Image 1: Iris Nebula with stars removed
Divide and Conquer – processing the nebulosity image
We previously created two files, one with only the stars and another with only the nebulosity. The star-only image is ready to go.
Now we turn our attention to the nebulosity image. Run “Astra Image” and use the “levels” sliders first. I move the black (leftmost) one to the right to kill the low light level noise. I then take the middle slider and move it to the left slightly to bring up the intensity a bit. Work with this until the dimmer features are clearly visible but the image doesn’t have large areas of saturation or background light. The nebula should have colour, so typically I increase the colour saturation and perhaps alter the hue/tint a tiny amount to enhance this. I usually don’t run deconvolution, instead I run wavelet sharpen and look very closely to ensure I haven’t added large amounts of noise. Denoise. Take a good look at the resulting image and make sure that you haven’t created a mess. Save the intermediate images as you go (always hit “apply permanently”, then save).
At this point the large scale image is nearly ready but there may be weird anomalies arising from the starnet process (e.g. rings where stars had been removed). There may be faint traces of satellite tracks or other problems. I load the image in Photoshop and use the magic healing brush on the worst and biggest messes. This may take a minute or two, or it may take an hour, depending on how bad things were. My image of the North America nebula had little donuts everywhere that I wanted to remove… it took over an hour of careful manual effort to do this. When finished zoom out to different sizes and review the situation to make sure there no problems were introduced and all the worst spots were cleaned up.
It may be useful to remove some features altogether. A chunk of light half outside the frame is a nasty distraction and can be removed. I don't do this very often but sometimes faint junk is barely visible and distracts. This can be done in Photoshop at the end of the process. Warning: Some purists may want to tar and feather you for this.
That should be it for the nebulosity image. Next we will merge them.
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