What are the advantages and disadvantages of the different types of chop-nod operation?
At Keck, “chop-nod” mode is defined as using a secondary chop throw and telescope nod throw of equal amplitudes but of opposite directions, 180° apart. The result is that for each nod position one of the other chop beams is in the same xy location (within the accuracy of the telescope guiding/tracking system, typically 0.06″ radial RMS) on the detector. For LWS, if the chop-nod throw amplitude is greater that 5.2″ then the opposite chop beam will be off the detector (assuming your chop direction is aligned with detector rows; if you chop at a 45° angle relative to the detector “up” direction, you can stretch this to about 7 arcsec). It might seem that nodding 90° from the chop and keeping all four beams on the detector (defined as “quad-chopping”) would increase the signal-to-noise ratio (S/N). However, analysis shows that there is no gain in S/N (for the same elapsed time) from using “quad-chopping” versus using “chop-nod”. See proof. This is counter-intuitive since quad-chopping incr
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