In the last part of this series, you learned the basics of processing the PNG image of the Orion Nebula for astrophotography. You brought out the details in the nebula and brighter areas. That was a relatively easy adjustment.

This image is a little more challenging. This image is of the Trifid nebula taken using the Canary One telescope. Slooh.com produced four images and here are two of them:

This is one of the luminance images:

This is the color image:

Unlike for the Orion Nebula, which has specialized processing performed on it, Slooh.com performs just a little processing on these images, and you can see the result here – the color image has very little detail in it.

With some work, you can end up with an image like this:

This image is much more suited for astrophotography since there’s much more detail in it and more tonal range. I could imagine myself sharing this image with others, perhaps in Slooh.com observations or on a social media app like Instagram.

The rest of this article explains how I got this result.

But before we get started you need to learn about a new process called DDP.

Introducing the Digital Development Process

The Digital Development Process is an algorithm invented by Dr. Kunihiko Okano that performs a nonlinear gamma stretch and unsharp masking to simulate the effect of photographic development on digital images.

That was a lot in one sentence! Let’s break it down:

  • An algorithm: a repeatable process
  • Nonlinear gamma stretch: a stretch is an adjustment using Curves or Levels using software like Photopea.com, Photoshop or GIMP. A nonlinear stretch is a specific type of adjustment that brings out the mid-tones in an image; mid-tones are important in astrophotography
  • Unsharp masking: a method for sharpening images that you can perform in software like Photopea.com, Photoshop

When you stretch an image using a Curves adjustment, you’ll come to a point where you can no longer improve on bringing out the mid-tones in an image without affecting the really dark and bright areas, as shown in the following image:

In the preceding image, there’s more detail in the nebula; however, we have also boosted the really dim parts of the image at the bottom right and are close to saturating the center part of the image.

The Digital Development Process, or DDP, addresses this by automatically performing some adjustments that move the histogram of your image to the right, thereby boosting the mid-tones and improving the overall image.

The problem with DDP is that its availability was limited to expensive software like MaxIM DL and other paid astrophotography processing software.

Fortunately, I found free software that we can use to perform DDP stretches on our images for great results.

Introducing DeepSkyStacker

DeepSkyStacker, or DSS, is free software you can use to process your astronomical images. DSS is actually meant for processing FITS files from your observations but it fortunately works with regular images too. If you want to learn more about processing FITS files, download my free book.

Fortunately for us, DSS implements DDP and we’ll use it to enhance our color images.

For now, download and install DSS, following the directions on its website.

Converting the Color Image for use with DSS

Before we can use DSS, we have to convert our color image of the Trifid nebula into the TIF format because that’s one of the image formats DSS can work with.

Save the luminance and color images at the beginning of this article somewhere convenient.

Start Photopea.com and open the color file.

From the menu, select File – Export As – More – TIFF. Your file will be in the Downloads folder of your browser – copy it to the same location as the luminance and color images.

Using DDP On Color Images

We’ll use DDP to enhance our color images to make the mid-tones more prominent.

The interface in DSS is a little different at first, but you’ll get used to it because everything is arranged logically.

Start DeepSkyStacker and select ‘Open picture file…’:

Open the color image that you saved in TIF format.

Next, select ‘Raw/FITS DDP Settings…’ under the Options category on the left side of the screen, and click Ok on the box that pops-up.

Your screen should look something like this:

Next, click the Luminance tab, locate the Midtones slider, move it all the way to the right, and click the Apply button. Your image should have more detail and color in it now.

Save the file by clicking the ‘Save picture to file…’ link, select a location, enter a filename, and ensure that ‘Apply adjustments to the saved image’ is selected.

We are now ready to move back to Photopea.com.

Adjusting the Luminance Image

Open Photopea.com and open the luminance image.

Open the Curves adjustment and place control points at the locations shown (for more about the Curves adjustment, check Part 1 of this series, or download by book):

Now move the control points to these locations:

The curve is a very gentle curve and the first control point near the bottom is placed to minimize the highlighting in the very darkest areas of the image. Your screen should look similar to the one shown in the screenshot.

Combine the Luminance with Color and Align

Next, we combine the luminance image with the color image.

On the luminance image, select from the menu Select – All, then Edit – Copy.

Open the color TIF image that you saved from DSS, and from the menu select Edit – Paste. Your image should appear to be a monochrome image. You should now have two layers in your image.

Set the blending mode to Luminosity as shown:

Click the eye icon next to the luminosity layer and note the change in your image.

The color and luminance images are slightly out of alignment.

From the menu, select View – Zoom In and do it a second time. Alternately hold down CTRL and ALT and scroll your mouse wheel to zoom in. Find an area near the center of the image and note that the stars are duplicated.

Ensure the luminosity layer is selected, and then select the Move tool at the top of the toolbar on the left side of the screen.

Click and drag the image until the stars are aligned.

Remove Green From Image

There is some green in this image that needs to be removed.

From the menu, select Layer – New Adjustment Layer – Curves.

Select Green in the Channel dropdown and set and move the control point as shown:

While zoomed-in you’ll notice that the green in the stars and nebula reduces when you make this change; it is a very small adjustment.

Save Your Final Image

Use the File Save PSD or File – Export menu to save your image. If you save in PSD, Photopea will save your image with the layers intact so that you can edit them again later.

Conclusion

In this article you learned about the DDP algorithm, you downloaded and installed DSS, you used DDP to stretch a color image, you adjusted the luminance image, you combined the luminance and color images, and you removed the green cast from your image.

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