Creating striking color CCD images using LRGB composites has become the image processing technique of choice for those who image the night sky. The theory and practice of the LRGB method have been particularly well presented in two publications: The Handbook of Astronomical Image Processing by Richard Berry and James Burnell discusses the theory and The New CCD Astronomy by Ron Wodaski covers the practical aspects. The websites of Robert Gendler and William McLaughlin contain excellent 'how-to' discussions of the LRGB layers method which I found helpful when I began to experiment with the process.
The L (Luminance) image is obtained in unfiltered light using a high resolution CCD camera, such as the ST-10E binned at 1X1, to obtain a grayscale image. Deep exposures are required to give a high signal-to-noise ratio (S/N) in the L image. Red, Green and Blue images are obtained using RGB color filters with the camera binned at 2X2. Alternatively, one uses a different CCD camera with large pixels and/or higher sensitivity (such as the FLI-DM). These measures are necessary to overcome the weaker signal of the filtered light which results in low S/N of the RGB images. The four images, L;R;G &B are combined using image processing software. The final LRGB composite reveals the detail of the Luminance image as well as the color information present in the noisier, lower resolution RGB images. This is due to the fact that our visual processes perceive detail almost exclusively in the Brightness or Luminence component not in the Color components of the image. Moreover, the eye ignores noise in the Hue and Saturation (Color) components. Therefore, LRGB compositing produces a color image with a lot of detail...the Holy Grail of astronomical color imaging. The same principal is used in the Component Video feature present in digital television images.
Processing the Luminance Images:
High signal-to-noise images are obtained by combining many shorter exposures to yield long total integration times. Typically, 12 ten min. exposures (or 24 five min. exposures) from the ST-10E, binned at 1X1, are combined to give a total integration time of 2 hours. Ten (10s) flat fields are obtained at the conclusion of an imaging session. Sets of 10 to15 raw dark frames at each exposure time are kept as a library which is turned over every few months. On summer evenings, when external water cooling is required to maintain the camera at -20 degrees C, better dark frames are obtained when they are taken immediately after the imaging session. The raw images are processed as follows:
Processing the RGB Images:
RGB exposures are taken with the FLI-DM binned at 1X1(with R:G:B exposure ratios determined from white balancing). Three sets of 5 exposures for each of the three filters are collected. Five flat fields (10s) are obtained with each filter at the conclusion of the imaging session. Sets of 10 dark frames are kept as a library with a 3 month turnover. The raw RGB images are processed as follows:
Creating the LRGB Image:
Please refer to Robert Gendler's article, Color CCD Imaging with Luminance Layering in the July 2001 issue of Sky & Telescope, pp133-136, or to his web site for a nice tutorial on creating LRGB images using Layers in Photoshop.
At this point the L image is an 8-bit 2184X1472 TIFF file and the RGB image is an 8-bit 1024X1024 file. Both files are approx 3 mB in size. Before combining the two images into an LRGB image they are aligned using Registar (Auriga Imaging). The L image is used as the reference and the new aligned RGB image (RGB-reg) is saved.
The L image and the RGB-reg image are opened in Photoshop with each image enlarged by the same %. The entire L image is cut and pasted onto the RGB image. Don't forget to check No to "Do you want to save changes?" before closing the L image! In the Layers Menu the opacity of the top layer (L) is reduced to 50% and precisely aligned (at high image magnification) over the bottom layer (RGB) using the Move tool and/or the arrow keys. After alignment, the opacity is brought back to 100% and the LRGB image is saved a photoshop file (.PSD) with two layers. Since the RGB and L components are in separate layers, they can be manipulated and enhanced independently.
This process can be repeated (LLRGB): The opacity of the top layer (L) is kept at 50% and the RGB layer color saturation is increased. The file is then flattened and used as a new, enhanced RGB layer for the next LRGB composite. This technique enriches the color of the new LLRGB composite as well as serving to equalize the contrast between the two layers.
When I'm satisfied with the result I flatten the image and save it as a TIFF file. Grain Surgery for Photoshop (Visual Infinity) is used to eliminate residual noise in the image.
STL-11000: Image processing is essentially as above. The camera is binned 1X1 for Luminance and H-alpha exposures. Individual exposures of 10- to 30-minutes are used (the STL-11000M is anti-blooming). These exposures are combined in CCDSoft to yield long effective integration times to reduce the S/N. The camera is binned 2X2 for the RGB data. Generally 3 to 5 individual 10 minute exposures for each color filter are combined. LRGB, LLRGB, (R)RGB or (RB)RGB, and (HA)RGB layering are carried out as described above.