The PAU Survey

Enrique Fernández

The Physics of the Accelerating Universe Survey (PAUS) is an ongoing extragalactic survey carried out with the William Herschel Telescope in La Palma, Canary Islands, equipped with the PAUCam Camera. The distinctive feature of PAUS is the ability to measure redshift of galaxies by photometric methods, with roughly an order of magnitude higher accuracy than that provided by other past and existing photometric surveys. This is accomplished by equipping PAUCam with 40 narrow-band filters, each 130A wide (FWHM) and equally spaced by 100A, spanning the region from 4500A to 8500A [1]. In addition the camera also has a set of six standard broad-band filters, in the six bands u, g, r, i, z, Y, and an external large size filters (full field of view), also in those bands.


PAUS originated in the context of the PAU Project, funded, in 2007, by the Consolider Ingenio 2010 Program of the (at the time) Spanish Ministry of Research and Innovation.

The PAU project was approved in 2007 and ended in 2014. Its main deliverable was the PAUCam camera, built by 5 of the 7 groups that were originally in the Consolider Project, namely from CIEMAT and IFT (in Madrid), and from IEEC, PIC and IFAE (in Barcelona). These groups also developed the large amount of software needed for the control of PAUCam and for the data processing from their production at the Telescope to their analysis at the labs [3,4]. The same groups also collaborate closely in other projects such as DES, DESI and EUCLID, described elsewhere in this report.

PAUCam operates as a Visitor’s Instrument at the prime focus of the William Herschel Telescope (WHT) in the Canary Island of La Palma. Starting in 2016 other groups have joined the PAUS Collaboration, namely from Durham University, Plymouth University and University College of London in the UK, from Leiden Observatory in the Netherlands, from ETH in Switzerland, and from Bonn University in Germany. The observing nights are granted from the Isaac Newton Group of Telescopes (ING), a Consortium of United Kingdom, the Netherlands and Spain, that operates several telescopes at the La Palma site, the WHT among them. The proposals for observation periods are submitted twice per year to the TACs (Time Allocation Committees) on those three countries, that advise the ING Management.

Figure 1: The PAUCam

PAUS Operation

Since first light in 2005 to the end of 2019 PAUS has observed for about 215 nights, with only 8.9 effective nights lost (5.6 due to WHT technical problems and 3.3 due to problems with PAUCam), but unfortunately with very bad weather conditions, particularly in the fall and winter of the last two years. The effective

number of good observing nights is only half of the above, namely 101 nights of good data. PAUS has chosen to observe in fields where data exists from other observations, either photometric or spectroscopic. These include the COSMOS field [5], containing over one million galaxies, collected from several telescopes (in satellites and ground-based), with a coverage of 2 square degrees in the equatorial region, and in the W1, W2, W3, W4 fields of the CFHTLS [6]. The COSMOS field has already been covered while the CFHTLS fields are not yet completed. In terms of square degrees we do not quote detailed numbers, as they depend on the particular analysis being pursued and also vary depending on the observation strategies. A rough number is 0.7 square degrees per good night of observation.

Scientific results

The PAUS objective is to survey an area of about 100 deg 2 down to magnitude $i_{AB} \approx 22.5$ with a redshift error $\sigma_{68}=0.035(1 + z) $. The redshift precision has already been achieved for the best sources, selected on the basis of a quality cut, as published in [2] and illustrated in Fig.1. A higher photo-z precision, of $\sigma_{68}=0.001(1 + z) $, is actually obtained for a bright and high quality sample of galaxies with detectable emission lines.

Since the first light in 2015 a big effort has been made to calibrate many aspects of the camera and to process the data (as published in references [1], [3] and [4] ). Analysis of these data is in progress and some of it will be published in the near future, for example on measuring intrinsic alignment, crucial for weak-lensing studies, and on validation of redshifts of DES data.

During 2010 some elements of the WHT will be modified to adapt to the installation of the future WEAVE spectrograph. We expect that PAUCam will again take some data in 2021 and in further years, to complete the $100 deg^2$ goal.

Figure 2: The $\sigma_{68}/{(1 + z)}$ (multiplied by $10^3$), top, and outlier fraction, bottom, for several quality cuts as a function of the cumulative magnitude. The solid lines show the results when 100, 80, 50 and 20 percent of the sample remains after a quality cut. The dashed line shows the COSMOS results without any quality cuts (from the public COSMOS2015 catalogue). The green line at the top shows that for 50% of the sample the redshift precision is better than 0.0035(1+z).
[1] The Physics of the Accelerating Universe Camera Cristobal Padilla et al., arXiv:1902.03623 [astro-ph.IM] (submitted to The Astronomical Journal).
[2] The PAU Survey: early demonstration of photometric redshift performance in the COSMOS field M. Eriksen et al., Mon.Not.Roy.Astron.Soc. 484 (2019) no.3, 4200-4215
[3] The PAU Survey: Operation and orchestration of multi-band survey data Nadia Tonello et al. Astron.Comput. 27 (2019) 171-188.
[4] The PAUS Survey: Data Reduction of Narrow Band Images. PAUS Collaboration (in preparation for publishing).
[5] COSMOS survey page:
[6] The Canadian French Telescope page: