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Narrowband Data Processing
Images obtained with a Monochromatic CCD Camera

The purpose of this tutorial is combination an processing of narrowband images (Hα en OIII) in an RGB end result. This is only applicable on emission nebulae and planetary nebulae as they mainly emit light in these narrowbands. The color balance is depending on the intensities of the Hα, OIII and Hβ lines. Often the Hβ channel is obtained by a specific combination of the available Hα and OIII data. The obtained Hβ channel is called the "synthetic Hβ" channel (see here for more information). As the use of a synthetic Hβ channel generates often huge color balance problems and most astroimagers mostly only have Hα and OIII filters available I was looking for a way to generate a natural RGB end result. In this tutorial I want to show that a natural looking end result can be obtained with only Hα and OIII data without creating a synthetic Hβ.

The data used for this tutorial is :
  • 9 Hα frames of Sh2-132, each exposed for 600 sec
  • 9 OIII frames of Sh2-132, each exposed for 600 sec
The images are made with an 300mm F/5 Newtonian telescope with 0,73x ASA focal reducer/corrector. An 7nm Baader Hα filter and an 8,5nm Baader OIII filter where used. The used CCD camera was the famous ATIK383L+ @ -25̊C. The Hα and OIII where registered, calibrated and stacked in PixInsight.
Image001
The registered, calibrated and stacked Hα en OIII images
Combining RGB channels
1. Open “StarAlignment” tool
2. Select reference image in (Hα image in this tutorial)
3. Select target image(s) in (OIII image 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
Image002
A RGB image can be build with these registered images.

1. Open the Hα image
2. Open the registered OIII image
3. Open the “LRGB Combination” tool
4. Select the Hα image in the red channel
5. Select the OIII image in the green channel
6. Select the OIII image in the blue channel
7. Click on the circle to start the process (RGB image will opened automatically)
Image003
Dynamic Crop
The RGB image is still unprocessed and it looks like a black image with some small white spots (stars). Before I start processing I will use the "Dynamic Crop" tool to cut off the black edges of the image.

1. Open the “ScreenTransferFunction” tool and then click the Auto Stretch button
2. Open the “DynamicCrop” tool
3. Select that part of the image you want to use
4. Click on the green sign to start the process
Image004
Histogram
Lave the “ScreenTransferFunction” still active and open the “HistogramTransformation” tool. 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.
Image005
Sharpening Fine Details
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.
Image006
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.
Image007
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.
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.
Image008
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.
Image009
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.
Image0010
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.
Image011
Select now the starmask and open the “Morphological Transformation” tool. Change the “Operate” to “Morphological Selection”.
Image012
Apply the “Morphological Transformation” tool to the masked image and deactivate the mask once done.
Image013
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.
Image014
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.
Image015
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 :
Image016
Sh2-132
Other results obtained with this work flow :
Image017
NGC6820 / 6823
Image0186
NGC6888
© Copyright Rob Kantelberg
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