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LRGB Data Processing
Images obtained with a Monochromatic CCD Camera

The purpose of this tutorial is combination an processing of separate L, R, G and B images taken with a monochromatic CCD camera into a LRGB end result.

The data used for this tutorial is :
  • 24 Luminance frames of IC1318, each exposed for 300 sec
  • 24 Red frames of IC1318, each exposed for 300 sec
  • 24 Green frames of IC1318, each exposed for 300 sec
  • 24 Blue frames of IC1318, each exposed for 300 sec
The images are made with an 72mm F/5 Megrez telescope with WO corrector. The L, R, G and B images where registered, calibrated and stacked in PixInsight.
Combining RGB channels
1. Open “StarAlignment” tool
2. Select reference image in (Luminance image in this tutorial)
3. Select target image(s) in (L, R, G and B images in this tutorial)
4. Select the target directory where the registered image(s) will be saved
5. Click on the circle to start the process
Image001
A RGB image can be build with these registered images.

1. Open the registered Red image
2. Open the registered Green image
3. Open the registered Blue image
4. Open the “LRGB Combination” tool
5. Deselect the "L" channel
6. Select the Red image in the red channel
7. Select the Green image in the green channel
8. Select the Blue image in the blue channel
9. Click on the circle to start the process (RGB image will opened automatically)
Image002
Histogram (RGB)
Open the RGB image and apply the “ScreenTransferFunction” to it. Open the “HistogramTransformation” tool and transfer the parameters calculated by "ScreenTransferFunction" to "HistogramTransformation" tool by drag and drop the "New Instance" button from "ScreenTransferFunction" on to the bottom bar of "HistogramTransformation" tool. Clicking "Reset" on " ScreenTransferFunction" removes the auto-stretch on our image and then clicking "Apply" on "HistogramTransformation" tool applies the same parameters to the image.
Image003
Background Neutralization
Open the RGB image and select a preview of the background (without stars). Open the "BackgroundNeutralization" tool and select the background preview under "Reference Image". Apply the "BackgroundNeutralization" tool to the RGB image.
Image004
Color Calibration
Select a second preview of some stars in the RGB image. Open the "ColorCalibration" tool and select the background preview under "Background Reference Image" and select the second preview (stars) under the "White Reference Image". Apply the "ColorCalibration" tool to the RGB image.
Image005
The RGB image is ready and can be save for now.
Histogram (Luminance)
Open the Luminance image and apply the “ScreenTransferFunction” to it. Open the “HistogramTransformation” tool and transfer the parameters calculated by "ScreenTransferFunction" to "HistogramTransformation" tool by drag and drop the "New Instance" button from "ScreenTransferFunction" on to the bottom bar of "HistogramTransformation" tool. Clicking "Reset" on " ScreenTransferFunction" removes the auto-stretch on our image and then clicking "Apply" on "HistogramTransformation" tool applies the same parameters to the image.
Image006
Sharpening Fine Details (Luminance channel)
The next step is Sharpening of fine details and structures in the image. There are many ways in PixInsight to do this but I mainly use the “HDRMultiscaleTransfer” tool which is a rather good and fast tool. This process is a “trail and error” process and therefore I often select a small “preview” area which contains some of these fine details and structures.
Image007
First of all its important to select “To Lightness” and “Lightness Mask” in the "HDRMultiscaleTransfer" tool. By standard the number off layers is 6 and the number off iterations 1. Normally this will work very good and robust but I will play a bit with these values for the best result. Once satisfied the "preview" can be closed and the "HDRMultiscaleTransfer" tool can be applied to the image.
Image008
You will see that the fine details and structures are more prominent in the images but the brightness and contrast suffer, this will be restored at the end of the tutorial.
Combining the L and RGB image
A LRGB image can be build with the registered L and RGB images.

1. Open the registered Luminance image
2. Open the registered RGB image
3. Open the “LRGB Combination” tool
4. Select the Luminance image in the L channel
5. Deselect the Red, Green and Blue channels
6. Drag the little triangle over the RGB image (LRGB image will opened automatically)
Image009
Dynamic Crop
I will use the "Dynamic Crop" tool to cut off the black edges of the image.

1. Open the “DynamicCrop” tool
2. Select that part of the LRGB image you want to use
3. Click on the green sign to start the process
Image010
Noise Reduction
Next step is noise reduction, also here are several ways to do it, I use the “ACDNR” tool. To provide protection to high signal areas, we first apply a mask. The mask applied is generated by duplicating the linear image. If you are working with a color image, the same procedure applies but it is best to extract a Lightness image from the color image. First action is to apply the “MultiScaleLinearTransform” tool in order to blur the image for a nice gradual protection of the high contrast features.
Image011
Now I apply the auto-stretch parameters permanently, by using the "HistogramTransformation" tool. When needed I tweak the white point (right slider) and the black point (left slider) of the image without chancing the mid tones.
Image012
Activate now the image and select the masker (Ctrl+M). Open the “ACDNR” tool. Normally I open a small “preview” in order to try different settings.
Image0013
The setting "StdDev" defines roughly the noise structure size you are wanting to eliminate. For the usual small scale noise, you may wish to use values ranging between the default 1,5 and about 2,5. Normally I use 2,0 for StdDev to start with. A setting for the "Amount" of 1,00 keeps purely the noise reduced image as the end result. The default setting of 0,90 keeps 90% of the noise reduced image with 10% of the original image, giving a smoother end result. Apply the "ADCNR" tool to the masked image and deactivate the mask once done.
StarMask
The "StarMask" process is very capable of producing mask images for use in the protection of stars, or perhaps to attack stars alone (e.g. for color saturation or to reduce star sizes). Open the “StarMask” tool and apply this to the image. The standard values of this tool are already quite good and robust for most of the images. Applying this tool will generate a new image (mask) with only stars.
Image014
Select now the starmask and open the “Morphological Transformation” tool. Change the “Operate” to “Morphological Selection”.
Image015
Apply the “Morphological Transformation” tool to the masked image and deactivate the mask once done.
Histogram and Curves
Last step in this tutorial is to recover the brightness and contrast of the image. This can be done with the histogram and curves tools. With the “HistogramTransformation” tool we need to move the left slider in order to define the black point without clipping the image. When needed we can also change the mid tones by moving the center slider. The right slider (white point) needs to be untouched.
Image016
Now we can tweak the image with the “Curves” tool. For this image only some minor tweaks to the RGB curve and thee saturation curve have been done.
Image017
Results
This is my work flow which I use and is not necessarily to only one available which will bring good results.

End result of this work flow :
Image018
© Copyright Rob Kantelberg
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