Hubble Legacy Fields (HLF) Data Release 1.5 for the GOODS-S region (HLF-GOODS-S)

• Introduction
• Information on Observations
• Data Products, Zero Points, and Download Options

Introduction

This describes the 1.5 version data release of the Hubble Legacy Fields (HLF) project for the GOODS-South (ECDF-S) region from the HST archival program AR-13252. The release combines exposures from Hubble's two main workhorse cameras, the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) and the Wide Field Camera 3 InfraRed Channel (WFC3/IR), taken over more than a decade between mid-2002 to the end of 2016. The HLF includes essentially all optical (ACS/WFC F435W, F606W, F775W, F814W and F850LP filters) and infrared (WFC3/IR F098M, F105W, F125W, F140W and F160W filters) data taken by Hubble over the original CDF-South region including the GOODS-South, CANDELS, ERS, ECDF-S and many other programs. Given that this dataset combines all images in the archive on the CDF-S to date, the AR proposal identified the product under the global name "Hubble Legacy Field". Given the large area, a global astrometric solution was bootstrapped from the smaller datasets. All the image mosaics have been produced using the same tangent point as the original GOODS-S dataset. The total incorporated in the HLF-GOODS-S is 5.8 MSec in 7211 exposures from about 2442 orbits. This is ~70% of a HST full cycle!

The HLF Team is Garth Illingworth, Daniel Magee, Rychard Bouwens, Pascal Oesch, Ivo Labbe, Pieter van Dokkum, Kate Whitaker, Bradford Holden, Marijn Franx, and Valentino Gonzalez. Please credit “Illingworth et al (2017), in preparation” when using these data.

The 1.5 version of the HLF dataset provides ACS/WFC and WFC3/IR images covering a 25 x 25 arcminute area plus (smaller) cutouts of four deep areas in the GOODS-South field. All the data covering the original HUDF field within the GOODS-South area were previously released as a high level science product - the HUDF/XDF field.

What's new in v1.5

The 1.5 version of the HLF GOODS-South dataset includes improved background subtraction in the WFC3/IR filters and improved correction for images affected by a time- and spatially-variable background (see section Data Processing). This version adds data taken with the WFC3/IR F098M filter data which covers the ERS and HUDF. Also, additional exposures were added to the ACS/WFC F435W from the HDUV (13872) program.

The HLF-GOODS-S Field

The HLF-HUDF Parallel Deep Fields

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Observations

Description

Observations included in this version of the HLF-GOODS-S Field were taken from July 2002 to October 2016 from 20 different HST programs.

Programs Used for HLF-GOODS-S Field

Data Processing

The process begins with individual flt files requested from the MAST HST archive. These images have already been subject to bias correction, dark subtraction, flat-field correction, and cosmic ray rejection. The flt files obtained from the MAST are reasonably well calibrated, given that the on-the-fly pipeline processing by STScI already takes advantage of darks and flat fields constructed from latest on-orbit calibration data.

All images used to produce the HLF were first visually inspected to identify any data quality issues (loss of guiding, excessive background, pointing accuracy) and any image which could not be corrected was rejected for processing. During this visual inspection we also identify images affected by satellite trails and optical ghosts from filter reflections generated by bright stars (ACS) and updated the data quality array to ensure these artifacts were masked during final processing.

For ACS/WFC data taken after HST Servicing Mission 4 (SM4) we used images that have been correct for charge transfer efficiency (CTE) degradation.

ACS/WFC Data

The HLF ACS/WFC dataset were reduced by the HLF data processing pipeline HLFRED (built on DrizzlePac) which can be found on Github.

WFC3/IR Data

The HLF WFC3/IR dataset were also reduced by the HLF data processing pipeline HLFRED

The HLFRED processing of the WFC3/IR observations in the HLF follows the procedure as described in (Magee, Bouwens & Illingworth 2011) but with a few additional corrections in order to improve the reductions.

While most WFC3/IR flt files requested from MAST can be used readily, in some cases CALWF3 can falsely flag nearly all pixels as cosmic rays, due to a varying background during the exposure. For WFC3/IR MultiAccum mode observations, CALWF3 assumes that accumulating background counts over the entire observation is a linear function. This assumption may not be the true for all observations. Depending on circumstances of the observation the background count rate may vary over the duration of the observation. In order to determine if the background count rate is sufficiently non-linear, we compared the average exposure time in a flt file's TIME array extension with the exposure time listed in the header EXPTIME keyword. If the we find the exposure time varies by more than 2% we correct the background signal in the raw image using code developed by the Frontier Fields team. This additional step assures the background count rate is linear before reprocessing the image with CALWF3. For more info on time-varying background see Instrument Science Report WFC3 2014-03.

We also mask out pixels on WFC3/IR images that were affected by source persistence. This masking was performed by utilizing persistence models generated by the STScI WFC3 Persistence Project. A persistence model is created for each WFC3/IR exposure which incorporates an internal persistence model (persistence within a visit) and an external persistence model (persistence from earlier visits). Using this model of the total persistence we flagged all pixels above a threshold of 0.2 electrons/s thus ensuring that our final reductions are not significantly affected by source persistence.

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