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10.2.2 Total Intensity while Chopping
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HAWC+ imaging observations may be performed using a symmetric nod-match-chop (NMC), which is a variation of the standard two-position chop with nod (C2N) mode described in the FORCAST section of this manual (for further details see the FORCAST Observing Modes document).
The chop is symmetric about the optical axis of the telescope with one of the two chop positions centered on the target. The nod throw is oriented 180° from the chop, i.e. anti-parallel, such that when the telescope nods, the source is located in the opposite chop position. A standard ABBA position sequence lasting a total of ~1- 2 minutes is used for these observations. Users have a choice of chop/nod angle with respect to the sky, which can be useful if extended flux exists in some directions but not others.
- If the source of interest has little extended flux then a user may wish to choose a chop throw smaller than the HAWC FOV. In this case the chop/nod subtraction results in two negative beams on either side of the positive beam, which is the sum of the source intensity in both nod. An example of an observation taken in this mode is presented in the left panel of Figure 29 (FORCAST).
- If significant extended flux exists then a chop throw larger than the HAWC FOV should be chosen up to a maximum throw of 10'. In this case no negative beams will appear in the image as they are always outside the array FOV.
- When choosing chop throws and angles one should carefully inspect other observations to avoid chopping into extended flux. The Herschel Space Observatory and WISE mission archives are good places to examine extended regions at HAWC+ wavelengths.
The C2N mode also requires small dithers in order to mitigate the effects of bad and missing detector pixels. The baseline HAWC+ dithering mode consists of a four-point dither at the corners of a square with size of ~1- 2 HAWC+ beams. Therefore, the minimum time for a single C2N observation with dithering is ~5 minutes.