This is the last article in the series about processing Slooh.com PNG images. In this article you’ll learn how to recover an image that has a radial gradient on it.
A radial gradient is a circular region of uneven illumination that leaves the center overexposed and the edges of the image underexposed.
In this tutorial we will be performing the nonlinear stretches of the luminance and LRGB images, plus we’ll use a mask to independently adjust the center and edges of our image and we’ll learn about the Shadows/Highlights adjustment.
Result
This is the final image you’ll create using the steps in this tutorial:
This is a very faint subject and I think this is a good result considering that there is uneven field illumination.
Source Images
Use these images to follow along in this tutorial. The only processing that has been done on them is make them smaller to fit on this webpage.
This is the Luminance image:
This is the LRGB image:
In both cases, right-click the image and select Save As.
Using DDP to Stretch the LRGB Image
As before, we need to convert our LRGB PNG image to a TIF. Using Photopea.com, open the LRGB image and export it as a TIF. Consult the preceding tutorials for details on how to perform these things in case you are not familiar with them.
Next, open the image in DSS and perform a DDP stretch on the image:
- Select Raw/FITS DDP Settings
- Click Ok
- On the Luminance tab, click Reset and click Apply
Looking at the image, the radial gradient is apparent, and we’ll correct that later in this tutorial.
Perform a Nonlinear Stretch on the Luminance Image
In Photopea.com, open the luminance image and perform a nonlinear stretch as follows:
Perform the following Curves adjustment:
Perform the following Levels adjustment:
Perform the following Curves adjustment:
Perform the following Levels adjustment:
And finally perform the following Curves adjustment:
Combine the Luminance and LRGB Images
Combine the luminance image with the LRGB image you saved using DSS, as you have done before. Ensure that you select the Luminosity blending mode.
From this image, you can see a radial gradient surrounding the nebula, which makes it appear to be washed out. The next series of steps adjust the image so that it appears to be more evenly illuminated.
Adjusting the Uneven Field Illumination
Start by flattening the image so that there’s only one layer – select Layer – Flatten Image.
- Duplicate the background layer by right-clicking it, and then select Duplicate Layer
- Disable the background layer by clicking the eye next to it
- Ensure that Layer 1, above the Background layer, is active
Now we are going to use a mask to reveal the center of the image. But we want the mask to have a gradual change so that the change between the center and edges of the image is not harsh.
- From the menu, select Layer – Raster Mask – Add (Reveal all)
- Activate the Gradient tool from the toolbar on the left, or press G on your keyboard
- At the top of Photopea, ensure that the gradient goes from black to white and that Radial is selected:
4. Zoom out of the image a little so that the whole image appears on the screen
5. Drag from the center of the image to about 1/4 an image width to the outside of the image as shown:
Your screen should look something like the following:
The squares represent transparent areas and the goal is to capture as much of the radial illumination as possible.
Once you have done that, click the eye next to the Background layer and you should have the complete image back on the screen.
What we have done here is create a mask with a gradual change from the center to the edges of the image. Our changes will be more gradual at this point.
Press and hold your CTRL key on your keyboard and click the black and white circle in Layer 1. This selects the range of pixels associated with the mask we just created. At the moment, we have the pixels selected that represent the center of the image – we need to select the outer edge pixels.
From the menu, select Select – Inverse – you should now see only a circle selected on the image.
We’ll make a Levels adjustment; however, we need to target just the selected pixels, so from the menu select Layer – New Adjustment Layer – Levels.
Adjust the Levels of this selection by moving the middle control point as shown:
As you make the adjustment, note how just the pixels you have selected change. The idea is to make the center of the image a little darker.
Now we need to adjust the pixels at the edge of the image.
1. From the menu select Select – Inverse
2. From the menu, select Layer – New Adjustment Layer – Levels
3. Adjust the middle point by moving it slightly to the right as shown:
The idea is to make the pixels on the outer edge match those in the center.
Now that they are about equal there’s one final adjustment to make.
Adjust the Vibrance
We want to make the nebula a bit more vibrant. From the menu, select Layer – New Adjustment Layer – Vibrance.
Set the Vibrance at 100.
Adjust Brightness of only the Nebula
The nebula is a little underexposed and we can improve that with a brightness adjustment.
Here, we’ll use the Magic Wand to select the nebula and then apply an adjustment to it.
1. Select the 4th tool from the menu on the left, and click and hold down your mouse button until another menu pops-up beside it
2. Select the Magic Wand
3. At the top of Photopea, set the Tolerance to 20
4. Zoom into the image to make the nebula larger
5. Click somewhere on the red part of the nebula
You want your selection to look something like this:
If your selection doesn’t look quite like that, hold down the Shift key on your keyboard and click elsewhere on the red part of the nebula to include more of the selected area and repeat as necessary until the selected area looks something like the preceding screenshot.
6. Click the Vibrance layer to make it active
7. From the menu, select Layer – New Adjustment Layer – Brightness/Contrast
8. Set the brightness to about 20 and the Contrast to about 25
Slightly Reduce the Noise in the Nebula
The nebula is a little noisy and we can attenuate that with a Gaussian Blur. With the nebula still selected do the following:
1. Select the Background layer to make it active
2. From the menu, select Filter – Blur – Gaussian Blur
3. Enter 2.2 px and click Ok
4. From the menu, select Edit – Fade, enter 40% and click ok
5. From the menu, select Select – Deselect and view your adjustment
Optional – Slightly Sharpen the Overall Image
You can slightly sharpen the image to reduce some blurring if you can see it. Do the following:
- Select Layer – Flatten Image
- Duplicate the Background layer
- From the menu, select Filter – Other – High Pass
- Enter 3px in the Radius field
- Set the layer’s blending mode to Soft Light
Save your image as a PSD and export it in your preferred format.
Conclusion
In this article, you learned a technique for recovering an image with uneven field illumination using masks as well as selections. You were able to independently adjust the center and edges of the image to compensate for the uneven field illumination. You also learned about adjusting individual parts of the image through selections, and you learned about one method of sharpening an image. This concludes the four part series of processing Slooh.com PNG images for astrophotograhgy.
Other Articles In This Series
There are a number of other articles in this series – here is the link to the main article that has links to the other articles in this series: Processing Slooh.com PNG Images Series
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