10.2 Planning HAWC+ Observations

10.2.1 Total Intensity Scan Mapping

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).

Example scan patterns for HAWC+ OTFMAP mode

Figure 10-4: Example scan patterns for HAWC+ OTFMAP mode. These patterns show the location of the central array pixel, which moves along the paths at a user-defined rate. The upper panels are Lissajous patterns. The top-left panel is shortly after starting an integration, while the top-right panel is after a longer time period. The lower-left panel shows a series of linear scans used to cover a larger region. The lower-right panel also shows the required cross-scan in the case of linearly scanned areas. Plots taken from Kovács (2008).

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.

10.2.2 Total Intensity while Chopping

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.

10.2.3 Polarization while Chopping

For Cycle 5, HAWC+ polarization observations may only be performed using the C2N (NMC) observing mode (see Section 10.2.2 above). In this mode, four standard C2N (NMC) observations are taken, one at each of four angles of the HWP (relative angles 0, 22.5, 45, and 67.5 degrees). This is followed by dithering, where the HWP cycle is repeated again for a total of four dither positions. We currently estimate an additional overhead of 90% efficiency associated with moving the HWP between positions. This has been incorporated into polarization sensitivities in Figure 10-3. The minimum time for a single polarization C2N observation with dithering is ~20 minutes.

As in the case of TOTAL INTENSITY C2N, one must take care to avoid chopping into regions of bright, extended flux. Additionally one must consider the polarization state of that reference flux, in both percent polarization and angle. Typically, neither of these values will be known for HAWC+ observations (although users may want to consult the latest Planck data release). This "polarized reference" beam will produce additional systematic uncertainties in the data. In the case where the source and reference beam have the same polarization level, the systematic polarization uncertainty is linearly proportional to the reference-to-source intensity ratio. For further discussion, see Schleuning et al. (1997) and Novak et al. (1997).

10.2.4 Mosaic Maps

If the source has an angular extent larger than the HAWC+ FOV in C2N mode, or larger than can be accommodated in a 10 minute OTFMAP, the central position of each HAWC+ field must be specified, with due consideration of the desired overlap of the individual frames. For mosaic observations, proposers should ensure that they request the total integration time required for all fields.