beam, calibration, sample variance and instrumental noise.

Null tests {#sec:jackknife}
==========

We use null tests to search for systematic errors in the data. In each null test, we divide the set of maps into two halves and difference the halves to remove true astrophysical signals. We then apply the cross spectrum analysis described in the last section to the differenced maps, which should result in a spectrum consistent with zero.

In the limit of perfect knowledge of the instrument, any remaining signals should not be of astrophysical origin; the residual astrophysical signal in simulated null spectra is much smaller than the uncertainties. The various ways in which the data are split in half are chosen to maximize the sensitivity to likely sources of systematic errors. We compare the resulting spectra to the expectation value of zero, with a significant deviation from zero implying the presence of a systematic error. We look for systematic effects using the following data splits:

-   Scan direction: We difference data from left-going and right-going scans (or in a minority of cases, upward and downward-going). This tests for effects sensitive to telescope velocity or acceleration. Examples include errors in the estimated detector time constants or microphonic 