Mission Overview

A PSF-Based Approach To Kepler/K2 Data (PSFK2)


Primary Investigator: Mattia Libralato

HLSP Authors: Mattia Libralato

Released: 2020-09-22

Updated: 2020-09-22

Primary Reference(s): Libralato et al. 2016aLibralato et al. 2016bNardiello et al. 2016

DOI: 10.17909/t9-1a6b-jk76

Citations: See ADS Statistics

Read Me

Example heartbeat star using PSFK2 photometry.


The Kepler/K2 mission offered to the science community the opportunity of obtaining high-precision light curves for a large variety of stellar fields, including stellar clusters. However, most Kepler/K2 data analyses reported in the literature are based on aperture photometry. Aperture photometry is perfectly suitable to investigating stars in sparse fields but it suffers from severe limitations in crowded environments like the central regions of stellar clusters. Thus, the wealth of information included in stellar clusters analyzed with the Kepler/K2 mission is often unexplored. The team has made use of their experience with undersampled Hubble Space Telescope images and developed a new method to analyze crowded regions with the Kepler/K2 data.

The combination of a high-angular-resolution catalog and accurate point-spread-function (PSF) models allows them to pinpoint a star in a Kepler/K2 exposure and measure its flux after all detectable nearby stars are PSF-subtracted from the image. This PSF-based technique: (i) increases the number of analyzable objects in the field, (ii) provides an unbiased flux measurement for each source, (iii) extracts stellar light curves in a crowded environment and (iv) improves the reachable photometric precision for faint stars. This technique is designed to exploit the huge potential offered by the "super-stamps", but it is also perfectly suitable to analyze single, isolated stamps. The team releases the light curves for stars in the open clusters M35 and NGC2158 observed during Campaign 0, and M44 and M67 in Campaign 5. They also provide the high-angular-resolution input catalogs used in their works, the lists of the variable stars identified (only for M 44 and M67 clusters) and the Kepler/K2 stacked images for each cluster.

Data Products

The files are sorted into subdirectories based on the following convention:



  • <campaign> = The Campaign the data come from, as a zero-padded, two-digit number prefixed with a "c". Example: "c00" for Campaign 0.
  • <cluster> = Name of the cluster (main object of interest), one of "m35", "m44", or "m67".  Note this level contains the catalog files, image stacks, and README files.
  • <channel> = The two-digit, zero-padded channel an object lands on, e.g., "47" for Channel 47.  Note that only the light curve files are in this level. 

Data file naming convention for stack images:



  • <target> = Name of target, one of "m35ngc2158", "m44", or "m67".
  • <ch> = Channel number, e.g., "81" for Channel 81.

Data file naming convention for catalog files:



  • <target> = Name of target, one of "m35ngc2158", "m44", "m67-ch13", or "m67-ch14". Note that Campaign 5 M67 splits the catalog by channels, but Campaign 5 M44 does not.
  • <type> = Type of catalog, "catalog.fits" for the catalog of all sources, and "catalog-var" for the variable sources.
  • <ext> = Extension type, either "fits" or "txt" (each catalog is avaialble in both FITS and text format).

Data file naming convention for light curve files:



  • <number> = the PSFK2 target identifier, as a 6-digit, zero-padded number
  • <ext> = type of light curve file, either "fits" or "txt"

Data file naming convention for README files:



  • <campaign> = Campaign number as a 2-digit, zero-padded number prefixed with "c".  Example: "c00" for Campaign 0.
  • <target> = Name of primary target, either "m35", "m44", or "m67".

Data file types:

_llc.fits,  _llc.txt Light curve file for a target, either FITS or text format.
_readme.txt README file for a given cluster.
_stack.fits Stack image.
_catalog.fits Catalog of all sources.
_catalog-var.fits Catalog of variable sources.

Data Access

PSFK2 light curves are available in the MAST Portal (web-based, cross-mission search interface) and Astroquery (Python package to search for and download files from Python scripts you write).  For those who want to download all K2SFF light curves for a given cluster, we provide bulk download scripts in the table below.

Link to the MAST Portal Webpage

MAST Portal

A web-based interface for cross-mission searches of data at MAST or the Virtual Observatory. Download PSFK2 light curves for a few targets.

MAST Astroquery

Search for, and retrieve, PSFK2 data products programmatically based on a list of coordinates or target names.
Cluster Campaign Bulk Download Script Stack Images (FITS) Catalog Files README File (TXT)
M35 0 M35 CH81 All: (FITS | TXT) | Var: (FITS | TXT) README
M44 5 M44 CH45 | CH46 | CH47 | CH48 All: (FITS | TXT) | Var: (FITS | TXT) README
M67 5 M67 CH13 | CH14 CH13 All: (FITS | TXT) | CH13 Var: (FITS | TXT) CH14 All: (FITS | TXT) | CH14 Var: (FITS | TXT) README



Please remember to cite the appropriate paper(s) below and the DOI if you use these data in a published work. 

Note: These HLSP data products are licensed for use under CC BY 4.0.


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