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Nebulosity Multi-Color Processing

Tutorial for beginners By Bill Logan © 2010 Stark-Labs Nebulosity is without a doubt, the best astrophotography software on the market for the money. Are there better software programs? Yes, but at a considerable higher cost, but for the frugal imager, Nebulosity cannot be beat. Nebulosity is a great camera control, image capture, calibration and stacking program. It is ideal for those shooting images with a one-shotcolor camera, but it is also great for those multi-color images using LRGB and narrowband filters with a monochrome CCD camera. This tutorial assumes that you already know how to capture, calibrate and combine and stack light, dark, flat, and bias frames. If you don't know how, please refer to my basic Nebulosity tutorial. Before we begin, there are few basic things about color imaging that we need to understand. The following is from the Handbook of Astronomical Image Processing (HAIP) written by Richard Berry and James Burnell. "Human Color Vision Although celestial objects emit light over a range of wavelengths, it takes the human eye to perceive color in the universe. Because it enables us to distinguish subtle features of the natural world, color vision is valuable to humans. Color vision is the result of comparing and ratioing the signals from the different types of cone cells in the retina and in the brain. However, the very low light levels typically encountered in astronomy fall below the threshold of vision for cone cells. Instead, low-light-level vision depends on their monochrome counterparts, the rod cells. It would serve little purpose to make "Realistic" color images showing objects in shades of gray or dark greenish gray. Therefore, in making astronomical images, our goal is to recreate the colors that we would see if celestial objects were bright enough to stimulate full color vision. Thus astronomical color images have the ability to reveal a Universe that we can never see directly." Pg 514 As human beings, we really don't know the "true" color of the objects that we find fascinating in the Universe. Your guess is as good as mine as to the true color of an object. No two pairs of eyes sees the same color, but color wavelengths give us clues to the real colors. Again from the HAIP, the wavelengths of the colors. Color Ultraviolet Violet Blue Blue-green Green Yellow-green Wavelength below 380 nm 380nm ­ 450nm 450nm ­ 480nm 480nm ­ 510nm 510nm ­ 550nm 550nm ­ 570nm

Yellow Orange Red Infrared

570nm ­ 590nm 590nm ­ 630nm 630nm ­ 700nm 700nm and above

Note: Color boundaries are subjective; colors blend gradually into one another. "One important key to the successful recreation of color images is choosing filters that isolate the right wavelength bands in the spectrum. Another key is accurate processing of the filtered images to correct for atmosphere extinction and small errors in the choice of filters. Finally, the output device we use to view the color image must emit a spectrum that recreates, in the eye of the beholder, the sensations that the original celestial object might has produced." Ibid pg 514 Now for the step-by-step procedure for processing multi-color images. The multi-color can be Luminance, Red, Green and Blue (LRGB) or just RGB without luminance or any number of different filters such as the narrowband group consisting of Hydrogen-Alpha, triple ionized oxygen (OIII), double ionized sulfur (SII) and many other specialized filters such as HII, He I and He II, calcium (K) just to name a few. The following steps assume that you have taken several light frames, darks and flats in LRGB. Step 1. Stacking. After calibrating your light frames and added color with Batch Demosaic + Square RAW Color, stack each color separately. When stacking, I like to use Translation + Rotation because no polar alignment is perfect and you will be aligning all your images using to opposing stars. There is always a small amount of rotation from frame to frame and we want all the stars to be sharp and round. First, stack all the luminance frames and save it with "Stacked Luminance." Stack the other colors as, "Stacked Red," "Stacked Green" etc. You should now have four stacked files in the folder. Now we need to stack the stacks. This is where we combine all the colors into one frame. Step 2. Stacking the color stacks. This step differs from one-shot-color camera processing and is the place where most people will make their mistake. Click the "Batch" drop-down menu. Select "Align and Combines Images." In the Align and Combine Images window, check the "Save each file" button. Next, select Translation + Rotation. Click OK. This will open your file folder. Select "Stacked Luminance, Stacked Red, Stacked Green, Stacked Blue" files. Using the cursor, stack the four files using two opposing stars as you did with the normal light frames. When it is finished stacking, the message in the lower left hand corner will say, finished stacking. Although you don't see the files yet, each individual color has been saved as "Align_Stacked"; "Align_Stacked" etc. Step 3. LRGB Color Synthesis. Now click the "Image" drop-down menu. Select LRGB Color Synthesis. In this window, you will have three choices of stacking; RGB, LRGB: Color Ratio, and LRBG: Traditional HIS. For now, click the LRGB: Color Ratio button. At this point, if you only took three colors, RGB or narrowband H-a, OIII, SII, you would click "RGB." Now click the "Red" button, the file folder opens and you'll see your saved "", etc files. Select the file. The red button will now turn red. Click the green button and select file. Do the same for blue and luminance. When all

four colors have been selected, click "Apply". Do not adjust the color sliders. The image will not look good. Don't worry about that now. Click "OK". Now click on the "File" drop-down menu and click "Save 16-bit color TIFF'. You may also want an insurance file so click file again and "Save Current File" to be saved a FIT file. You can use either TIFF or FIT when processing in your favorite post processing program. If you have any comments, suggestions, recommendations to improve this tutorial, let me know at [email protected] Clear skies, Bill Logan Logan Observatory Eagar, AZ, USA


Microsoft Word - Nebulosity Multi-Color Tutorial

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