I have written before about how to boost areas of faint signal; however, that process is rather involved because you have to create and use a mask using the time and energy-consuming Starnet, StarNet2, or StarXTerminator process to create the mask, stretch it, and then use the mask to boost the faint parts of your image. The results may vary based on how you create your mask.

Two methods can produce similar results to using a traditional mask with less effort and time: the STF and ArcsinhStretch processes and the GHS process. The method you choose will depend on the characteristics of your image and the desired outcome. Still, the STF and ArcsinhStretch process is generally the simpler option requiring less effort, while the GHS process may produce better results for certain subjects.

Sample Download

I’ve made a sample observation of the unstretched luminance image of SH2-240, the Spaghetti Nebula, available for download (9.5 Mb). The image is derived from a narrowband image of SH2-240 that was processed using the RGB color palette and then DBE and SPCC were used on the resulting image before extracting the luminance.

Result of Using Only the STF to Stretch an Image

This is the luminance image of SH2-240 that’s been stretched using only the STF:

The nebula itself is barely visible due to a lack of contrast; this is understandable because I’ve only transferred the default STF to the image using HistogramTransformation as shown in the screenshot below (HistogramTranformation on the right and STF on the left):

The default STF is not optimal for this image, but it’s a good start.

Result of Using the STF and ArcsinhStretch Processes to Boost Areas of Faint Signal

The following image was stretched as above using the STF and then I used the ArcsinhStretch to bring out the nebula’s details:

The increased contrast in the image makes the nebula more apparent, and the darker background serves to highlight its features and enhance its visibility.

Using the ArcsinhStretch process to stretch an image like this is simple: open the real-time preview and click the Estimate Black Point button, and then adjust the Stretch factor parameter until you see some contrast appear in your image. This works for color images, too, and you don’t have to mask the image to get this result.

Another approach is to use multiple applications of the GeneralizedHyperbolicStretch (GHS) process, discussed next.

Result of Using the GHS Process to Boost Areas of Faint Signal

The following image was stretched using several iterations of the GHS process without a mask:

There’s a little less contrast in this image compared to the one created using ArcsinhStretch, but the result is still good, and I didn’t have to use a mask to bring out the details of the nebula.

Using the GHS Process to Improve Contrast

In this particular case, I used the GHS process as follows:

  1. Make the image you want to stretch the active image and ensure it is in a linear (unstretched) state
  2. Disable the STF by clicking the computer icon in the STF window or by pressing CTRL+F12 on your keyboard
  3. Start the GeneralizedHyperbolicStretch process and click the Zoom In button (it should say 250 in the box to the left of the magnifying glass icons)
  4. Carefully drag the scrollbar below the black histogram display until you see the bump in the histogram. The scrollbar moves the display quite a bit, and you only need to move the scrollbar a very small amount to see the bump in the histogram. This is what my screen looks like:



  5. Click within the histogram window at the point where the bump on the graph starts to rise from the x-axis; a yellow line will appear. This is what my screen looks like:




  6. Click the Send to SP button (if you don’t see the button on the screen, expand the process’s Readout Data section)
  7. Set the Local intensity parameter to around 10
  8. Click the Zoom 1:1 button to adjust the histogram window
  9. Click the real-time preview button (the open circle) to bring up the real-time preview
  10. Slowly drag the slider next to Stretch factor until the bump on the histogram reaches the first quarter; this is what my screen looks like:



    There’s very little contrast in the image at this point and we’ll correct that in the following steps.
  11. Close the real-time preview window and make the stretch permanent by clicking the Apply icon (the square), or by pressing F5 on your keyboard
  12. Reset the GHS process by clicking the reset button on the lower-right of the GHS process window
  13. Open the GHS real-time preview window
  14. Click an area of the nebula, or your object, on the real-time preview that’s hardly visible
  15. Click the Send to SP button
  16. Set the Local intensity to around 10
  17. Drag the Stretch factor until the point just before you see the histogram bifurcate

    This is what my screen looks like when the histogram bifurcates:



    The green arrow points to the point on the histogram where it is beginning to split into two distinctive peaks. To adjust the Stretch factor, drag the slider until the histogram barely starts to split into two distinctive peaks – try to find the point at which it starts to split.
  18. Close the real-time preview window and make the stretch permanent by clicking the Apply icon (the square), or by pressing F5 on your keyboard
  19. Repeat the steps from 12-18 and select another hardly visible region
  20. Improve the image’s contrast by making the background darker using the HistogramTransformation (HT) process. Open the HT process and select the real-time preview and click the button shown on the following screenshot:



    Check the real-time preview and evaluate the result; if the result is too dark, reset the HT process and drag the dark tones slider from the left of the histogram to the point where the bump on the histogram starts to rise from the zero line as shown in the following screenshot:


  21. Close the HT real-time preview and make the stretch permanent by clicking the Apply button or by pressing F5 on your keyboard

You now have your final image.

ArcsinhStretch or GneralizedHyperbolicStretch

Your approach will depend on your image’s subject and the contrast between it and the background. Both processes are relatively quick, so you may want to try both and compare the results, as demonstrated in this article. It is worth noting that the steps for these processes are easier to follow and perform than they may initially seem.

Conclusion

In this article, you were introduced to using the ArcsinhStretch and GeneralizedHyperbolicStretch processes as effective techniques for enhancing images, and have had the opportunity to compare the results of using both approaches. A noteworthy aspect of these methods is that they do not require a mask, making them convenient and easy to apply.