SAOImage DS9 is a FITS file viewing and manipulation application from the Chandra X-ray Science Center (CXC) and the High Energy Astrophysics Science Archive Center (HEASARC). SAOImage DS9 is used for various analysis of Chandra X-Ray as well as other FITS data.

This article walks you through creating an RGB image using DS9.

Why Use SAOImage DS9?

With options like the online FITS viewer JS9 and JS9-4L and FITS Liberator, why would you use SAOImage DS9?

Each of the applications implements their algorithms differently which produces different results. Let’s look at the results of basic processing of a planetary nebula. NGC 3195 is a small target sized at just 40 x 35 arcseconds, so the image manipulation software must have good resolution for the target to show up in the final image.

I followed my own directions in Using Slooh.com’s JS9 FITS Editor To Create RGB Images to create a final image of NGC 3195:

Although the image is cropped and zoomed in quite a bit, it is difficult to make out the nebula in this image – the image is noisy and grainy.

I used the directions in Using FITS Liberator To Create An RGB Image to create the following image:

The details of the nebula are more easily resolved, yet the image is slightly over-exposed. The image, however, is less noisy and grainy even though the same underlying FITS files were used to create this image as were used to create the JS9 image.

I created the following image using SAOImage DS9:

Clearly this image is far better than the other two with more colors and more details in the nebula.

And finally, for the sake of comparison, I processed the FITS files using MaxIM DL and came up with this image:

This image reveals the most detail of all of the images; however, MaxIM DL is expensive so you should expect good results (assuming you have good data to begin with, which is the case here).

So clearly, SAOImage DS9 is superior to both JS9 and FITS Liberator. The nice thing is that the process of creating an RGB image is relatively straight-forward. The rest of this article describes the process.

Downloading the Samples

I have made sample FITS files available for download for this tutorial – these are the same files I worked with to create this tutorial.

I took these images using itelescope.net, telescope 33, on December 25, 2018.

Acquiring SAOImage DS9

SAOImage DS9 is freely available for a number of platforms including Windows, Mac, and Linux. Visit the following page to download the application:

http://ds9.si.edu/site/Download.html

Follow the installation directions for your platform – it should be relatively straight-forward since SAOImage DS9 is self-contained.

Creating an RGB Image

To create an RGB image, you have to open all three images in SAOImage DS9 and then adjust the image’s scale so that it looks good. Use the following steps to open the images and adjust the scale.

1. Start SAOImage DS9 and select Frame – New Frame RGB

A new smaller window pops-up – move the SAOImage DS9 window so that both windows are visible on the screen but not overlapping.

2. From the menu in the smaller window, select Lock – Scale as shown:

3. Configure the smaller window as shown:

4. From the main window, select File – Open and locate and open the red image.

5. Configure the smaller window as shown:

6. From the main window, select File – Open and locate and open the green image.

7. Configure the smaller window as shown:

8. From the main window, select File – Open and locate and open the blue image.

Your window might look like the following with the image mostly black:

Select the Scale function – from the menu, select Scale – Log and your display should look something like this:

We can adjust the scale to bring out some more detail.

From the menu, select Scale – Scale Parameters. A new window pops-up:

Move the window so that it does not overlap the image.

We’ll now adjust the black point of the scale – it is currently set to zero, which is too low.

Hover your mouse over various dark parts of the image and note the number shown in the Value field as shown:

Look through your image and try to find the lowest value. You don’t have to be completely accurate as the black point will be around a certain number, and you can always adjust it later.

Once you have found a low number, enter it in the Limits Low field in the Scale Parameters window. Also enter the High value of 65535 if it is not already set, and then click Apply to commit the changes.

Check the effect on your image and adjust as necessary to bring out the details but leave the background as black as possible.

Once you have completed your changes, click Close in the Scale Parameters window to close the window.

Exporting Your Image

From the menu, select File – Export – TIFF to retain the most fidelity.

Editing The Image In Photopea

The image is almost complete – you can apply a Curves adjustment to make the background darker and highlight the details of the nebula.

Use your browser to navigate to https://www.photopea.com/, and select File – Open to open your file.

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

Adjust the Curves to make the image darker and to highlight the nebula – try the following setting:

Save your image as a PSD to retain the layers or save in some other format to simply export the image.

Conclusion

In this article, you learned about how different applications implement algorithms that produce different results for the same input data. You also learned how to create an RGB image using SAOImage DS9.

Do you like this article? Consider downloading my Free book about Slooh.com – Remote Astrophotography Using Slooh.com – A Handbook.

Update: I published a new article that shows you how to incorporate the luminance data into your final image: Adding Luminance Images to SAOImage DS9-Processed RGB Images