Image Processing Tutorial #4
Object C31, The Flaming Star Nebula Processed in Photoshop CS.
Constellation Auriga. Mag. 9.2, Size: 37x19 arc min.,1600 LY distant.
This tutorial assumes you have a pre-stacked group of raw files. The example uses a stack of C31 images captured thus:
Optics: Celestron C102 refractor, focal length 500mm, f/5.
Orion broadband light pollution filter.
Mount: Celestron CG5 goto, unguided.
Camera: Unmodified Canon 400D (XTi), ISO 1600, auto white balance.
Frames captured: 161 at 2 minutes each (5 hours, 22 minutes).
Canon raw files processed in Nebulosity, dark frames applied, stacked in adaptive mode. Stack saved as 16-bit linear TIFF file.
This raw stack is available for download as a TIFF or archived ZIP.
There is moderate light pollution at my patio site in suburban Goleta CA. Shooting date: October, 2007.
Skies were clear with good seeing. Temperature 55 degrees, humidity 70%.
This tutorial is aimed at producing images from underexposed raw files. Such files are common when shooting unguided
exposures or as a means to inhibit overexposure due light pollution. Both require short exposures. Underexposed raw files
require a few extra processing steps.
This example relies heavily on Noel Carboni's Photoshop Action Tools.
His website is here.
The processing program Nebulosity is produced by Stark Labs. Their site is here.
There is a tutorial on processing images using Nebulosity here.
Before beginning adjust your monitor until you see 20 shades of grey:
1. This is the stack of 161 images produced by Nebulosity. Note the image is very dark and the histogram shows very little information.
This is characteristic of linear files, the data has not been lost or compressed. The nebula information is hidden in those thin histogram
lines. They need to be expanded carefully so that no data is lost. Also hidden therein is light pollution and camera noise. As the data
expansion proceeds this noise must be removed or reduced.
2. Photoshop's Curves function (CTRL+M) is used to begin stretching the histogram data. The bottom-left portion is pulled to the left (white dot) to brighten the dark shadows. The upper right portion is pulled down (black dot) to prevent stars and highlight areas from burning out. Applying this function three times to the image above yielded the image below. Note the data lines in the histogram have moved away from the left edge and are somewhat thicker. The image has brightened considerably but has a green tint and no sign yet of the nebula.
The red line hasn't moved as far as the green and blue, indicating the red is subdued. This is due to the broadband light pollution filter used on the camera and will be compensated for.
3. Photoshop's Levels function (CTRL+L) is used to balance color and darken the sky. This is called "setting the blackpoint."
It's done for each individual channel (red, green and blue) as shown in the dialogue boxes below. The left slider for each channel
is moved just short of the spike. In addition, the red midtone slider is moved slightly left to compensate for the loss of red from
the broadband filter used on the camera.
This has darkened the image considerably. The first traces of the nebula are now visible.
Scroll over the image to
compare it with step #1.
4. Another Curves function is applied as in step #2. In the histogram, the red channel is now stronger than the green and blue, indicating the nebula information is developing. If the target were a blue nebula, the blue channel would have been brought out instead of the red.
5. Do you see the dark horizontal bands in this pixel view? The noise is there, scroll over the image to see
the effects of its removal with Noel Carboni's Horizontal Banding Noise Reduction.
6. With some of the noise removed the contrast can now be improved. Another Carboni tool, Local Contrast Enhancement,
is applied. Scroll over the image to see its effect.
7. Curves are now used to further increase the contrast on all channels, and to dim the deep reds in the background.
Scroll over the image to see the effect.
8. Contrast enhancement has increased the graininess of the image. Carboni's Space Noise Reduction is effective at reducing it. Scroll over to see the results.
The decrease in noise allows Local Contrast Enhancement to again be applied (image not shown).
9. The Saturation function (CTRL+U) is used to increase the intensity of the red. It should be used sparingly.
10. Curves is now used to increase contrast and make small color adjustments to the red and green channels.
12. Another Curve is used. Scroll over to see the effect and note the histogram. After the Curve is applied the green channels loses information (it's called clipping). This can result in a loss of color detail.
14. The stars in the image are large and distracting. Three applications of Make Stars Smaller will help. Overuse of this tool can cause blotchiness. Scroll over to see the effect.
15. One final Curve is used to brighten the image and to reduce the blue. With that, the image is finished.
In the end, develop the color and contrast until it is appealing to you! Examine images of the same target taken by others to get an idea of how it should look. However, trying to make your shot look just like theirs can be futile.
Remember that each image, even of the same target, requires its own variations on processing. The example shown here was the result of trial and error. If I processed this image again I would try different techniques and would likely get different results.
