Merge pull request #231 from jkrick/add_text_spherex_psf

added text about PSF

Author: troyraen

tutorials/spherex/spherex_psf.md

@@ -271,6 +271,22 @@ ax1.imshow(psf)
 plt.show()
 ```
 
+## 9. Using the SPHEREx PSF in Forward Modeling (e.g., Tractor)
+
+The PSF returned by this notebook is oversampled relative to the native SPHEREx detector pixel grid. 
+This is intentional: the PSF is evaluated on a fine sub-pixel grid so that it can represent different intra-pixel source positions accurately.
+
+Tools such as Tractor do not expect an oversampled PSF directly. 
+Instead, they require a PSF that is pixel-integrated at the native detector resolution and evaluated at the correct sub-pixel phase of the source. 
+If you pass the oversampled PSF directly into Tractor without resampling, the effective PSF width and normalization will be incorrect, which can lead to systematic differences relative to the SPHEREx Spectrophotometry Tool.
+
+To use this PSF for forward modeling or fitting, you must:
+1. Shift the oversampled PSF to the source’s sub-pixel position,
+2. Downsample (integrate) it onto the native SPHEREx pixel grid, and
+3. Normalize the resulting PSF before passing it to Tractor.
+
++++
+
 ## Acknowledgements
 
 - [Caltech/IPAC-IRSA](https://irsa.ipac.caltech.edu/)