The Parallel Mode AOT is offered with only one observing mode: scanning. It is the only compatible mode for operating SPIRE and PACS simultaneously: in fact, scan mode is the only option for both SPIRE and PACS to map large areas of the sky, even when they are operated alone.

The scanning strategy has been optimized for both instruments. Scan maps are performed by slewing the spacecraft at a constant speed along parallel lines to cover a large area. The lines follow great circles on the sphere which approximates parallel lines over short distances. When finishing a scan leg, the spacecraft has to perform a turn maneuver and continue observing along the next scan leg in the opposite direction: the time required for this turn is about half a minute. Note that scan mapping does not make use of chopping, being the signal modulation provided by the spacecraft motion.

Two scan speeds are offered: the fast mode applies 60 arcseconds per second, the slow mode is performed at 20 arcseconds per second. Applying the fast mode (default value), PACS PSF is degraded due to on-board data averaging and detector memory effects.

The Parallel Mode AOT does not allow to do cross-scans in a single observation. However, it is strongly suggested to perform two scan maps of the same area, one with nominal coverage and the other with orthogonal coverage, in order to remove more efficiently the stripping effects due to the 1/f noise and to get better coverage redundancy, especially for SPIRE. For this purpose two AORs shall be concatenated in HSpot to be performed with a different scan direction. An example of the better coverage achievable using cross-scans is presented in Figure 2.8.

Figure 2.1.  PACS and SPIRE footprints overlayed on the Herschel focal plane image. The field-of-views of the two instruments are separated by 21 arcminutes respectively.

As the SPIRE arrays are not fully filled, the telescope scans in Parallel Mode are carried out at +42.4º ("Nominal" scan direction) or -42.4º ("Orthogonal" scan direction) with respect to short axis of the arrays (angle from instrument +Z-axis to +Y-axis, see Figure 2.1). In order to provide homogeneous coverage over the mapping area and to optimize PACS performances in the two scan directions (see Section 4.1.3.2 of the PACS Observers' Manual), the step size between consecutive lines is 168" when scanning in the "Nominal" direction and 155" when scanning in the "Orthogonal" one. This small step size for SPIRE (being 348" the default one SPIRE-only large scan maps, see Section 3.2.1 of the SPIRE Observers' Manual) results in a high oversampling factor and a gain in sensitivity, without compromising map uniformity.

When selecting "SPIRE PACS Parallel Mode" from the "Observation" menu in HSpot (see Chapter 14 of the HSpot Users' Guide), a window like the one shown in Figure 2.2 appears on the screen.

As a first step, the PACS band has to be selected in the blue channel (60-85 µm or 85-130 µm). The other four Parallel Mode bands are fixed.

In the second step, selecting the scan speed the observer could have control over the sensitivity of the measurement. Note that there is no possibility to repeat Parallel Mode maps within one single observation request (AOR) to enhance the survey depth.

Figure 2.2.  HSpot window for setting up Parallel Mode AOR

	Note
	The depth of observation in Parallel Mode mode can be controlled by three ways: (i) adjust scan speed; (ii) add "Nominal" and "Orthogonal" coverages on the same area of the sky; (iii) repeat AORs.

The dimensions of the area to be covered is set by the length and height of the common SPIRE-PACS survey area (see Figure 2.3). The figure illustrates how the common SPIRE-PACS survey area - the grey box - is actually built up. The number of scans required to cover the common area is higher than would be required in a single instrument coverage. The number of extra scans and the required length of the scan legs both depend on the scan angle and the footprints separation. Taking into account the 42.4º scan angle and the fixed 21' separation of the SPIRE and PACS focal planes' footprints on the sky, the Parallel Mode observations perform five or six extra scans and extend the scan leg length with 22' respectively. Hence, the actual area observed with each instrument will be bigger than what was requested.

Note

Using the SPIRE Photometer AOT, the Parallel Mode settings can be reproduced in scan map mode simply enlarging the requested area by ~15' in both length and height. This applies to PACS scan map mode too: however, in this case the observer must set a scan step of 168" and +42.4º orientation angle in "Array" or "Array with sky constraint" reference frame to get the same Parallel Mode "Nominal" scan direction coverage. The scan step must be set to 155" and the orientation angle to -42.4º to reproduce the Parallel Mode "Orthogonal" scan direction coverage.

Figure 2.3.  Illustration of Parallel Mode scanning scheme. The grey area is the common SPIRE-PACS survey field what the observer specifies in HSpot. The scanning angle with respect the arrays' short axis is 42.4º. Red colour represents the SPIRE scanning path, blue colour is used for PACS.

The centre of the common survey area is at the coordinates given by the target position, unless an offset along the axes Y or Z of the spacecraft coordinates is given. The map size along a scan leg can be specified by the map "Length" parameter. In perpendicular direction, the common survey size is defined by the "Height" parameter. In case the scan direction is set to "Orthogonal", then the observer has to change the "Length" and "Height" parameters and the spacecraft will perform an observation along -42.4º scan angle over the same survey area specified for the "Nominal" direction. The pairs of nominal and orthogonal AORs can be concatenated in HSpot (see further details in the HSpot Users' Guide).

An illustration of how a Parallel Mode AOR footprint is visualized in HSpot can be seen in Figure 2.4 and in Figure 2.5. Pink represents the PACS and green the SPIRE footprint: the observer requested common survey area is painted by both instruments. SPIRE and PACS coverage maps as taken from real observations is shown in Figure 2.6, Figure 2.7 and Figure 2.8.

	Tip
	It is recommended to concatenate Parallel Mode AORs if the same target applies for all measurement in a sequence. In such a group of observations the observatory overhead will be charged only once and HSpot will render this time to the first AOR of the concatenation chain.

Figure 2.4.  Parallel Mode footprint image example as displayed in HSpot. NGC 7293 is imaged in "Nominal" scan direction with a coverage area of 54'x72'. A comparison between observer's requested area and real instrument coverages is presented in Figure 2.6.

Three optional parameters are available:

	Note
	Setting constraints on when the observation can be performed make scheduling and the use of Herschel less efficient, hence the observer will be charged extra overheads to compensate (see Herschel Observatory Manual).

Figure 2.5.  Parallel Mode footprint image example as displayed in HSpot. NGC 7293 is imaged in "Nominal" plus "Orthogonal" scan directions. However, the "Length" and "Height" parameters are the same in the 2 AORs (instead of being inverted): the result is that the area covered by both instruments, in both directions, is only 54'x54' (see Figure 2.7).

Figure 2.6.  SPIRE (darker area, shift to left) and PACS coverage maps, normalized to 1, as obtained from observation of NGC 7293 in "Nominal" scan direction only. Overplotted in green is the original observer requested area of 54'x72'.

Figure 2.7.  SPIRE (darker area, shift to left) and PACS coverage maps, normalized to 1, as obtained from observation of NGC 7293 in "Nominal" and "Orthogonal" scan directions. Overplotted in green is guaranteed area observed by both instruments, in both scanning direction. Since the "Length" and "Height" parameters were the same in the 2 AORs, this area corresponds to a square of 54'x54'.

Figure 2.8.  SPIRE coverage comparison between single-scan (figure behind) and cross-scans (figure in front) Parallel Mode observations. Note the better coverage of the latter, especially if some pixels have been masked out during data reduction (circled area).