Add resumable all-sky 2MASS acquisition

assets/2mass/README.md

@@ -50,5 +50,29 @@ python3 scripts/process_2mass_psc.py \
 The resulting M44 CSV contains 7,802 sources after the documented three-band
 photometry and `rd_flg` selection.
 
+## Full-sky acquisition
+
+`scripts/download_2mass_psc_all_sky.py` provides a resumable full-sky
+acquisition without asking IRSA for a cone larger than 1 degree. Inspect its
+storage plan before starting the long download:
+
+```sh
+python3 scripts/download_2mass_psc_all_sky.py --plan
+python3 scripts/download_2mass_psc_all_sky.py --download --workers 2
+```
+
+It covers each latitude band with integer-RA cone sectors, wider at high
+latitudes, while outputs are cleaned and retained by unique 1-degree RA/Dec
+ownership cells. Cone overlap cannot create duplicate stars. Temporary raw
+tables and cleaning intermediates are stored under `/tmp/2mass_psc_all_sky/`
+and deleted after each successful cover. The generated full-sky catalogue has
+one CSV per ownership cell under `processed/all_sky/`, named
+`tile_raRRR_decDDD.csv`. `RRR` is
+`floor(RA_deg mod 360)` and `DDD` is `min(179, floor(Dec_deg + 90))`; the
+corresponding range is `[RRR, RRR+1) x [DDD-90, DDD-89)` degrees. It is
+intentionally not a single CSV because that would duplicate a roughly 15--17
+GiB result during merging.
+See that directory's README for restart and raw-retention behavior.
+
 Provenance: [IRSA Gator program interface](https://irsa.ipac.caltech.edu/docs/howto/gator_prog_interface.html),
 [2MASS PSC column descriptions](https://irsa.ipac.caltech.edu/2MASS/download/allsky/format_psc.html).