Don't allow picket fence obs to be calibrated.

hyperdrive detects picket fences, but despite claiming that it'll only
use the first "fence", it actually just crashes. Rather than attempting
to calibrate these observations, just abort instead.

Information has been added to the online documentation.

CHANGELOG.md

@@ -13,6 +13,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   `solutions-apply`.
 
 ### Fixed
+- Picket fence observations were previously detected, but not still handled
+  correctly. Until they are fixed, hyperdrive will not attempt to do anything
+  with a picket fence observation. A doc page has been added for picket fences.
 - `solutions-apply` would incorrectly apply solutions when flagged tiles were
   present. Thorough testing suggests that this is no longer the case and cannot
   happen again.

mdbook/src/SUMMARY.md

@@ -42,6 +42,7 @@
   - [Dead dipoles](defs/mwa/dead_dipoles.md)
   - [mwaf flag files](defs/mwa/mwaf.md)
   - [Raw data corrections](defs/mwa/corrections.md)
+  - [Picket fence obs](defs/mwa/picket_fence.md)
   - [mwalib](defs/mwa/mwalib.md)
 - [Sky-model source lists](defs/source_lists.md)
   - [Flux-density types](defs/fd_types.md)

mdbook/src/defs/mwa/picket_fence.md

@@ -0,0 +1,25 @@
+# Picket fence observations
+
+A "picket fence" observation contains more than one "spectral window" (or
+"SPW"). That is, not all the frequency channels in an observation are
+continuous; there's at least one gap somewhere.
+
+`hyperdrive` does not currently support picket fence observations, but it will
+eventually support them properly. *However*, it is possible to calibrate a
+single SPW of a picket fence obs. with `hyperdrive`; e.g. MWA observation
+[1329828184](https://ws.mwatelescope.org/observation/obs/?obs_id=1329828184) has
+12 SPWs. If all 24 raw data files are given to `hyperdrive`, it will refuse to
+interact with the data. But, if you supply one of the SPWs, e.g. coarse channels
+62 and 63, `hyperdrive` will calibrate and provide solutions for the provided
+channels, i.e.
+
+```shell
+hyperdrive di-calibrate \
+    -d *ch{062,063}*.fits *.metafits \
+    -s srclist_pumav3_EoR0aegean_EoR1pietro+ForA_phase1+2_TGSSgalactic.txt \
+    -n 100 \
+    -o hyp_sols.fits
+```
+
+For this example, the output contains solutions for 256 channels, and only one
+channel did not converge.

src/cli/di_calibrate/params.rs

@@ -813,7 +813,10 @@ impl DiCalParams {
                 if f.chanblocks.is_empty() {
                     return Err(DiCalArgsError::NoChannels);
                 }
-                warn!("\"Picket fence\" data detected. Only the first contiguous band will be used as this is not well supported right now.");
+                // TODO: Allow picket fence.
+                eprintln!("\"Picket fence\" data detected. hyperdrive does not support this right now -- exiting.");
+                eprintln!("See for more info: https://MWATelescope.github.io/mwa_hyperdrive/defs/mwa/picket_fence.html");
+                std::process::exit(1);
             }
         }
         let fences = Vec1::try_from_vec(fences).map_err(|_| DiCalArgsError::NoChannels)?;