HDRMultiscaleTransform is a great tool for increasing contrast in areas of high signal. It’s an easy to use tool, but it relies on having a suitable mask in place to protect areas you don’t want to affect with the process.

Creating A Mask To Protect The Image

This is an image of Messier 51, made up of two galaxies:

In this example, we’ll use HDRMultiscaleTransform to make the cores of the galaxies better resolved. To do that, you’ll need a mask to expose only the cores of the galaxies. To create a mask for only the cores, I used the GAME script to create regions that selected those areas. Refer to this article for how to install the GAME script and for an overview of how to use it.

I created a gradient mask for both regions and came up with this mask:

The mask protects the entire image (shown in red), except for the cores of the two galaxies (shown in white). As I mentioned earlier, I created this mask using the GAME script to create a gradient mask.

Another way to create a mask for the brightest parts of the image is to create a range mask using the RangeSelection process. Using the RangeSelection process is straight-forward: start the process and click the real-time preview button (the circle button at the bottom right of the process window) and move the Lower limit slider until you only see the bright parts of your image

Once you have isolated the bright parts of your image, use the RangeSelection‘s Smoothness parameter to hide the stars and add a smoothing effect to your selection. Once you are satisfied with the mask, create it by dragging the triangle icon from the RangeSelection window to your image, and apply the mask to your image using Mask – Select Mask.

Use HDRMT To Improve Contrast

Run HDRMultiscaleTransform using the default settings. This is the image before running HDRMT:

Note that the cores of both galaxies are very bright. This is after running HDRMT:

The cores are more well-resolved and there is more detail in them.

You can also use HDRMT on the galaxy itself. Create a mask that protects the background and run HDRMT with these settings for this particular image:

I derived the settings by experimenting with the Number of layers and Scaling function settings. This is what the image looks like after applying the process:

There is more detail throughout the galaxy.

Sample Download

If you’d like to work with the images used for this tutorial, you can download a sample observation of M51 here:

Download Sample Observation of M51

Conclusion

You learned about the HDRMultiscaleTransform process and learned how to use it.

More Articles In This Series

This article is part of a whole series of articles about processing images using PixInsight: