Hi Lena,

I think you forgot to attach the picture. Would you mind sending it again?

The Huygens PSF is going to give the PSF of one field point. The software can then make the sum of different field points. So if the field points are defined at the slit, then maybe the Image Simulation would be a good tool. You can define a rectangular image representing the slit at the object. The software can then calculate a grid of Huygens PSF and make the correlation with your image. Have a look at that article that explains how it works: How to simulate high resolution images

I don't think POP would be needed if your source is just ambient light. POP treats the beam as an array of electric fields and considers diffraction to propagate the beam. POP is only needed in very specific instances when traditional ray-based geometric analysis or wavefront-based diffraction analysis are not valid. There is an article introducing POP here:Exploring Physical Optics Propagation in OpticStudio

If you can share your file (File...save as...zar), please open a case and we can have a look.

Thank you.

Sandrine