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10.2.1 Total Intensity Scan Mapping
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HAWC+ imaging observations may be performed in on-the-fly (OTF) mode, sometimes referred to as scan-mapping. In this mode the secondary mirror remains stationary on the optical axis of the telescope while the telescope assembly itself slowly moves with respect to the sky. This scan motion modulates the celestial source with respect to the atmosphere in a manner similar to chopping the secondary mirror. Scan rates must reach (~2 Hz) x (HAWC beam width) in order to remove the source from the atmospheric background. This implies rates ~10 - 80 arcseconds per second depending on the band-pass.
In order to ensure absolute flux calibration in this mode, observers must carefully plan observations so that some of the mapped region contains no extended flux. Otherwise, one can only measure a differential flux with respect to the lowest measured intensity level. Further removal of residual atmospheric signal is performed by removing common-mode noise observed in all HAWC+ detectors. This averaging amounts to a spatial filter with size equal to the HAWC FOV. Therefore, while large maps may be necessary to reach a true zero-intensity level, users should be aware that one cannot also recover all spatial scales in a given region.
HAWC+ will offer two types of OTFMAP scan patterns (Figure 10-4). Lissajous scans are small and meant for sources smaller than the HAWC+ FOV while Linear Cross scans are more efficient at mapping large areas several times the FOV. The patterns in Figure 10-4 show the two-dimensional location of the array center where movement along any curve is movement in the time dimension. Two-dimensional scans are necessary in order to reconstruct all spatial scales in a map. The Lissajous scans are 2D by definition. However, linear scans require multiple scans, even in the case where a source fits completely in the HAWC+ FOV. The secondary (or cross) scan direction should by rotated with respect to the initial scan (while orthogonal scans are best, they are not absolutely necessary).
While proposers must request an area for scan mapping, they do not need to specify any specific pattern in Phase I proposals. Successful proposers will work with a SOFIA Support Scientist to choose an optimal scan pattern and strategy for their observations. For the purposes of the proposal, scanmap time estimates should be made using the sensitivity estimates in Table 10-1. For sources smaller than the HAWC FOV use the MDCF or NESB. For larger maps one may use the Mapping Speed.
In order to avoid inefficiencies such as computer crashes and the like, we do not recommend scan durations longer than 10 minutes. If a given map area and sensitivity cannot be achieved in that time, then multiple pointing positions should be used. This is an estimate based on previous ground-based experience and may differ once HAWC+ is commissioned on SOFIA.