Hi,
That's very easy. You can use DynamicPSF. Use it first on the linear image; once you've set the measurement stars, create a process icon. Then, stretch your image and double click on the icon, it will measure exactly the same stars.
DynamicPSF has also a button the calculates the average PSF of the selected stars (just below the star list, the second button from the right with a capital epsilon). You can retrieve the average psf before and after stretching and compare.
There are also automated scripts, but with this tool you're sure you're comparing exactly the same stars.
Best regards,
Vicent.
