ISIS data not aligning with the image

I’m trying to find the distinction between pixels that contain SPICE data and ones that contain NULL data along Jupiter’s polyline.

Ex framelet with a red line representing the boundary to search over and find it pixels have data:

I use the (x,y) pixel coordinates of each red dot to find the latitude, longitude, and slant distance of the corresponding pixel, and expect to see NULL data when the limb ends and camera pixels hit empty space, but when I run the campt program using the pysis library, every pixel along that red line has data. There is no NULL data to be found.

The question is, am I missing something simple? If not, do I continue to try and debug using pysis and ISIS, or switch gears entirely and just try to build a similar program with SpiceyPy and SPICE, and no ISIS at all.

It is hard to say if this error is due to the camera model or errors in the underlying SPICE data. There’s similar issues on the github repo but I have yet to see consistent enough offsets to say it’s the filter CCD offsets in the camera model. Looking more into this has been on my to do list for a while but I’m currently overwhelmed with lots of government code compliance work.