Here is the PixelMath end of this. I did not save a project of the previous data so I am having to do this all over again. Someone good with PixelMath would do what I did much more efficiently as I am terrible with PixelMath.
Anyway the sequence is first make a black image the same size as the original. This is the base for your circle mask.
Next clone your black image.
Next create the white circle using the inellipse function in PixelMath. If you open the expressions editor and go to the inellipse function you will see the parameter format for this function.
Last apply the circle mask to the black image clone and with PixelMath use the original image and copy to the black image.
The TGVInpaint part requires modifying PI with a new, not released TGV module. You can find out more about it here. https://pixinsight.com/forum/index.php?topic=8970.0
Here is a link showing one of it's uses. https://drive.google.com/file/d/0B13r3kYqTf8NcXQzVnJuVmlUeGs/view?usp=sharing
Now the RangeSelect mask take some real work to get things just right for this image. This is not a plug in some numbers and your there. I actually use the RangeSelect tool on the masks created by the RangeSelect tool. I also had to use Curves on the masks created by the RangeSelect tool.