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7.2 Planning Observations
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The HAWC instrument has two main observing configurations: Total Intensity Imaging and Imaging Polarization. The instrument also has two standard observing modes for imaging: the Nod Match Chop (NMC) mode combines traditional chopping with nodding and On the Fly Mapping (OTFMAP) mode keeps the secondary mirror fixed as the telescope primary is scanned across the source. The NMC observing mode is used for polarization observations; this mode includes chopping and nodding cycles in multiple half wave plate (HWP) positions.
The standard NMC mode is a subset of the standard two-position chopping with nodding mode (C2N). NMC consistis of several steps, listed below and illustrated in Figure 7-5.
- Chop, where the secondary mirror of the telescope is moved at some frequency and angle.
- Nod, where the telescope nods back and forth, each chopper beam being placed on the desired source.
- Dither, which is a set of Nods at small offsets on the sky (nominally four positions), with each position having a Chop/Nod observation taken.
Total Intensity observations with OTFMAP mode produce a continuous telescope motion with a two choices of pattern shape, the sizes of which are selected by the proposer. The first pattern (Box) is a series of linear scans used to map some rectangular region on the sky. The second pattern (Lissajous) is a curvilinear shape meant to cover a small region whose size is less than the array field of view.
18.104.22.168 On the Fly Mapping
In OTF 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 bandpass.
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 emission from the science target. 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+ offers two scan types for OTFMAP scan patterns: Lissajous and Box. Lissajous scans are recommended for soucres smaller than the HAWC+ field of view (FOV) at a given bandpass, while Box scans (analogous to traditional raster scanning methods) are more efficient at mapping large areas several times the FOV. The patterns in Figure 7-6 show the two-dimensional location of the array center during the progression of a scan, with Lissajous scans depicted in the two top images and Box scans shown in the two bottom images. Two-dimensional scans are necessary in order to reconstruct all spatial scales in a map. The Lissajous scans are two-dimensional by definition, however Box scans require multiple scans, even in the case where a source fits completely in the HAWC+ FOV. The secondary (or cross) scan direction of a Box scan should by rotated with respect to the initial scan (orthogonal scans are best, although 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, scan map time estimates should be made using the sensitivity estimates in Table 7-1. For sources smaller than the HAWC+ FOV, use the MDCF or NESB. For larger maps one may use the Mapping Speed.
Scan durations shorter than 10 min are recommended to ensure the stability of continuous OTFMAP observations for large periods of time. If a given map area and sensitivity cannot be achieved in that time, then multiple pointing positions should be used.
In Lissajous observations, the telescope is driven to follow a parametric curve at a non-repeating period; as the scan progresses longer in time, more and more of the area defined by a scan amplitude will be covered.
Figure 7-7 demonstrates the actual scan modes used in flight. The white box shows the Total Intensity FOV, the orange line shows the actual path as taken by the telescope, and the background images are the resulting image after the scan data is reduced.
In Box scans, the telescope is driven in a linear fashion at the specified rate in one direction for the given length and then moved perpendicularly before scanning in the reverse direction—similar to how one would mow a very large lawn. This is continued until the desired area is covered, after which the process repeats in the perpendicular direction to cross the same areas in the perpendicular direction. For optimal reduction and coverage, three scans are performed, each at a slightly different starting angle to improve coverage and provide reduction robustness against systematic effects.
Figures 7-8, 7-9, and 7-10 demonstrate the actual scan modes used in flight. The white box shows the Total Intensity field of view, the orange line shows the actual path as taken by the telescope, and the background images are the resulting image after the scan data is reduced.
22.214.171.124 Nod Match Chop
In NMC mode, four dither points are created symmetrically about the central object coordinates; an example is given in Figure 7-11. At each dither position, chopping is started at the given throw and angle and the telescope is nodded between two symmetrically located points 180 degrees seperated from each other (positions A and B).
NMC mode observations, are very time intensive and are subject to large observational overheads waiting for the telescope and/or secondary mirror assembly to complete chop/nod/dither movements. For total intensity observations, OTFMAP is strongly encouraged. This mode provides better sensitivity and lower overheads, which provides better overall observing efficiency.
If the source has an angular extent larger than the HAWC+ FOV in NMC 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.
7.2.2 Polarimetry Observations
For Cycle 8, HAWC+ polarization observations may only be performed using the NMC observing mode. In this mode, four standard NMC observations are performed, 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 Table 7-1. The minimum time for a single polarization NMC observation with dithering is ~20 min.
As in the case of NMC observations, chopping into regions of bright, extended flux must be cafefully avoided. Additionally, the polarization state of that reference flux must be considered in both percent polarization and angle. Typically, neither of these values will be known for HAWC+ observations (although proposers 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).
If the source has an angular extent larger than the HAWC+ FOV in NMC mode, 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.
126.96.36.199 Nod Match Chop
In NMC mode, a nominal four dither points (with a maximum of 9-point dither points) with a three detector pixels offset are created symmetrically about the central object coordinates. At each dither position, chopping is started at the given throw and angle and the telescope is nodded between two symmetrically located points 180 degrees seperated from each other (positions A and B). Observations are performed in sequence at position A, then B, then B again, and back to position A. After one of these ABBA nod sequences, the HWP is rotated to the next angle; this continues until the HWP has gone through four angles, after which the telescope moves to the next dither position and repeats.
Figure 7-12 below shows both the change in source RA (illustrating the nodding of the source) and the half-wave plate angle for a single dither position of a polarization sequence. The dashed lines denote the completion of a half-wave plate observing sequence. This same sequence (four half-wave plate angles) is repeated for each dither position to move the source appreciably and assist in the correction of bad/missing pixels. The source is being chopped during this entire sequence (and accounts for the thickness of the blue source RA line) but is not specifically highlighted here. The standard ABBA nod sequence and the half-wave-plate angles are highlighted.
NMC mode observations, are very time intensive and are subject to large observational overheads waiting for the telescope and/or secondary mirror assembly to complete chop/nod/dither movements. These overhead factors are included in USPOT after the on-source time estimation in SITE.
Note: OTFMAP in polarimetry mode will be offered as shared-risk during Cycle 8. This observing mode is under comissioning during Cycle 7.