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Fork Information

This document describes how this fork relates to its upstream project, the Tractor: what changed, why it exists, and how it is kept in sync. For general usage, see README.md.

Upstream

Fork Point

This fork currently branches from the upstream at:

Knowing the branch point makes divergence auditable and helps when rebasing or merging future upstream changes. (The upstream's default branch is main.)

Why This Fork Exists

This fork was created to support features and performance needed for the forced photometry performed as part of the NASA SPHEREx mission pipelines and associated IRSA tools. The code was developed as part of the pipeline development by the SPHEREx Science Data Center (SSDC) at Caltech/IPAC.

Upstreaming of some of the changes is under discussion; other changes are SPHEREx-specific code which will not be upstreamed, but because of its heritage in tractor with its GPLv2 license, cannot be held in the differently-licensed SPHEREx pipelines repositories.

What Differs From Upstream

  • pyproject.toml and requirements.txt: added dependencies for pip installation
  • tractor/lsqr_optimizer.py: memory optimized derivative handling
  • tractor/optimize.py: time optimized image addition; computed covariant errors
  • tractor/psf.py: numba optimized Lanczos shifting
  • tractor/spherex_utils.py: created new module for SPHEREx-specific utilities; added function to support arbitrary PSF oversampling when fitting shapes

Sync Policy

We import changes from the upstream into the fork periodically as part of preparation for major SPHEREx processing cycles.

For maintainers, keeping the upstream as a second remote makes syncing straightforward:

git remote add upstream https://github.com/dstndstn/tractor.git
git fetch upstream
git merge upstream/main        # or: git rebase upstream/main

License & Modifications Notice

Copyright for modifications in this fork:

Copyright (C) 2023-2026 California Institute of Technology

This fork remains licensed under the GNU General Public License, version 2, the same terms as the original (see LICENSE and COPYING).

Note that the upstream's LICENSE and COPYING files contain only the verbatim stock GPLv2 text - they do not carry the project's own copyright statement. The Tractor's actual copyright notice lives solely in the "authors & license" section of the upstream's README.md:

Copyright 2011-2023 Dustin Lang (Perimeter Institute) & David W. Hogg (NYU / Flatiron)

That section is therefore the canonical copyright notice that GPLv2 requires to be kept intact. This fork preserves it verbatim: the fork notice is prepended above the upstream's content in README.md precisely so that block is never edited, moved, or removed. (The license is stated only as "GPLv2", with no "or later" clause in the README, the per-file headers, or the license files, so this fork treats it as GPLv2 specifically.)

As required by the GPL, modified files carry a notice indicating that they were changed and when.

Contact

Questions about the SPHEREx pipelines should be directed to the IRSA help-desk (https://irsa.ipac.caltech.edu/docs/help_desk.html).