Figure 3 is an example of this curve... and don't forget to set the black point afterwards.


Fig. 1


Fig. 2


Fig. 3


It is possible to get very creative and use curves to adjust specific parts of the image.  You may just want to increase the contrast in just the dimmer areas.  Figure 4 is a curve I use to stretch the image only above the level of 28.

So get creative and experiment.  You will be amazed of what you can do with just curves.

Fig. 4