The Screen Transfer Function process in PixInsight stretches an image so you can see it on your screen. It’s also an important part of processing your images since it allows you to see your progress without affecting the image’s data. Changes you make with the STF are temporary and affect only the image you see on the screen.

Later in this article, I’ll show you how to make the STF a permanent part of your image.

For now, open an image (preferably an RGB image but a monochrome one is fine as well) and select the ScreenTransferFunction process from the Process menu to open it.

Click the Track View button as shown:

This button causes PixInsight to track the active process to the active image. So if you had another image and clicked to it instead, PixInsight would make the STF process window follow to the new image (note that the settings are unique for each image, so even though the STF process would follow the active image, its settings would be unique to the image).

Enable the STF by clicking the button shown:

Next click the Auto Stretch button to have PixInsight calculate the optimal stretch:

In my case, the Auto Stretch has produced a significant green cast, so I can unlink the channels by clicking this button:

Click the Auto Stretch button again to allow PixInsight to calculate the stretch of the individual channels:

You can modify the stretch of the individual colors by dragging the individual colors’ arrows as shown:

You can zoom in to your view of the channel’s color by clicking the Zoom In Mode button…

…and then clicking within the channel – each click zooms in more into the channel’s magnification.

Click the Edit STF Mode button (the arrow) to edit the individual channels:

You can zoom out again by clicking the Zoom 1:1 button:

The STF Scroll Mode button works on zoomed-in views of the channels, allowing you to scroll the view to the left and right as required.

The New Instance and Apply buttons are not applicable to the STF because we enabled the Track View option and changes made in the STF are visible immediately.

Making the STF Permanent

You can easily make the STF a permanent part of your image. Note, however, that what I’m about to show you makes your image non-linear. Generally, you make your image non-linear after applying various processes, including Image Integration, Star Alignment, noise reduction, and color balance, so keep this procedure in mind for when you have completed those processes. Here’s how to apply the STF to your image:

1. Adjust the STF so that your image appears to be as you want it to appear

2. Open the HistogramTransformation process from the Process menu

3. Drag the New Instance icon from the STF to the toolbar on the HistogramTransformation as shown

4. Click the STF Enabled button in the STF process window

5. Drag the New Instance icon from the HistogramTransformation window to your image

Your STF is now a permanent part of your image. If you export your image to some other format at this point, the image exported will be the same as shown on your screen.

Update

Conclusion

This article has covered the essentials of using PixInsight’s Screen Transfer Function (STF) to stretch and adjust astrophotography images. Additionally, it has guided how to make these adjustments permanent, ensuring that your final image looks exactly how you want it to present it.