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5.3.1 Observing Mode: Symmetric Chop
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If possible this observing mode should be used, because it is the most efficient mode. This mode combines chopping symmetrically to the telescope's optical axis with a matched telescope nod to remove the residual telescope background. This mode is also known as nod-match-chop (NMC) mode (cf. FORCAST Sect. 7.2.1) or beam switching (BMSW, cf. GREAT Sect. 9.1.1).
When observing using a symmetric chop, large chop amplitudes degrade the image quality due to the introduction of coma. This effect causes asymmetric smearing of the PSF in the direction of the chop. However, the effect is small (effect on SNR less than 10%) in the red channel for all chop throws and in the blue channel for total chop throws less then 5' and wavelengths longer than 63 μm. For wavelengths shorter than 63 μm, we recommend total chop throws of less than 4'. Generally, it is recommended to use a chop as small as possible, but keep the FOV in the off-positions outside of any detectable emission.
The position angle of the chop can be specified relative to equatorial coordinates or telescope coordinates (e.g. horizontal). Keep in mind that the telescope nod matched to the chop creates two off-positions symmetric to the on-position (Figure 5-5, Left).
The total overhead in this mode is about 1.6N ton + 300 s, since the source is only observed during 50% of the observation and additional time is required for telescope moves, plus 300 s for the setup. This overhead estimate assumes that the on-source exposure time per map position ton is at least 30 s. If the on-source exposure time per map position ton is less than 15 s, the Bright Object Mode should be used. For values of ton in between, one needs to enter an alternate overhead in SPT. The total alternate overhead is N(ton + 20 s) + 300 s.