Sometimes your image includes some areas of faint signal like the nebula in the following image (Sh2-240):

In this case, the nebula has a relatively low signal.

If you begin your workflow by integrating and then stretching the image using the Auto Stretch provided by the STF, you end up with the following luminance image:

The nebula is faint and this could be improved by boosting the signal of the nebula to make it more visible.

Overview of Boosting Areas of Faint Signal

The process involves the following major steps which I discuss in the sections that follow:

  1. Create an object mask that isolates the area of faint signal
  2. Use CurvesTransformation to bring out the faint detail
  3. Use CurvesTransformation to darken the background

The following sections describe each step in detail.

Create An Object Mask To Isolate Areas of Faint Signal

Do the following to create an object mask:

  1. Stretch your luminance image using HistogramTransformation based on the STF
  2. Clone your luminance image (drag the image’s name):


  3. Rename the cloned image ‘object_mask’ (double-click the image’s name and ener the new name):


  4. Use the StarNet process to create a starless image (drag the new instance/triangle icon onto the object_mask image):


  5. Using HistogramTransformation, stretch your object_mask image so that it has a dark background and try to boost the midtones (adjust the midtones slider)


  6. Stretch the resulting starless image so that the faint details are as bright as you can make them. This is what my luminance image looks like:

Use CurvesTransformation To Bring Out Faint Detail

Mask your luminance image using the object_mask and use CurvesTransformation to increase the nebula’s contrast – use the RGB/K adjustment, select a point on the middle of the diagonal line, and drag the point to the left to see the boost in contrast.

Use CurvesTransformation To Darken The Background

Do the following:

  1. Invert the mask so that the nebula is protected (select Mask – Invert Mask)
  2. Use CurvesTransformation to darken the background – use the RGB/K adjustment, this time dragging the selection point to the right.

This is what my luminance image looks like:

The nebula is much brighter now and the stars have not been brightened at the same time.

Ensure your luminance image is called L_HT and continue on the main workflow.

Conclusion

In this article, you learned how to enhance the fainter areas of an image.

More Articles In This Series

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