In the tutorial Creating An RGB Image Using SAOImage DS9 I showed you how to create an RGB image from three input files (one for each color). The problem is that many observations include Luminance data as well, and you essentially ignore that valuable data when focusing only on RGB.

In this article, I show you how to incorporate a luminance image into your RGB image for more contrast and detail.

Creating Your RGB Image

Download the sample files and follow the directions in Creating An RGB Image Using SAOImage DS9 to create your RGB image for this tutorial.

Once you have done that, download this sample, which is the luminance image from the NGC 3195 observation – I’ll show you how to incorporate this image into the RGB image.

Preparing Luminance Data

We’ll have to convert our luminance FITS into a TIFF, so we’ll use SAOImage DS9 to do that.

1. Start SAOImage DS9 and locate and open the luminance FITS file.

2. Under Scale, select Log and now we’ll adjust the black point.

3. As in the other tutorial, from the menu, select Scale – Scale Parameters, find the low value in your image by hovering your mouse over dark parts of your image and observe the number in the Value field.

4. Enter the low value in the Low field of the Scale Parameters window, and enter 65535 in the High field if it’s not already there.

Your image should look something like this:

5. Zoom out to fit the image into the display. From the menu select Zoom – Zoom to Fit

6. Export the image as a TIFF

From the menu select File – Export – TIFF and save the image

Incorporating Luminance Data

We’ll use Photopea to incorporate the luminance data, but you can use Photoshop or GIMP too.

Navigate to photopea.com and select File – Open to open the RGB image you created in the other tutorial.

If you saved your RGB image as a PSD, go ahead and merge all of the layers by selecting Layer – Flatten Image.

From the menu, select File – Open and Place and locate the TIFF file for the luminance image. It might take a while to load the file.

Once the file is placed, press Enter to commit the file to a new layer in your image.

Set the blend mode to Screen to combine the images. From the Layers panel at the bottom-right, find where it says Normal and change it to Screen as shown:

You now have an image that incorporates the luminance data.

Performing Final Edits

The image’s background is a little too light and the nebula is a little over-exposed at this point. We can perform some adjustments to enhance the image.

I applied a Curves adjustment to darken the background and highlight the nebula. Then I added a Hue/Saturation adjustment and boosted the Saturation.

The image was a little too blue, so I applied another Curves adjustment on the blue channel to slightly reduce the blue in the overall image.

The last adjustment I made was an Exposure adjustment where I slightly modified the Exposure and Gamma variables.

I include the final PSD in the sample download for this article, for you to look at – you can open this in Photopea, Photoshop, or GIMP (link is above in the section “Creating Your RGB Image“). Take a look at the various layers to see how I made the adjustments I mentioned above.

Conclusion

In this tutorial you learned how to incorporate luminance data into an RGB image using SAOImage DS9 and Photopea.