2.2.  Scientific benefits of Parallel Mode

As mentioned earlier, there is no evident degradation of SPIRE data between the SPIRE-prime and the Parallel Mode configuration, as long as the on-source integration time is the same. This implies that when the program involves large areas (bigger than 30'x30') and the observer is interested mainly in SPIRE data, it would be advantageous to use Parallel Mode in respect to SPIRE-only observation, basically obtaining PACS data for free. In order to get the same on-source integration time as the SPIRE-only observation, the observer might require to repeat the observation multiple times, including cross-scans observations.

Instead, for PACS we do see a degradation of the PSF in respect to PACS-only observations, due to the higher compression factor in Parallel Mode. Hence, if the program mainly relies on PACS-data, it might be better to schedule PACS and SPIRE observations separately.

As a consequence, for programmes involving shallow galactic surveys over large areas of sky, Parallel Mode offers a major efficiency gain and major additional scientific data with respect to two separate PACS and SPIRE coverages. For other mapping programmes like deep galactic surveys or extragalactic surveys, there may be some advantages with careful design of mosaicking, although not so great.

2.2.1.  Shallow galactic surveys

For very large shallow programmes, where the requested coverage is typically in the order of 100 square degrees or more, the observer could save a significant time compared to using the two instruments separately and covering the survey area twice. This is the most likely instance in which Parallel Mode could be beneficial.

2.2.2.  Deep galactic surveys

Deep surveys of nearby molecular clouds mapped with SPIRE will have a benefit using Parallel Mode mode. Instead of doing SPIRE observations only, it is conceivable that PACS is operated in parallel without compromising SPIRE data quality, providing a shallow PACS survey of the complete regions. This would effectively provide additional data for the same observing time. The benefits in terms of mapping efficiency and additional data would not be as great as for a larger shallow survey, but PACS data could be worthwhile nonetheless.

2.2.3.  Deep extragalactic surveys

For such a programme the Parallel Mode gain on mapping efficiency is not a great benefit. This is particularly true under the assumption that the survey area of deep extragalactic mapping programmes is considerable smaller than for galactic surveys. Moreover, these observations will cover a range of depths where both PACS and SPIRE sensitivities are demanding. As with the galactic surveys, it is possible that PACS could be operated in parallel to provide additional data but in considerable lower sensitivities.

2.2.4.  Additional benefits of Parallel Mode

Operating SPIRE and PACS together increases the load on the helium tank of Herschel by much less than a factor of two, because the total load is largely from the cryostat parasitics. Parallel operations is thus very favorable in terms of science per litre of helium, increasing the overall lifetime of the observatory.

Moreover, observations made in Parallel Mode will have more accurate relative PACS/SPIRE astrometry, because the relative angular offsets between the two arrays are fixed and accurately known and the pointing error will be the same for the two instruments throughout the observation.

The efficiency of scientific mission planning is also increased because the scheduling of PACS and SPIRE observations of the same sky area can be done within a single block. This reduces the total telescope slew time over the mission lifetime in favor of science time.