Hi Rob,
I have downloaded and checked your project, thank you for uploading it.
There is no bug
![smile :)](http://pixinsight.com/forum/Smileys/default/smile.gif)
What you're getting with PixelMath is correct and the expected result. You are executing PixelMath in the context of the current view, which is "debayer_masterbias_50d_20110529_80f_iso1600_3_125C_Preview01" because the rest of image windows in the current workspace are shaded ('shaded' is equivalent to 'hidden' in this context). Consequently, PixelMath automatically interpolates the images being operated (subtracted in this case) to match the geometry of its target image (336x280 px).
What you want to do requires executing PixelMath either on one of the large images, or much better, in the global context. To run this PixelMath instance globally, specify the following in the Destination section of the PixelMath tool:
- Create new image: enabled
- Image width: 4770
- Image height: 3177
- RGB Color: selected
- Sample format: 32-bit floating point
Now you can run PixelMath globally, that is independently on any image, by clicking the Apply Global button or pressing F6.