Add resumable all-sky 2MASS acquisition

assets/2mass/processed/README.md

@@ -13,11 +13,14 @@ the source-sky direction.
 
 ## Quality selection
 
-All three J/H/Ks magnitudes must be finite and every character of `rd_flg` must
-be `1`, `2`, or `3`.  These are the 2MASS default-magnitude origins that
-generally indicate the best detections, photometry, and astrometry.  Thus the
-script rejects nondetections/upper limits (`0`), poor aperture photometry (`4`),
-inconsistent band deblends (`6`), and missing brightness estimates (`9`).
+RA/Dec must be finite and inside their ICRS ranges; all three J/H/Ks magnitudes
+must be finite; and every character of `rd_flg` must be `1`, `2`, or `3`.
+These are the 2MASS default-magnitude origins that generally indicate the best
+detections, photometry, and astrometry.  Thus the script rejects
+nondetections/upper limits (`0`), poor aperture photometry (`4`), inconsistent
+band deblends (`6`), and missing brightness estimates (`9`).
+The IPAC literal `null` is treated as a missing numeric value, the same as a
+blank field, and is therefore rejected by these finite-value checks.
 
 ## Two-parameter fit