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 Image Processing
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Pulling dim Nebulae from the background using PhotoShop

 If  you are proficient with  PhotoShop this might be old hat to you. However, I'm still trying to  pickup the various tricks and features available in PhotoShop for processing Astro-images. I recently saw an image where the dim nebulae were clearly visible and asked Duncan McCloud How he did that. These are the steps he uses.

  • First,  create a Curves adjustment layer.

  • Then hold <ctrl> and click on faint gas in the image.

  • Then hold <ctrl> and click on dark background.
    You'll see the points you just selected on the Curves graph.
    The two points can be very close together, that's OK. If Photoshop shows them as the same point, choose slightly brighter gas as your first point.

Duncan likes to set a control point just below the dark background point; this keeps the background from getting darker when you start moving the curve. To do this just click on the curve below the background point and press the up and right arrows on your keyboard until it is snuggled up to the background point. Make sure the X and Y values are the same.

  • Now the fun part; select the point on the curve for the gas (<ctrl>-tab cycles through them)

  • Then press the up arrow on your keyboard and watch the gasses get brighter while keeping the background a constant level. Don't get carried away, a little movement on the curve goes a long way.

Duncan  usually set a control point in the mid-range so it doesn't get too bright . He also anchor it with a second point right next to it so the highlights don't get lowered. Without the second anchor point, the curve would flip all over the place, creating an unwanted look



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