The SPHEREx Science Data Center (SSDC) has discovered an error in the header of the PSF layer extension in the SPHEREx Spectral Image FITS files. The PSF extension contains a cube, with each plane representing the PSF for a different zone. The zones correspond to an 11×11 grid across the detector. The PSF cube planes are stored in order of increasing x-index across the grid, but the header incorrectly indicates increasing y-index (see below). As a result, software that relies on the header information to select a PSF for a given location on the detector may retrieve the wrong PSF cube plane. The magnitude of this error depends both on the source location and the variation of the PSF across the detector. The error is largest in two of the corners and can result in a photometry error of +/-5% as compared to using the correct PSF.

The issue is illustrated below with two small 4x4 grids (16 zones). The number in each cell of the grid represents the cube plane (i_plane) corresponding to that zone. Note that the actual SPHEREx PSF cubes are computed for an 11x11 grid (121 zones).

13 14 15 16 9 10 11 12 5 6 7 8 1 2 3 4

In the actual PSF cube, the FITS plane number, from 1 to 121, corresponding to a source in the zone at zero-indexed grid coordinates (x,y) is iplane = y × ncols + x + 1 where ncols is the number of columns in the grid. The origin of the grid coordinates (x,y) = (0,0) is in the bottom left corner of each detector, such that the (0,0) zone contains the FITS origin pixel (1,1).

4 8 12 16 3 7 11 15 2 6 10 14 1 5 9 13

The incorrect PSF headers indicate that iplane = x × ncols + y + 1

The SSDC is working with IRSA to correct the SPHEREx data files and will release updated Spectral Images and calibration files no later than April 3, 2026. See below for specific impacts and mitigations.

Impacts for users

If you only use the IMAGE (flux), FLAGS, VARIANCE, and ZODI extensions for image analysis such as aperture photometry, there are no impacts as these extensions are correct.

If you use the Spectrophotometry Tool hosted at IRSA, this tool currently suffers from the same issue and an updated version will be available with the corrected data release.

If you use the PSF extension from the Spectral Image files, the Python notebook "Understanding and Extracting the PSF Extension in a SPHEREx Cutout" will be updated to:

• Check the pipeline version to determine if the PSF header needs to be updated.
• Provide a function to update the header.

This notebook will be available by March 19.