Consequences of constant elevation scans for instrumental systematics in
  Cosmic Microwave Background Experiments
  Instrumental systematics need to be controlled to high precision for upcoming
Cosmic Microwave Background (CMB) experiments. The level of contamination
caused by these systematics is often linked to the scan strategy, and scan
strategies for satellite experiments can significantly mitigate these
systematics. However, no detailed study has been performed for ground-based
experiments. Here we show that under the assumption of constant elevation scans
(CESs), the ability of the scan strategy to mitigate these systematics is
strongly limited, irrespective of the detailed structure of the scan strategy.
We calculate typical values and maps of the quantities coupling the scan to the
systematics, and show how these quantities vary with the choice of observing
elevations. These values and maps can be used to calculate and forecast the
magnitude of different instrumental systematics without requiring detailed scan
strategy simulations. As a reference point, we show that inclusion of even a
single boresight rotation angle significantly improves over sky rotation alone
for mitigating these systematics. A standard metric for evaluating
cross-linking is related to one of the parameters studied in this work, so a
corollary of our work is that the cross-linking will suffer from the same CES
limitations and therefore upcoming CMB surveys will unavoidably have poorly
cross-linked regions if they use CESs, regardless of detailed scheduling
choices. Our results are also relevant for non-CMB surveys that perform
constant elevation scans and may have scan-coupled systematics, such as
intensity mapping surveys.
