Basic LRGB workflow part five - introduction to enhancements
Protecting elements of the image before enhancement
When you want to being to refine and enhance your basic LRGB image (or narrowband image for that matter), you want to protect elements of the image that you do not want to adjust with that particular process.
For this image of Messier 81, I would like to enhance the colour of the spiral arms and core by warming the colour and to also accentuate the spiral arm and dust structures that are otherwise obscured to a small degree by the dynamic range of the data.
To do this, we are going to create a Galaxy Mask so that the background sky and stars are protected from whatever process we want to apply to the galaxy.
I have first created a star mask using the DSO Mask script, see the separate tutorial on Starless technique for Hubble Palette narrowband images, here.
Then using PixelMath we subtract the star mask from the luminance image to produce a galaxy mask.
Minimise the resultant galaxy mask image and apply this image to the LRGB image.
You can click on/off the hide/reveal mask button on the menu bar to see what is protected and what is revealed.
Local Histogram Enhancement
I find a very effecttive technique to bring control to the enhancement stage of processing is to enhance a cloned image and then blend the enhanced image once complete with the original un-enhanced LRGB image. You can then iterate and experiment easiy with the degree of enhancement.
A clone is easily created by dragging the image tab onto the desktop of the workspace.
With everything but the galaxy now protected you can use the powerful tools of the Local Histogram Enhancement process to enhance the galaxy in your cloned image.
Note how small the value is in the "Amount" - I typically use values in the range 0.10 - 0.20. Keep "Contrast Llimit" at 2.0. The slider on the "Kernal Radius" option applies the contrast enhancement to either large scale structures (slider to the righthand side for large values >100) or smaller scale structures (slider to the lefthand side for values <100). I rarely use values over 250 (except perhaps on a large mosaic) or values less than 30 (for structures in a galaxy core).
An iterative approach can be used, with two or three applications at different Kernal Radius values. I find it better to work from large values to small values.
Note though, excessive application of Amount or too many iterations will produce quite artifical enhancements that perhaps do not represent real astronomical features. This is a very powerful tool and the maxim "less is more" is very true.
Best to use the preview window and view the outcome of the chosen Amount before clicking the square apply icon.
Second iteration of LHE
Third iteration of LHE
Enhancing the dust
A very effective tool to enhance dusty and dark regions is the dark Structure Enhancement script.
Note once again that this is a powerful tool and too great an application can produce artificial results.
A boost of the colour warmth with the "C" curve and saturation with the "S" curve.
Small adjustment of the "A" curve to lift the red content of the galaxy yellow-ish core.
Curve adjustment for contrast
A very gentle introduction of an "S" shape into the RGB curve will aid contrast enhancement of the overall galaxy structure.
This completes the basic enhanacements and we now want to blend this image with the un-enhanced LRGB to achieve the desired final enhancement.
The enhancements outlined in this tutorial are an introduction only and I plan a further tutorial to explore enhancements more fully.
Protecting the LRGB image with Mask015
Using the techniques described in part four for LRGB combination, we protect the LRGB image with the 0.15 mask "Mask015" so we can iteratively apply the "LRGB_enhanced" image using PixelMath.
Selecting the Mask015
Then type the name of the enhanced image into the "RGB/K" line of PixelMath and selecting "Replace image" apply the instance icon to the LRGB image. Evaluate the resultant change and repeat until you are satisfied with the result.
Next we want to apply some subtle sharpening to the galaxy to aid with the fine detail. Renove the Mask015 and select the Galaxy Mask.
Create a preview window of the galaxy.
Sharpening using Mulitscale Linear Transformation
The Multiscale Linear TRansformation process is a versatile tool, useful for noise reduction and sharpening as well as other more advanced techniques.
By selecting "Detail Layer" and then adding decreasing values from Layer 1 through to Layer 4 we can subtly sharpen the image at various scales.
Note the small values I have entered by highlighting each Layer in its row.
Using the preview window of the MLT tool will open a preview of the active image preview and you can evaluate the effect. Alternatively apply the instance icon to the preview and undo/do to evaluate the effect.
There are a number of noise reduction tools available in PixInsight. A very effective tool is TGVDenoise and I regularly use it after making adjustments to its defualt settings.
The key parameter to adjust is the "Edge protection" and I have found for my CCDs that reducing the number in the box to the right to -4 gives me the right 'exponential' value.
I then decrease the value in the slider to 1.40 by sliding it to the left and repeat the same slider move to the left for "Smoothness".
The default number of iterations is 100. Use this at first, evaluating the result on a preview window containing fine structure detail you wish to preserve along with background sky in which you want to minimise the fine-grained noise.
You can see in the process I have settled on a value of 250. I will regularly use values up to 500.
Select the Local Support option and this enables the process to use the image as an aid to the noise reduction. You do not need to adjust anything else.
Apply the triangular instance icon to the image.