Cycle 5 HAWC+ in SSpot

Introduction

HAWC+ is a 2nd generation instrument that will be commissioned in 2016, during Cycle 4. HAWC+ specific instructions and reminders of general issues are given in the following topics below. Note that all menu selections and selectable options are boldfaced , field names appear as in SSpot, and text to enter into SSpot is displayed in Courier New . The brief instructions below will be expanded to include details after the observing modes have been characterized in-flight. Before preparing detailed HAWC+ observations in SSpot, please read the HAWC+ section of the Observer's Handbook .

HAWC+ observing modes are covered in three Astronomical Observation Templates (AOTs) in SSpot, HAWC PLUS Total Intensity,  HAWC PLUS Polarization, and HAWC PLUS OTFMAP, which correspond to standard imaging with two position nod-match-chop, polarization with two position nod-match-chop, and scan mode imaging respectively.

In the main panels for each AOT, the PassBands correspond to filters with the characteristics given in Table 10-1 . The mean wavelengths are as follows:

HAW_A: 53 μm
HAW_C: 89 μm
HAW_D: 154 μm
HAW_E: 214 μm

The Astronomical Observation Requests (AORs) should be created as described in the SSpot User Guide . The most critical information we require at this time are accurate target coordinates and any desired chop angles to avoid areas of extended emission (as seen in Herschel images, for example).

Many of the fields in the AOTs are self-explanatory, and many general instrument-independent parameters are covered here in the FORCAST page for SSpot. Please read through the brief descriptions below and consult with your support scientist for help with filling out the remaining fields in the AORs.  There are many of the AOR parameters that are kept as constants to simplify observations with HAWC+.  These can be changed by the support scientist after discussing the observations, but the defaults either in SSPOT or given in the sections below are the values that are preferred.

If you have any questions, please contact the SOFIA help-desk .

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HAWC+ Available Field of View

For polarimetry observations, the current configuration of HAWC+ lacks a second T polarization state array; as such, the field of view is reduced to approximately half in the detector's X direction (the first element of the Field of View in Table 10-1). Total intensity observations are unaffected and can use the whole field of view via the R polarization state. To state it clearly, these are the available field of views for both TOTAL_INTENSITY and POLARIZATION observations.

The images below are included to illustrate and underscore this difference. In both, the HAW_A, HAW_C, HAW_D, and HAW_E FOV's are shown by the pale blue, green, orange, and red boxes respectively.

The above image shows the available FOV in TOTAL_INTENSITY observations using a 400" chop throw at 0 degrees in a typical C2N/NMC observation.

The above image shows the available FOV in POLARIZATION observations using a 400" chop throw at 0 degrees in a typical C2N/NMC observation.

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Observing Condition & Acquisition / Tracking

To better assist in our observations of your target, please fill out the relevant details in the Observing Condition & Acquisition / Tracking window.

Of particular importance and interest are the final 6 parameters in that window for tracking purposes; please choose representative values for your target when possible. The visible wavelength will help us quickly assess whether the source itself can be used as a tracking source for the SOFIA visible tracking cameras of if we should plan for an alternate tracking star/source.

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Total Intensity Imaging - NMC/C2N

Overall description: four dither points 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. As commissioning of HAWC+ progresses, this sequence will be refined and may include additional calibration observations during the sequence described.

Real World Example

The image below shows a source being dithered between 4 positions. The green crosshairs give the position of one of the sources and is at the same physical location for each image, showing the image movement.

An aside on NMC/C2N overheads with HAWC+

NMC/C2N observing modes, either with or without polarization 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. Estimated overheads before first flights were purposefully large (factors of 10!) and these large overhead factors remain in SSPOT 3.3.0.1. Reduced overheads based on flight data analysis are becoming available and are near factors of 3 or 3.5; the support scientist(s) for HAWC+ in Cycle 5 will be using better estimates of overheads for observational planning as they become available.

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Polarization - NMC/C2N

Overall description: four dither points 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 separated from each other. After each ABBA nod sequence, 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. As commissioning of HAWC+ progresses, this sequence will be refined and may include additional calibration observations during the sequence described.

Real World Example

The plots below show 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 (4 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 is highlighted, and the half-wave-plate angles are highlighted as well.

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Scanning - OTFMAP - Box (a.k.a. raster, waffle)

Overall description: 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 a square 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. As commissioning of HAWC+ progresses, this sequence will be refined and may include additional calibration observations during the sequence described.

Real World Example

The following images 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.

In the above image, the scan starts at the bottom right, pauses midway through to obtain an estimate of tracking performance (the orange dot on the middle right) and then proceeds until the end.

In the above image, the scan starts at the upper right, pauses midway through to obtain an estimate of tracking performance (the orange dot on the middle of the top) and then proceeds until the end. The two directions are then combined to obtain the image below.

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Scanning - OTFMAP - Lissajous

Overall description: 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. As commissioning of HAWC+ progresses, this sequence will be refined and may include additional calibration observations during the sequence described.

Real World Example

The following images 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.