What is meant by the terms “chop” and “nod” in radio astronomy with regard to taking observations?
Chopping and Nodding for Mid-Infrared Astronomy Kevin Volk December 2007 Observations in the mid-infrared (or equivalently the thermal infrared, so called be-cause at these wavelengths the thermal radiation from the atmosphere and the telescope are significant) are often done in chop/nod mode wherein one points the telescope at a position slightly off the target and moves the telescope field of rapidly between the source position and a sky position, usually by rocking the secondary mirror at a set frequency, and then offsets the telescope to the other side of the position to repeat the process now sampling the sky on the other side of the central position. This is illustrated in Figures 1 and 2 below. However please note that the angles over which chopping takes place are small: at Gemini the maximum chop amplitude is 15 arc-seconds, which is about the same as the separation of the Mizar A/B binary star pair in Ursa Major–a large enough separation to be seen by those with good eyesight, bu
