Image Processing Tutorial- Page 3

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Copyright Steve Cannistra


Step 4- Levels Adjustment- Highlights:
In many ways, this is the most important step of image processing, and yet it is not always done correctly.  The purpose of this step is to adjust the highlights (white point) and shadows (dark point) in order to maximize the dynamic range, but without clipping either end (which will result in a loss of valuable data).  This is especially important for the shadows end of the histogram, where the faint pixels need to be enhanced, not thrown out!  We work so hard to capture faint detail during an imaging session, and yet this detail is often lost due to improper setting of levels.  

(Please click on image for a higher resolution view)
Levels adjustment, part 1

Steps for adjusting the highlights:
1.  Transfer the image back into Photoshop (after vignetting correction in ImagesPlus).  

2.  Once you have identified the brightest star (you could use the "threshold" function for this), the idea is to monitor the pixel intensity of each channel (R, G, B) to reassign the values to 255, 255, and 255 respectively (i.e., white).

3.  Use the "Color Sampler Tool" in the eyedropper menu to select one or two of the brightest stars for monitoring.  I suggest enlarging the photograph for this step, and also using a 5x5 sample area (click on the above photo for details), to permit more accurate sampling.  Notice the #2 target area in the photo, overlying a star.  There is a corresponding section in the "Info" box to the upper right that provides a read-out of the pixel intensity in this area.

4.  Now, open a Levels layer- "Layer, New Adjustment Layer, Levels."  For each channel, move the white point cursor (the one on the right) towards the left, while monitoring the Info box, until the pixel value of the chosen star (#2 in this example) reads 254-255.

5.  Repeat for the Green and Blue channels.

6.  You have just set the white point, without clipping the highlights!

7.  To remove the Color Sampler Tool circle, press the Alt key (Windows) and hold the cursor ovcr the region- it will morph into a scissors.  Click on the image once this happens, and the
Color Sampler Tool circle will disappear.

Step 5- Levels Adjustment- Shadows:
Clipping the shadows end is almost never a good idea for astrophotography of objects like nebulae, where we are interested in capturing faint detail.  We work hard to capture those faint photons, and we should strive to preserve and enhance them to obtain the best image possible.   Ever notice how some astrophotos have a stark black sky background, with nebulae that have a "cookie cutter" contour with an abrupt border that looks artificial?  In these instances, the histogram has almost certainly been clipped, with loss of faint nebulosity on the periphery of the nebula.  For relatively bright and well-defined objects like galaxies or even bright star clusters, a minor amount of shadows clipping is sometimes permissible in order to obtain a dark background sky (in this instance there is generally no loss of important data, since most of the detail within the galaxy is relatively bright and therefore far removed from the left side of the histogram).

(Please click on image for a higher resolution view)
Levels adjustment, part 2

Steps for adjusting the shadows:
1.  Open the same Levels layer that you created in Step 4, select the red channel, and move the shadows cursor (the one the left) towards the right so that it is positioned just before the abrupt rise in the histogram.  Notice how the image becomes "discolored."  This will be corrected once all of the channels have had their shadows point adjusted.

2.  Repeat for the Green and Blue channels.

3.  You have just set the shadows point, without clipping faint detail.  If you move the shadows cursor too much to the right, you will "clip" the faint pixels, meaning that they will be reassigned to values close to 0, 0, 0 (black).  If you clip the shadows, the histogram will "hug" the y-axis (BAD), instead of starting with an upward slope (GOOD).  Here's a quick test you can try on some of your older images:  If you examine the pixel levels in the background sky (using the eyedropper tool and watching the "Info" section for the values) and see lots of 0's and 1's for R, G, B values, you've probably clipped the shadows end.  Your sky background may initially look pleasing to you, but you will not (or should not) be satisfied with the final result.  Once you clip the data, it cannot be recovered!
  Here is the final result of Levels adjustment:

(Please click on image for a higher resolution view)
Levels adjustment, final

Step 6- Curves Adjustment- the fun part! (next page):

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