Often when taking exposures you end up with multiple files per color. DeepSkyStacker is perfect for stacking multiple images. This article is a step-by-step tutorial that walks you through using DeepSkyStacker, JS9, and Photopea.com to create a very good quality final image.

While I cover using GIMP in another article, this article uses a more modern approach and results in a higher-quality image.

Result

This is the image you’ll produce by following the directions in this article:

Overview of the Process

We’ll first stack each color image using DSS. This will result in one FITS file for each color and this file is sometimes referred to as a master/primary file.

Next, we use JS9 to stretch the FITS file and then download a PNG of the stretched image.

Following that, we use Photopea.com to combine each of the colors and the luminance image to produce a final result.

Acquiring and Installing DSS

Download DeepSkyStacker from here: http://deepskystacker.free.fr/english/index.html

Follow the installation directions and take note of the directions at the top of the page.

Downloading The Sample Observation of NGC 1514

I have made my observation of NGC 1514 available as a sample download. The FITS files I used to create this tutorial are contained in the ZIP file, so you can follow along in this tutorial.

Stacking Using DSS

The first thing we’ll have to do is stack the images. You stack images per color, so you end up with one stacked image for luminance, one for green, one for red, and one for blue – these are often called master files. Once we have these images, we’ll switch to using JS9 to stretch the image and then use Photopea.com to combine the master images and perform edits and enhancements.

Use the following directions to stack the FITS files:

1. Start DSS and select Open Picture Files

2. Select the five Luminance files (filenames with _l_ in them)

3. Click Check All

4. Click Stack checked pictures

Configure the stacking parameters

You need to go through this process just one time:

a. Click the link next to Stacking Mode and select Standard and click Ok to close the window

b. Click the link next to Alignment Mode and select Automatic and click Ok to close the window

c. Click the link next to Method and select Average

Continuing on…

5. Click Ok to start stacking

The stacking process can take some time and there are multiple phases. When stacking completes, you’ll see a screen with a very dark image on it. This is the stacked image.

6. Click Save picture to file

7. Enter the name ngc1514-lum and select the Save as type as FITS (16 bit/ch)

8. Click Clear List on the left and answer No to whether you want to save the list

Repeat the stacking process for the remaining colors. The general steps are as follows:

1. Select five color files (filenames have _r_, _g_, or _b_ in them)

2. Click Check All

3. Click Stack checked pictures

4. Click Ok to start stacking (there’s no need to configure stacking parameters because they are saved)

5. Click Save picture to file

6. Enter the name ngc1514-<color> and select FITS (16 bit/ch)

Replace <color> with the name of the color you are stacking.

7. Click Clear List

8. Answer No to the Save Changes question

Your folder will contain some extra files with a TXT extension and files beginning with ‘Autosave’ – you can safely delete these files.

Stretch Each Image Using JS9

Now that we have the master/primary FITS file for each color; however, these are in their unstretched state (linear). We need to stretch the images so that we can see the image on the screen and to be able to export a final image. We’ll use JS9, an online FITS editor, to stretch each image. JS9 makes stretching the image very easy – use the following steps:

  1. Start JS9 – point your browser to https://js9.si.edu
  2. Select File – Close – all images
  3. Using Windows Explorer (or Finder on a Mac), find the files you extracted
  4. Select the Luminance FITS file
  5. Drag the luminance file to the JS9 window
  6. Move your mouse around the dark parts of the image and note the value of the green number at the upper-left of the screen. You’re looking for the lowest number
  7. From the menu, select Scale
  8. Enter the number you found in step 6 (in my case the number was 930) in the low field, and press Enter
  9. Resize the image to the original image size by selecting from the menu, View – resize – set to image size
  10. From the menu, select File – Save – PNG
  11. Refresh the page to re-initialize JS9
  12. Rename the downloaded file to something meaningful like NGC1514-Lum.png

Repeat the above steps from step 2 for each of the colors.

Combine the RGB Files Using Photopea.com

Now that we have stretched the images, we need to combine them to produce an RGB image – we’ll also combine the luminance image for more detail. We use Photopea.com for this step because it is very similar to Photoshop, plus there’s nothing to install on your system. Use the following steps:

  1. Point your browser to Photopea.com
  2. From the menu, select File – Open and open the Luminance PNG
  3. From the menu, select Image – Image Size
  4. Note the pixel dimensions of the image (1360 x 1360 pixels)
  5. From the menu, select File – New
  6. 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.

  1. From the lower right of the screen where it says Layers | Channels, click the Channels tab
  2. Click the word Blue to make the blue channel active
  3. Find the blue image in the tabs across the top of the Photopea.com window
  4. From the menu, select Select – All
  5. From the menu, select Edit – Copy
  6. Switch back to your new image with the blue channel active, and from the menu, select Edit – Paste

Repeat the above steps from step two, choosing the red and green channel and the corresponding image for step three. 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

    The image is a little noisy, so we’re going to reduce the small-scale noise a little
  7. From the menu, select Filter – Blur – Gaussuan Blur
  8. Drag the slider or enter a value of 2 px and click Ok

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

This is your final image, you can save it by selecting from the menu File – Export As and select a format to export it.

Conclusion

In this article, you learned how to combine multiple observations using DeepSkyStacker, you learned how to stretch images using JS9, and you learned how to combine the channel images into an RGB image and you learned how to combine the luminance image.