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, notably in 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 the 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) in those three countries, that advise the ING Management.

PAUS Operation

Since first light in 2005 until the end of 2019, PAUS has observed for about 215 nights with high efficiency (only 8.9 effective nights lost), but unfortunately with very bad weather conditions, particularly in the fall and winter. For that reason the effective number of good observing nights has only been 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 the W1, W2, W3, W4 fields of the CFHTLS [6]. The COSMOS field has been completely 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. During 2020 no data has been taken, due to ongoing work in the WHT telescope needed to accommodate the future WEAVE spectrometer. Future running of PAUCam will be possible after the installation of WEAVE, but detailed plans are not yet developed.

Scientific results in 2020

The PAUS objective is to survey an area of about 100 $deg^2$ down to magnitude iAB ≈ 22.5 with a redshift error $\sigma_{68}/(1+z)=0.0035(1+z)$. The redshift precision has already been achieved for the best 50% of all sources, selected on the basis of a quality cut, as published in [2]. The code to accomplish this task was the template-based code BCNz2 [2]. The growing number of analysis of the data has also resulted in a better understanding of the many issues raised by the first usage of a new instrument, PAUCam, with distinctive features, such as the high number of filters. In particular photo-z determinations have also been approached with deep-learning AI (Artificial Intelligent) techniques. A code developed for that task, named DEEPZ [9], already obtains results superior to those mentioned above, as illustrated in Fig. 1 (see figure caption for an explanation). Studies of the use of PAUS data in weak lensing [10] and in measurements of Intrinsic Alignment [11] have also been published in 2020.
Figure 1: The $\sigma_{68}/(1+z)$ (multiplied by $10^3$), for all galaxies with secure redshift and for various codes. The dashed red line is the baseline performance corresponding to the BCNZ2 code. The rest of the lines show the results of DEEPZ for various options, as explained in the figure legend.

[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:
[7] The PAU Survey: photometric redshift using transfer learning from simulations. M. Eriksen et al., Mon.Not.Roy.Astron.Soc. 497 (2020) 4, 4565-4579.
[8] The PAU Survey: a Forward Modeling Approach for Narrow-band Imaging. L. Tortorelli et al., Mon.Not.Roy.Astron.Soc. 484 (2019) no.3, 4200-4215
[8] The PAU Survey: Intrinsic alignments and clustering of narrow-band photometric galaxies. H. Johnston et al., Astron.Astrophys. 646 (2021) A147