This article is a continuation of the article Creating an RGB Image Using DeepSkyStacker and GIMP, except instead of using GIMP this article demonstrates how to use Photopea.com, so there’s nothing extra to install on your system.

Using Photopea.com instead of GIMP provides you with more capabilities and also helps you learn Photopea.com, which is very similar to Photoshop, so you have a more portable skill set.

This article assumes that you have gone through all of the steps to stack the individual color images so that you now have four TIF files from DeepSkyStacker: one for each of the L, R, G, and B files.

Adjusting The Stretch of Your Images Using Photopea.com

  1. Point your browser to Photopea.com
  2. From the menu, select File – Open and open the luminance file, NGC1514-lum.tif
  3. From the menu, select Image – Adjustments – Levels to bring up the Levels dialog box
  4. Move the Levels dialog box so that it is out of the way of your image but still visible on your screen
  5. Move the middle slider so that it is at the foot of the spike in the graph window

    This is what my screen looks like:



    Note that your image becomes very bright and this is expected behavior.
  6. Click Ok to comit the chagne
  7. From the menu, select Image – Adjustments – Levels to bring up the Levels dialog box again
  8. Move the slider on the left, so that it is at the foot of the graph

    This is what my screen looks like:



    This action makes your image darker and you might be able to see the nebula on your screen.
  9. Click Ok to comit the change

Repeat the above steps from step three after opening each subsequent color file; perform the same adjustment on each image. Note that the nebula might not be apparent on the red image after you make the adjustment.

Combining the RGB Files

Now that we have adjusted the stretch of each file, we can combine all of the files to produce a color image.

  1. From the menu, select Image – Image Size
  2. Note the pixel dimensions of the image (1360 x 1360 pixels)
  3. From the menu, select File – New
  4. Enter 1360 in the Width and Height boxes and click Create

    We’re now going to copy and paste each color image into the corresponding channel of the new image.
  5. From the lower right of the screen where it says Layers | Channels, click the Channels tab
  6. Click the word Blue to make the blue channel active
  7. Find the blue image in the tabs across the top of the Photopea.com window
  8. From the menu, select Select – All
  9. From the menu, select Edit – Copy
  10. Switch back to your new image with the blue channel active, and from the menu, select Edit – Paste

Repeat the above steps from step six, choosing the red and green channel and the corresponding image for step seven. When you finish the process, you’ll have an image in each channel of the new image.

When you are finished, click the eye icon next to RGB to reveal the color image. Click the Layers tab to make it the active tab.

Combining the Luminance Image

Now we need to combine the luminance image with the RGB image to bring out more details. Do the following:

  1. From the tabs across the Photopea.com window, select the luminance image
  2. From the menu, select Select – All
  3. From the menu, select Edit – Copy
  4. Make the new RGB image the active image by clicking its tab at the top of the Photopea.com window
  5. From the menu, select Edit – Paste

    This creates a new layer over your RGB image. We’ll set the blending mode so that the luminance image gets combined with the RGB image.
  6. At the bottom right, where it says Normal, click the arrow next to it and select Luminosity

    Now flatten the image so that it has just one layer.
  7. From the menu, select Layer – Flatten Image

    Now perform an adjustment to make the image darker

  8. From the menu, select Image – Adjustments – Levels
  9. Move the slider on the left to the foot of the graph so that your image is darker
  10. Click Ok to comit the change

This is your final image. Note that the nebula is pretty faint; this image could use more work to bring out the faint nebula. There is another technique you can use to bring out more details in the image and the technique is described in this article.

Conclusion

In this article, you learned how to use Photopea.com to create an RGB image and combine the luminance image as well.