CLASH: Photometric redshifts with 16 HST bands in galaxy cluster fields
  The Cluster Lensing And Supernovae survey with Hubble (CLASH) is an Hubble
Space Telescope (HST) Multi-Cycle Treasury program observing 25 massive galaxy
clusters. CLASH observations are carried out in 16 bands from UV to NIR to
derive accurate and reliable estimates of photometric redshifts. We present the
CLASH photometric redshifts and study the photometric redshift accuracy of the
arcs in more detail for the case of MACS1206.2-0847. We use the publicly
available Le Phare and BPZ photometric redshift codes on 17 CLASH galaxy
clusters. Using Le Phare code for objects with StoN>=10, we reach a precision
of 3%(1+z) for the strong lensing arcs, which is reduced to 2.4%(1+z) after
removing outliers. For galaxies in the cluster field the corresponding values
are 4%(1+z) and 3%(1+z). Using mock galaxy catalogues, we show that 3%(1+z)
precision is what one would expect from the CLASH photometry when taking into
account extinction from dust, emission lines and the finite range of SEDs
included in the photo-z template library. We study photo-z results for
different aperture photometry and find that the SExtractor isophotal photometry
works best. Le Phare and BPZ give similar photo-z results for the strong
lensing arcs as well as galaxies of the cluster field. Results are improved
when optimizing the photometric aperture shape showing an optimal aperture size
around 1" radius giving results which are equivalent to isophotal photometry.
Tailored photometry of the arcs improve the photo-z results.
