Absolutely!
But, to make a mask of your image (in linear mode):
Right-click on the image and select "Duplicate".
Now, bring up the STF tool and press Ctrl+A to apply an autostretch to the duplicate image.
Bring up the HistogramTransformation Tool. Drag the little diagonal (apply button) from the STF tool to the base of the HT tool; it will now show you the STF curve.
On the STF tool, press the disable STF button and apply the HT to the duplicate image.
Next, either extract the luminance from the image or convert it to greyscale.
Once you've done that, go back to your original image, right-click and select Mask, then choose the luminance/greyscale image as the mask.
You will likely need to invert the mask, so press Ctrl+I to invert it to protect the stars (red areas are protected).
Now that the mask is in place, press Ctrl+K to hide/show it, you can run the MultiscaleMedianTransform tool and it will only apply to the background.
Let me know if this doesn't help.
H