In Parallel Mode (PM) PACS and SPIRE operates in photometry mode simultaneously, carrying out large-area mapping observation. PACS takes data in its red band (160 µm) and in one between the blue band (70 µm) and the green band (100 µm), while SPIRE is observing in its three photometric bands (250, 350 and 500 um). The SPIRE PACS Parallel Mode is treated as a 4th Herschel instrument.
Simultaneous observations in five bands are made possible without significant degradation in instrument performance. PACS and SPIRE have identical integration time, then the relative instrument sensitivities have to be carefully inspected to judge the real scientific benifit of using PM.
Parallel Mode will be used during dedicated Observation Days of 21 hours, although no single observation could last longer than 18 hours. The reason for this is to allow uninterrupted observation with the PACS and SPIRE photometers to reduce operational overheads caused by cooler recycling or switching between subinstruments.
The major advantage of SPIRE PACS parallel mode are for programmes involving shallow galactic survey. Indeed for very large shallow programmes, where the request coverage is tipically in the order of dozens square degrees, the observer could save a significant time compared to using the two instruments separately and covering the survey area twice. For other progammes the PM gain on mapping efficiency is not a great benefit. Indeed some observations , such as Deep Extragalactic surveys, will cover a range of dephts where both PACS and SPIRE sensitivities are demanding.
To have detailed information about the PACS/SPIRE Parallel Mode, see the Observers' Manual
PACS SPIRE Parallel Mode Template
The Paralel Mode is offered with only one observing mode (AOT).
Scan maps are performed by slewing the spacecraft at a constant speed along parallel lines to cover a large area.
Two scan speeds are provided: a Fast speed of 60 arcsec/s and a Slow speed of 20 arcsec/s. Using the Fast speed a degradation of the PACS PSF is expected due to on-board data averaging. Because of the limeted bandwidth available for data transmission, PACS data in PM are averaged coadding on-board 8 frames in the blue/green array and 4 frames in the red array (default averaging for the PACS scan mapping mode).
Thus, in the blue and green channels, the sampling frequency is reduced to 5 Hz (instead of 40 Hz) and, if the telescope scans with a speed of 60 arcsec/s, it corresponds to a sky movement of 12 arcsec, well above the telescope diffraction figure for these bands (5 arcsec and 8 arcsec for the blue and red channels, respectively).
When finishing a scan leg, the spacecraft performs a turn maneuvre and continues the observation along the next scan leg in the opposite direction. The time required for this turn is about half a minute. Scan mapping does not make use of chopping, the signal modulation being provided by the spacecraft motion.
It is suggested to perform two scan maps of the same area, one with nominal coverage, the other with orthogonal coverage in order to remove more efficiently the stripping effects due to the 1/f noise. For this purpose two AORs shall be concatenated in HSpot to be performed with a different scan direction.
See the dedicated sections regarding to the PACS scan mapping and SPIRE large mapping modes to have detailed information about these observing modes.
SPIRE PACS Parallel Mode 1σ sensitivities for a point source for a single map coverage.
| Scan Speed
|| 60 arcsec/s
|| 20 arcsec/s
| PACS 60-85 µm
| PACS 85-130 µm
| PACS 130-210 µm
| SPIRE 250 µm
| SPIRE 350 µm
| SPIRE 500 µm
Setting the PM Parameters
The PACS SPIRE Parallem Mode AOT (Astronomical Observation Template) is selected in HSpot from the "Observation" menu.
The following parameters must be specified:
- PACS blue filter: a PACS band has to be selected between the blue (70 µm) and the green (100 µm) channel. The other four bands are fixed (160 µm, 250 µm, 350 µm, 500 µm);
- scan speed: the Fast speed (60 arcsec/s) or the Low speed (60 arcsec/s) must be selected. Selecting the scan speed the observer could have control over the sensitivity of the measurement ("Observation Estimation" botton in HSpot);
- dimension of the area to be covered: the map dimension is set by specifying the length and height (in arcmin).
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 42.4 degrees scan angle (scan direction with respect to the Z-axis of the array) and the fixed 21 arcminute separation of the PACS and SPIRE footprints, the PM observations perform five or six extra scans and extend the scan leg length with 22 arcminutes respectively.
- scan direction: setting Nominal or Orthogonal, the scan direction (provided as scan angle with respect to the Z-axis of the array) is set to +42.4 degrees or -42.4 degrees, respectively. If the observer wants to perform two quasi-othogonal observations of the same sky region, two AOR with different scan direction must be updated. In this contest it must be noticed that, since the Length parameter specifys the leg length, while the Height parameter specifys the length of the orthogonal dimension, changing the scan direction (e.g, from Nominal to Orthogonal) implicates to change the Length and Height parameters in order to observe the same sky region of the quasi-orthogonal scan;
- map orientation: set allowed angles for the orientation of the array on the sky ("Array with Sky Constraint") or not provide any constraint. As the map is carried with a specific angle of the array and because the orientation of the array on the sky changes as Herschel moves in its orbit, the actual coverage of the map will rotate about the requested centre of the map, except for sources close to the ecliptic plane. However a constraint on the orientation of the scan map in the sky implies that the observation will not be able to be performed during certain periods.
Parallel Mode scanning scheme. Grey area is the common PACS/SPIRE field specified in HSpot.
Blue and red color represent the PACS and SPIRE path, respectively.
Example of Parallel Mode footprint for a "Nominal" observation.
The displacement between PACS path (pink) and SPIRE path (green) is evident.