In this article, you’ll learn how to process a Slooh.com PNG images of a faint nebula by performing a nonlinear stretch on the luminance image using Curves and Levels and you’ll perform a nonlinear stretch on the RGB image too. Finally, you’ll learn how to combine the L and RGB images for your final image.

Result

This is the image we’ll be creating:

This is NGC 6957 acquired using one Slooh.com mission.

Source Images

If you’d like to follow along in this tutorial, download the following source images. These images are from one reservation using the Generic processing recipe.

This is the luminance PNG image that Slooh.com produced:

This is the LRGB PNG image that Slooh.com produced:

There’s very little detail in both images, yet the processes I show you here make them more apparent.

Nonlinear Stretching The Color Image Using DDP

We’ll start by using the nonlinear DDP stretch using DeepSkStacker. Refer to Part 2 of this series for where to download DSS and for some details about how to use it.

Open the LRGB image using Photopea.com and save it as a TIF. Once you have done that, open the file using DSS.

Your screen should look something like this:

We’ll use the default options for the DDP stretch: click the Reset button on the Luminance tab, click Apply and save the resulting image.

There’s already some good detail; however, it is a noisy image and we can reduce that quite a bit with the luminance image.

Nonlinear Stretch of the Luminance Image

Open the luminance image in Photopea.com and perform a curves adjustment as you have before, except this time make the curve look something like this:

Next, perform a Levels adjustment by selecting Image – Adjustments – Levels.

Adjust the control point on the left so that you locate it at the beginning of the graph, as shown:

This adjustment will make your image darker.

Next, perform another Curves adjustment, making the curve appear like the following:

Perform another Levels adjustment, moving the control point on the left to the beginning of the graph, as shown:

Finally, perform one more Curves adjustment making the curve look like the following:

All of these adjustments have resulted in a nonlinear stretch of the luminance image.

Reduce Noise on the Luminance Image

The luminance image is pretty noisy, so let’s reduce it. Use the following directions:

  1. Select Filter – Blur – Gaussian Blur
  2. Enter 2.2 in the box or adjust the slider and click Ok
  3. Select Edit – Fade and enter 40 in the box, or adjust the slider
  4. Select Filter – Noise – Median
  5. Enter 8 in the box or adjust the slider and click Ok
  6. Select Edit – Fade
  7. Enter 30 in the box or adjust the slider and click Ok

We have reduced some of the noise in our image, but we have also introduced some blurring. We can fix that using the Smart Sharpen filter as follows:

  1. From the menu select Filter – Sharpen – SmartSharpen
  2. Enter 100 for Amount or adjust the slider
  3. Enter 1 for Radius or adjust the slider
  4. Click Ok to apply the changes

Combine the Luminance with the LRGB Image

You’re now ready to combine the luminance with the LRGB image as follows:

  1. Using Photopea.com, open the LRGB image you saved using DSS.
  2. Switch back to the luminance image and from the menu, select Select – All, then Edit – Copy
  3. Switch to the LRGB image and select Edit – Paste
  4. Change the blending mode to Luminosity
  5. Select Layer – Flatten Image

The image is a little washed out because the black point needs to be set correctly. Open the Levels adjustment and move the left control point to the point shown on the graph below:

We can move the control point so far over because the other parts we remove are noise and so can adjust the control point to the beginning of the large rise in pixel values.

Remove Green and Adjust Image

There is some green in the image and it needs to be removed to improve the color balance.

Open the Curves adjustment, select Green in the Channel and adjust the control point so that it is about the width of the dot away from the intersecting lines, as shown:

Perform a similar adjustment for the Red channel:

This is your final image, so you can now save it or export it.

Conclusion

In this article, you learned how to perform a nonlinear stretch of a luminance PNG image, you performed a DDP stretch of the color PNG image, you reduced noise in the luminance image, you combined the luminance and color images, you adjusted the levels of the color image and you removed the green cast in the final image, creating a much more detailed image than the one Slooh.com produced.

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