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5.3.2 Observing Mode: Asymmetric Chop and Fast Asymmetric Chop
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If the target's size or environment does not allow to use the Symmetric Chop Mode, one has to use the Asymmetric Chop Mode allowing larger chop throws at shorter wavelengths and is not creating symmetric off-positions around the source. The asymmetric chop keeps the on-beam on the optical axis. This results in an image unaffected by coma. Consequently, the off-beam is off-center by twice the amount compared to the symmetric chop with the same chop throw resulting in twice as much coma. But that is of no consequence as the off-beam should only see empty sky. The telescope is nodded to an off-position where the same chopped observation is executed to provide the residual background subtraction. Figure 5-4 illustrates this geometry. Note that this mode is similar to FORCAST's asymmetric chop-offset-nod (C2NC2) mode (see 5.3.1).
The total overhead in this mode is about 4.3N ton + 300 s, since the source is observed during 25% of the observation plus additional time for telescope moves and 300 s for the setup. This overhead estimate assumes that the on-source exposure time per map position ton is at least 15 s. For shorter values of ton, the Bright Object Mode should be used.