Adding noise

Thermal noise

noise_thermal, t_amb, and tau0 allow you to add thermal noise from the atmosphere and from the ALMA receivers according to current ALMA specifications according to observing band. This will be expanded for greater flexibility and other observatories soon.

Phase noise

simmos does not have its own atmospheric phase screen. Rob Reid wrote a python script to add phase noise, noisify_alma.py. noisify_alma gets its parameters from the ALMA specifications where it can, but our knowledge of the atmosphere is still not complete.

note that noisify_alma.py also does thermal noise. don't add twice as much as you intended.

Phase noise refers to the phase difference between antennas due to varying delays caused by the atmosphere. It is relatively simple to produce uniform phase noise (no correlation between antennas), but in reality the average size of the phase difference between antennas is expected to increase with baseline length. How exactly it behaves is not quite known, but even when using uniform phase noise you should consider what sort of weather your "observation" would receive with dynamic scheduling.

WARNING: the way noisify_alma generates Kolmogorov phase noise is to draw a set of phases which is consistent with Kolmogorov turbulence. Unfortunately that is not the same as preserving the expected behavior of phase difference vs. baseline length, and noisify_alma's Kolmogorov phases are currently a factor of ~2 too small. I am working on a different way of generating them, using a power spectrum.
Topic revision: r2 - 2008-11-26, RemyIndebetouw
 

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