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K2 Extracted Lightcurves ("K2SFF")

Vanderburg & Johnson 2014, PASP, 126, 948

See also: Vanderburg (2014),arXiv:1412.1827 | Vanderburg et al. (2015), ApJ, 800, 59

Introduction

Description of Data Products

Data Access

K2SFF Standard Search Form

K2SFF Multi-Target Upload Form

Download README

Introduction

An example of the uncorrected fluxes from K2 (blue) and the corrected K2SFF version (orange).
The lightcurves from K2 contain larger systematics than the original Kepler mission, due to the reduction in pointing precision as a result of having to rely on only two reaction wheels. Vanderburg & Johnson have created a technique to correct for the pointing-dependent nature of the pixel-level fluxes. This correction improves the photometric precision by typical factors of 2-5, and results in median photometric performance of K2 targets to within a factor of two of the original, 4-wheeled mission.

The team have released their extracted lightcurves (using a variety of photometric apertures), as well as diagnostic plots, for each target. Simplified, ASCII text tables of the corrected lightcurves, excluding observations during thruster firings, are provided for the aperture that results in the lowest rms. Most users will prefer to download the FITS lightcurves, which include all data points (with flags to indicate thruster firing), as well as multiple versions (each FITS extension is an extracted lightcurve for a different aperture). There are a total of twenty apertures provided: ten circular and ten based on the pixel response function. The final extension contains the summed image from all the postage stamp frames.

Description of Data Products

Each target is organized into a subdirectory structure, based on its EPIC ID, with the following format:

c<##>/<####00000>/<#####>/

where:
  • c<##> is normally the 2-digit, zero-padded Campaign number the target was observed in. There are some exceptions: the engineering campaign uses "cet" for this field. Campaign 10 was split into two sub-campaigns designated as "c101" and "c102". There are K2SFF light curves for c102 only, and in this case the campaign number is a 3-digit field.
  • <####00000> are the first FOUR digits of the target's EPIC ID
  • <#####> are the last FIVE digits of the target's EPIC ID
For example, the star EPIC 202059070 was observed in Campaign 0. It's subdirectory path is therefore:
<...>/c00/202000000/59070/

Several files are available for each target. Each file has the same prefix ("hlsp_k2sff_k2_lightcurve_{EPICID}-c##_kepler_v1"), followed by an extension. The extensions and types for these files are:

  1. _allfiles.tar.gz = A tarball containing all files for this target.
  2. _correction.png = A plot showing the corrected flux as a function of arclength distance across the detector.
  3. _image.png = A plot showing the summed image from all postage stamp frames.
  4. _llc-default-aper.txt = ASCII text table containing time and corrected flux values for the default ("best") aperture.
  5. _llc.fits = FITS file containing extracted lightcurves for all aperture sizes/shapes. The first extension contains a binary FITS table with the time, corrected flux, raw flux, arclength distance, a flag indicating whether a thruster firing was occurring, and the cadence number for all observations, using the "best" aperture size/shape. The next twenty extensions contain similar tables for every aperture size/shape. The 22nd and 23rd extensions contain tables that define each circular and PSF aperture, respectively. The final extension contains the summed image of all the postage stamp frames.
  6. _llc.png = A plot of the raw and corrected fluxes for the "best" aperture.
  7. _medians.png = A plot of the median background count per pixel as a function of time for the "best" aperture.
  8. _skypos.png = A plot showing the location of the target in the K2 field-of-view.

Data Access

Note: The Campaign 5 data include 1,504 targets extracted from the M67 super-stamps. These stamps consist of 72 individual apertures, aligned to create a contiguous 400x400 pixel region. You can locate these 72 stamps by setting "Object Type" to "M 67" in the MAST K2 Search Form. The 1,504 targets, which use smaller apertures within these stamps, all have EPIC IDs, although there are not mission-produced light curves for these objects.

MAST Discovery Portal

We strongly recommend using the MAST Discovery Portal as the primary search interface for the K2SFF lightcurves. The Portal allows you to search for, and access, all data at MAST (including the K2 mission data like target pixel files). It also features a variety of tools, such as an interactive plotter, a sky viewer with footprint overlays, and cross-match functionality.

wget Scripts

You can download extracted lightcurves in bulk using wget scripts. In the table below we provide wget scripts to download all the extracted lightcurves available for a given campaign. You can also generate custom wget scripts to download large subsets of the data using the K2SFF Search Form (see below). Just change the "Output Format" to be "FILE: WGET LC commands", then hit "Search". Instead of displaying your search results in a table, a wget script file will be downloaded to your machine, which you can then execute with an "sh" command in a terminal to sequentially download each lightcurve in the file.

wget Script Files By Campaign (Downloads FITS Files Only)
Campaign ENG Campaign 0 Campaign 1 Campaign 2 Campaign 3 Campaign 4
Campaign 5 Campaign 6 Campaign 7 Campaign 8 Campaign 9 Campaign 10

K2SFF Search Form

Most users will want to use the K2SFF Search Form to search for and retrieve lightcurves for targets. The Search Form allows you to search based on metadata (like magnitudes from the EPIC catalog), coordinates (via cone searches), or target ID (including the EPIC IDs). You can search for a single target or upload a list of targets. The following screenshots demonstrate how to conduct some common search tasks using the Form.

Once you have your search results, you may download the files for a target bundled in a tarball, interactively plot the extracted lightcurve for any of the aperture sizes/shapes, or visit the individual target webpages that contain links to download the extracted lightcurves along with the diagnostic plots for easy viewing. The following screenshot demonstrates how to perform these actions.

The interactive plotter allows you to plot one or more target lightcurves with zoom capability. You can also select different photometric apertures to preview, or compare the original and corrected fluxes. The following screenshot demonstrates some of the more common plot functions.