Hi Chao-Lin,
Regardless, this can be fixed by removing the same
ground template
from the 2 halves.
I do remove the same ground template from the two halves, and that's
exactly the reason for concern. This means that information from the
late half-scans goes into the early half-scan map, and vice versa.
Since the ground subtraction procedure mixes the signal between the two
subsets, it will introduce a ground-fixed anticorrelation even if there
was no correlation to start with. This can also be described as
filtering the sky differently in the two subsets, in a way that could
introduce jack failures. In our standard mapmaking we apply ground
subtraction separately to the left-going and right-going scans. There
are other reasons why we do this, but among those reasons it protects
the scan-direction jackknife test from this problem.
The safer thing would be to make separate ground templates for the two
halves. This would potentially make the jackknife maps less directly
comparable with our standard coadded maps, but that's OK. I can
compare them with jack0 coadds and sims using the same hacked code.
- Walt