The Herschel Lensing Survey (HLS) is a Herschel open-time key program targeting massive galaxy clusters. The strong gravitational lensing power of these clusters enable observations to penetrate through the confusion noise, which sets the ultimate limit on our ability to probe the Universe with Herschel. HLS will obtain deep images with PACS at 100 and 160 μm and SPIRE at 250, 350, and 500 μm for a sample of 44 massive clusters.

• Target Selection - Targets of the survey, were selected from the most X-ray-luminous clusters imaged by the the ROSAT X-ray all-sky survey. These X-ray-luminous clusters are also the most massive and therefore assumed to be the most effective gravitational lenses. The majority of HLS targets come from the sample of the Local Cluster Substructure Survey (LoCuSS), which adopts the following selection criteria:
  (1) LX > 2 × 10⁴⁴ erg/s, (2) 0.15 < z < 0.3, (3) NHI < 7 × 10²⁰ cm^-2, and (4) −70 < δ < 70. In addition, some number of clusters with spectacular lensed systems are included in the sample. For the majority of our target clusters we have well-constrained, accurate mass models which have been constructed through many years of intensive imaging/spectroscopic campaigns with HST, Keck, and VLT telescopes. Most of these clusters also have MIPS 24 μm images and many will be accessible from ALMA for future follow-up observations.


• Observing Parameters - Each target cluster is imaged by both PACS and SPIRE. With PACS we use the scan-map mode at medium speed. The scan leg lengths are 4 arcmin, cross-scan step is 2 arcmin, number of scan legs is 13. Each cluster is observed twice by orthogonal scan maps with 18 repetitions each. The total observing time is 4.4 h per cluster with an on-source integration time of 1.6 h (1500 s/pixel). With SPIRE we use the Large Map mode with the nominal speed. The scan direction was set to scan angles A and B. The length and height of the map are set to 4, which in practice will produce a map of 17 arcmin × 17 arcmin.With 20 repetitions, the total observing time per cluster is 1.7 h with an on-source integration time of 0.6 h (17 s/pixel).


• Coordinated programs -The Herschel Lensing Survey is directly coordinated with a few other observing programs. The most important of these is the Spitzer/IRAC Lensing Survey, which is a Spitzer Warm-Mission Exploration Science program. This program will obtain deep (5 h/band) Spitzer/IRAC 3.6 and 4.5 μm images of ∼50 massive clusters. By design, its target list is highly overlapped with that of HLS. Deep IRAC images will be essential for identifying optically-faint high-redshift infrared-luminous galaxies as well as for deriving accurate photometric redshifts. In addition, roughly half of the HLS clusters are imaged by HST/WFC3 through two on-going programs: “Are Low-Luminosity Galaxies Responsible for Reionization?” and MCT: “Through a Lens, Darkly – New Constraints on the Fundamental Components of the Cosmos”.
  
  The Herschel Lensing Survey is also closely related to two other Herschel open-time key programs: “LoCuSS: A Legacy Survey of Galaxy Clusters at z = 0.2" and “Constraining the Cold Gas and Dust in Cluster Cooling Flows”. The former will obtain wide (3 deg × 3 deg) and shallow PACS and SPIRE maps of ∼30 massive galaxy clusters at z ∼ 0.2, many of which are also targeted by HLS. The latter program will study the brightest cluster galaxies (BCGs) in a dozen coolingflow clusters, and the HLS data will provide PACS/SPIRE photometry for a much larger sample of BCGs.