ArcsinhStretch is a unique process in PixInsight because you can only use it with RGB data and it focuses on preserving color information.

You have to do a couple of things before using ArcsinhStretch:

  1. Set the RGBWorkingSpace so that all channels are equal
  2. Reset the STF (ScreenTransferFunction)

Setting the RGBWorkingSpace

Following the steps in this section won’t make a difference to how your image is displayed; however, it does affect PixInsight’s calculations. This is why you might consider running BackgroundNeutralization and ColorCalibration before using this process.

Do the following to use this process:

  1. Ensure you have an RGB image open on your PixInsight desktop and start the process by selecting Process – ColorSpaces – RGBWorkingSpace or locate it under <All Processes> in the Process menu
  2. If not already selected, select the RGB image at the top of the RGBWorkingSpace window:
  • Set the sliders for each of the red, blue, and green to 1.0
  • Drag the New Instance icon (the icon on the far left at the bottom of the RGBWorkingSpace window) to your RGB image to execute the process

Resetting the STF

If the STF is active when using ArcsinhStretch, it will affect PixInsight’s calculations, so be sure to reset it.

If it is not already open, start the ScreenTransferFunction process by selecting from the menu Process – IntensityTransformations – ScreenTransferFunction or locate it under <All Processes> in the Process menu.

Do the following to reset the STF:

  1. Ensure that the Track View option is enabled (the checkmark icon at the bottom-right of the STF window)
  2. Click your image to make it the active image
  3.  Click the Reset button at the bottom-right of the STF window

Using ArcsinhStretch

1. Start the ArcsinhStretch process by selecting from the menu Process – IntensityTransformations – ArcsinhStretch or locate it under <All Processes> in the Process menu.

2. ArcsinhStretch has a real-time preview (RTP), so click it to enable the RTP (the circle icon at the bottom-left of the ArcsinhStretch window).

3. Once the RTP is open, the button labeled ‘Estimate Black Point’ becomes active and you can click it to have the process estimate your image’s black point.

You may see some colors show up on your image when you click the button – these are areas where the black level is clipped (set to zero), as shown:

You can use this information about clipped pixels to adjust your image later.

4. Move the slider for the Stretch Factor to the right until you start to see the color in your image and adjust to taste. Note that you may have to apply ArcsinhStretch in a couple of iterations to get the desired result.

5. Adjust the upper or lower slider for the Black Point until the highlighted areas disappear (the upper slider is the coarse slider and the lower slider is the fine slider)

6. Once satisfied, close the RTP and drag the New Instance icon (the icon at the lower-left of the ArcsinhStretch window) to your image to execute the stretch

Rather than performing one large adjustment, try applying a couple of smaller adjustments. In case you need more of a stretch than is available on the Stretch Factor slider, use another adjustment to help get you there.

Also note that your results may vary a little between the RTP and your final stretch due to the way PixInsight calculates the RTP view.

Conclusion

In this article, you learned about ArcsinhStretch and learned how to use it effectively.

More Articles In This Series

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