Image Processing Tutorial #3
IC1848 Baby/Soul Nebula Processed in Photoshop CS.

This tutorial assumes you have a pre-stacked group of raw files. The example uses a stack of IC1848 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: 129 at 2 minutes each.
Frames stacked in Nebulosity, dark and flat frames applied. Stack saved as 16-bit TIFF file.

There is moderate light pollution at my site (suburban Santa Barbara, CA). Shooting date: September, 2007.

Astronomical image processing is a simple series of small steps. Stretch the histogram a little, apply noise reduction, repeat. This tutorial is aimed at producing images from underexposed raw files. Such files are common when shooting unguided exposures or as a mean to inhibit overexposure due light pollution. Both require short exposures.

This example relies heavily on Noel Carboni's Photoshop Action Tools. His website is here.

Image

Histogram

Comments

The unaltered stack, composed of 129 linear frames combined in Nebulosity. This stack is available for download on the Nebula page.

Note the almost non-existant information on the histogram. This tiny amount must be skillfully expanded to reveal the nebula hidden in the image.

The Curve function is used to begin expanding the thin histogram lines. The upper area of the curve is moved downward to prevent burning out highlights. In this image the function was applied 4 times to reach the desired brightness. The fuzzy edges caused by aligning 129 frames has been cropped.

Note the histogram has begun to expand. But there is still no evidence of any nebula in the image! Also note the red channel is displaced relative to the green and blue. This is caused by the light pollution filter necessary to remove stray light form nearby streetlights.

The Levels function is now used to set the black point of the image. For each channel (red, green, blue) the left slider is moved to the right, stopping just before the spike. In addition, the red channel midpoint slider is moved slightly to the left (input level 1.25) to enhance the red just a little. The background is now neutral, and faint traces of the nebula can be seen.

At his point, two of Noel Carboni's Photoshop Action tools are employed: Horizontal Banding Noise Reduction
and Vertical Banding Noise Reduction. This reduces faint red lines which show up in my images, resulting from long exposures at ISO 1600. Some cameras don't produce such noise. Some produce even more. Longer exposures make it worse. The effect of these reduction tools is not visible in the small images used in this example, so I didn't display them.

Another application of the Curves function as in the first step above further expands the histogram.

The Levels function was used to set the black point (re-darkening the sky) and to further stretch the histogram. The adjustments are shown for each color channel, note the positions of the sliders for each channel. Now the detail is starting to come out. Compare this histogram with the one for the unaltered stack (scroll over it to see the original):

Close examination of the image pixels reveals more noise has crept back in. The noise is faint and hard to see, but if not removed it will intensify as processing continues. Two of Carboni's Action Tools were applied:
Vertical Banding Noise Reduction and Color Blotch Reduction.
The left image shows the noise. Center has vertical banding reduced. Right shows color blotch reduction. The effect is subtle, and doesn't show very well in these small images.

Levels was used to reduce background brightness.

Again with Curves, the histogram is
stretched. This is a simple bow-curve without the downward swoop used in the beginning
of processing.

Note the stars are beginning to multiply and clutter up the image.

Curves was used to brighten the midtones. Note the input level = 12. This prevents the background from lightening too much.

Carboni's Make Stars Smaller was applied three times. This tool must be used with care as it can produce odd background noise.

A final Curve adjustment and the image is done. This is by no means a perfect image. Longer exposures would have given much smoother results. This example was chosen to demonstrate how to get an image from underexposed raw files.

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 most likely get different results.