CLASH Observational Details

Cluster Sample Size Determination:

The CLASH cluster sample was selected with three key considerations: (1) inclusion of clusters that are dynamically relaxed (as indicated by a circularly symmetric x-ray surface brightness distribution), (2) inclusion of massive clusters (as indicated by x-ray gas temperature >5 keV), and (3) selection of a sufficient number of clusters to enable us to measure the mean mass concentration, cVIR , to 10% accuracy given the statistical uncertainties arising from measurement errors, intrinsic scatter, and intervening large-scale structure.

Cluster Sample Selection:

The specific 25 clusters observed in the CLASH program are presented in the figure and table above. Our sample is drawn heavily from the Abell and MACS cluster catalogs (Abell 1958; Abell et al. 1989; Ebeling et al. 2001; Ebeling et al. 2007; Ebeling et al. 2010). Our cluster sample covers a wide redshift range, 0.15 < z < 0.9 (with a median of z ~ 0.4), spans almost an order of magnitude in mass (~5 to ~30 x 1014 solar masses), and all have X-ray temperature TX > 5 keV. Twenty of the 25 clusters strictly meet the X-ray surface brightness symmetry criterion and were not selected based on their lensing properties. Although these clusters are solely X-ray selected, one or more giant arcs are visible in at least 18, which indicates that the relaxed clusters in our sample have Einstein radii in the range 15 to 30 and, hence, we are assured of high quality strong lensing information from CLASH HST imaging (the existing data for the remaining two are of insufficient resolution to determine the presence of large arcs). The remaining five clusters were specifically selected because they are known to have very large Einstein radii (35" to 55"). These five high magnification lenses have the highest potential for the discovery of very highly magnified ultra-high redshift galaxies. These five clusters are not necessarily "relaxed" - indeed, MACS0717 is a well-known merging cluster system. The targets are listed in Table 1 along with a summary of the associated existing ground and space-based data and an itemized list of their allocated HST orbits.

Cluster Observations:

Each cluster target is observed with HST set at two different orientations. For each orientation, half of all the exposures for a given cluster field are acquired over the course of 4 epochs. This allows us to perform the SN Ia search component of our study. The combination makes a survey footprint around each cluster that is depicted in the figure above. The total area of complete 16-filter coverage in the cluster is 4.07 square arcminutes (88% of the WFC3/IR FOV). The orient rotation of ~30O is chosen to maximize the SN search area but still allow the observations to run as close to consecutively as possible. This allows us the option to drop the switch to the second orientation if we find a promising high-z SN candidate in the first orientation. In such a case, we can use the already allocated orbits for the SN search area while fulfilling scheduling constraints. At the end of the 4 epoch x 2 orientation sequence, the cluster core has been imaged with both ACS and WFC3 in 16 different passbands for a total integration of 20 orbits.


The parallel fields are not observed in 16-filters. The ACS parallels are taken in F775W and F850LP. The WFC3 parallels are taken in F350LP (UVIS), F125W (IR) and F160W (IR). The typical cadence between epochs is approximately 10 to 14 days. Thus, at each orient, the total duration of the SN search is ~30 to 45 days. The orientation angles are selected to allow the second orientation to be reprogrammed to follow-up any SN candidate found in the first orientation without requiring the use of ToO orbits. If, however, a promising SN candidate is found at the end of cluster observing sequence then the CLASH/CANDELS orbit reserve are used for the follow-up. Follow-up observations include exposures with the ACS G800L or the WFC3/IR grism (depending on the probable SN redshift) to obtain a spectrum of the SN candidate.

Exposure Times in Each Filter:

The exposure times for the primary camera (cluster center position) are set primarily by the need to achieve 80% photometric redshift completeness down to F775W = 26 AB mag. This requires ~18 orbits per cluster. The 5-sigma limiting magnitude in each of the 16 filters is >26 AB mag. However, we augment the total cluster exposure by 2 orbits per cluster to extend the NIR depth to a 10-sigma limiting AB mag of F160W = 26.7 as needed for our lensed high-z galaxy search. Thus, on average, we require 20 orbits per cluster for 25 clusters. However, since some clusters have usable archival data only about 15 of the 25 clusters need the full 20 orbits. The table below shows the total integration times in each filter on the cluster when the full sequence is completed.

Why 16 Filters?

Redshift estimates for multiply lensed images are crucial for breaking lensing degeneracy, by narrowing the allowable range in the gradient of the mass profile. However, most of the useful lensed images are much too faint for spectroscopy. The typical lensed source magnitude is 25 < I < 28, so that even with the largest facilities only the brightest arcs yield spectroscopic redshifts. Fortunately, we can now obtain sufficiently accurate redshift information using photometry (a.k.a., photo-z's) from both ACS and WFC3. WFC3 is key because of its unique UV and NIR sensitivity, which at last allows the Lyman-limit feature to be photometrically traced to redshifts as low as z ~1.5. The figure below shows how the effective photometric redshift depth (i.e., the 80% completeness limit) and the number of z > 1 arcs with photo-z's increase as one adds more broadband filters for a given total integration time. By distributing the total integration time over a set of 16 well-chosen filters we accomplish the key step to producing superb quality strong lensing maps: detecting a sufficient number of lensed objects in each cluster and obtaining accurate distance estimates for them. The 16-filters yield photo-z's that have an accuracy of ~0.02 x (1 + z). This requires photometry that is capable of producing reliable photo-z's down to a magnitude limit of F775W ~ 26 AB mag.

Above: The distribution on the sky (in Galactic Coordinates) of the 25 CLASH clusters. An dust IR emission map (Schlegel et al. 1998) is shown in the background.

Above: Footprint of the CLASH survey for a given cluster. ACS FOV shown in yellow, WFC3/UVIS in blue, and WFC3/IR in red. The actual footprints for each cluster may vary slightly depending on the actual ORIENT angle used.

CLASH Overview Paper available on-line:

Our survey overview paper is published: Postman et al. 2012, ApJS, 199, 25. Please read this paper for the most complete details about the CLASH survey. See News link for current observation schedule. The CLASH survey has been completed.