Estimating the error in a spectrum T following stray radiation removal, using repeated observations in the NEP field. Open circles give the standard deviation of ΔT_ij within bins of 2 × 10⁵ data points as a function of T, using spectra from the three pairs of NEP observations (dark gray: 3−2, light gray: 1−3, black: 2−1). These lie slightly in excess of the prediction based on only line noise plus baseline errors (dashed line). Also shown is the mean of ΔT_ij for each bin for each ij combination. Both the excess and non-zero means arise from imperfect stray radiation removal. Crosses show a simple estimate of the total error which combines 6% of the rms spectrum of the stray radiation correction in quadrature with the line noise.