Correlated Errors in the COBE DMR Sky Maps

Abstract

The {\it COBE} DMR sky maps contain low-level correlated noise. We obtain estimates of the amplitude and pattern of the correlated noise from three techniques: angular averages of the covariance matrix, Monte Carlo simulations of two-point correlation functions, and direct analysis of the DMR maps. The results from the three methods are mutually consistent. The noise covariance matrix of a DMR sky map is diagonal to an accuracy of better than 1\%. For a given sky pixel, the dominant noise covariance occurs with the ring of pixels at an angular separation of $60 \deg$ due to the $60 \deg$ separation of the DMR horns. The mean covariance at $60 \deg$ is $0.45\% ^{+0.18}_{-0.14}$ of the mean variance. Additionally, the variance in a given pixel is $0.7\%$ greater than would be expected from a single beam experiment with the same noise properties. Auto-correlation functions suffer from a $\sim 1.5\; \sigma$ positive bias at $60 \deg$ while cross-correlations have no bias. Published {\it COBE} DMR results are not significantly affected by correlated noise. COBE pre-print 94-