HDUV Data Release 1.0

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• Introduction
• Information on Observations
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• Imaging Data Products
• Source Catalogs
• Citation Information
• General README File

Introduction

This summary describes the Hubble Deep UV (HDUV) Legacy Survey data release. The HDUV is a 132 orbit imaging program with the WFC3/UVIS camera onboard the Hubble Space Telescope (HST) designed to obtain deep UV images in the F275W and F336W bands over a total area of ~100 arcmin² in the central parts of the two GOODS/CANDELS-Deep fields (Grogin et al. 2011). The survey was approved in HST Cycle 22 (program 13872).

This image release also includes all the F275W imaging data taken by the CANDELS survey, which were not previously available as high-level science products due to alignment difficulties. Using a novel approach which relies on other images taken in the same HST orbit, we successfully aligned these CANDELS F275W images and combined them with the HDUV survey data.

Similarly, to facilitate the most efficient scientific exploitation this release incorporates the UVUDF v2.0 data release from Teplitz et al. (2013) and Rafelski et al. (2015). The UVUDF data were taken directly from MAST and added to the F275W and F336W mosaics in GOODS-S.

The addition of the HDUV data further enhances the legacy value of the two GOODS/CANDELS-Deep Fields, which now include deep 11-band broad-band imaging from HST as well as very deep ancillary data at all wavelengths from the X-ray to the radio, enabling unique multi-wavelength studies.

A detailed survey overview can be found in Oesch et al 2018, ApJS, accepted as well as on the HDUV team page at http://www.astro.yale.edu/hduv.

Survey Design

The HDUV survey was designed to efficiently complement and build on previous datasets. In particular, the HDUV is placed in the GOODS/CANDELS-Deep regions which have the best multi-wavelength coverage of all extragalactic fields that are not limited to a single pointing (the latter including the HUDF and the Hubble Frontier Fields, which are covered by different UV programs).

The primary observations of the HDUV survey include 10-orbit deep F275W and 8-orbit deep F336W filter images over a total of 13 WFC3/UVIS pointings split over the central parts of GOODS-N and S for a total coverage of ~100 arcmin². The specific layout in the two fields is shown in the figure below. The pointings were chosen to most efficiently provide coverage in the CANDELS-Deep areas (blue outline), while avoiding overlap with and building around the existing deep UVUDF data (pink). In GOODS-N, the CANDELS/F275W imaging was acquired during the bright part of CVZ visits, resulting in a more extended footprint around the CANDELS-Deep area (pink outline).

The following table provides an overview of the F275W and F336W data included in this release. They include the UVUDF data in GOODS-S and the CANDELS/F275W data in GOODS-N.

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Observations

Description

The 132 orbits of HDUV data were obtained in 42 independent visits between 2014 November 25 and 2015 November 16. In GOODS-S, these visits were composed of 4 or 5 full-orbit exposures in either F275W or F336W. The exposure time of these images was ~2600-2800 s. In GOODS-N, all visits made use of the CVZ. Thus to reach an equivalent 4-orbit exposure, only 2 actual orbits were required for each filter (at the expense of an increased background during the bright part of the orbit).

Data Processing

A special pipeline for the reduction of these data was written based on previous WFC3 and ACS data reduction pipelines developed by our team and used, e.g., for the public image release of the XDF and the HLF (Illingworth et al. 2013, 2016). The pipeline is written in python and makes heavy use of the drizzlepac software package developed by STScI, which includes the basic processing steps required for HST data reduction.

CTE Correction

The WFC3/UVIS images need to be corrected for charge transfer (in)efficiencies (CTE). A pixel-based correction method is provided by STScI, which is applied directly to the raw images.

Sky Darks

The WFC3/UVIS images were found to show significant structure in their dark images, which clearly shows up when combining images at a depth of several orbits as available here. In particular, the combined WFC3/UVIS images exhibit a significant "striping" along the readout direction when using the standard, non-CTE corrected dark frames (that were available at the time of these observations). To remedy this, a sky dark was computed based on all the exposures for each filter, which was then subtracted from each individual exposure. This procedure resulted in very flat and uniform images.

Note that the spirit of the sky dark procedure is similar to the use of the newly developed "self-calibration" by STScI (see Anderson & Ogaz 2014), which constructs delta-darks to be subtracted from each image.

Image Alignment

Image alignment was performed relative to the F435W images from the GOODS dataset. The HDUV survey was designed to allow for straightforward alignment by obtaining several images of the same filter in a given visit. The relative positions of exposures within a visit are accurate enough that they can easily be combined and cosmic ray cleaned, before a global shift is determined for each visit.

The CANDELS UV images required additional steps for alignment, since they were taken exclusively in the bright part of each CVZ orbit, with only one F275W image (of about 1500 s) per visit. A simple visit-by-visit alignment strategy is thus not applicable. However, the CANDELS survey obtained a short ~400 s long-pass F350LP image in each visit immediately before the F275W exposure. Those images contain enough high-S/N sources for an accurate alignment and were used to compute relative shifts that were applied to the F275W exposures.

Pixel Grid

The processed images were drizzled to the same 60mas pixel grid as the 3D-HST images (available here), to enable the most efficient use of these data. The images have dimensions of 17500×19700 pixels in GOODS-S and 20480×20480 pixels in GOODS-N.

Programs Used

The table below lists all the programs from which data have been included in this image release.

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hlsp_hduv_hst_wfc3-uvis-60mas_goodsn_f336w_v1.0_drz.fits