Stokes Q image spectrally degraded with Gaussian PSFs and random noise. From left to right, the spectral degradation increases as a function of the FWHM of the Gaussian, from Δλ = 25 mÅ to Δλ = 100 mÅ. From top to bottom, the integration time is ∞, 16 s, 4 s and 1 s, respectively. The images are scaled to  ± 0.15% relative to the continuum intensity. For a fixed exposure time the noise in each image increases with decreasing width of the instrumental profile.