k2-dataset/s3://stpubdata/k2/public/ffi

K2 Full Frame Image Files (FFIs)

The FFIs contain values for every pixel in each of the 
operational output channels. Nominally 2 FFIs are taken each
campaign and are used to confirm the proper orientation 
of the detector on the sky. The files are stored in FITS 
format with each file containing 84 image extensions, that is, 
one extension for each channel. Currently 2 of the 21 modules 
have failed resulting in 8 channels (i.e., channels 5-8, and 
17-20) containing null values.  
See the Kepler Archive Manual for more information on FFIs 
at archive.stsci.edu/kepler/manuals/archive_manual.pdf.
Updated information can also be found in the K2 Data Release Notes
(archive.stsci.edu/k2/data_release.html).

The project produces a calibrated FFI file 
(e.g., ktwo2014203150825-c01_ffi-cal.fits)
an uncertainty FFI file 
(e.g., ktwo2014203150825-c01_ffi-uncert.fits)
and a raw FFI file (e.g., ktwo2017162122209-c14_ffi-orig.fits).
The uncertainty file contains the propagated uncertainties 
on the flux for each pixel in the calibrated FFI.
The raw files, which became public in February 2018, 
are uncalibrated and do not contain WCS parameters. Later,
the mission provided software that added WCS parameters to
the raw FFIs, which are available as "_ffi-orig2.fits" files.
The only difference between these and the "_ffi-orig.fits" files
should be the updated WCS solution based on the processed FFIs.

Note the raw FFI names change when the calibrated versions
are created. Initially the names do not include the campaign number.
For example, the initial FFI for C15 is kplr2017246053350_ffi-orig.fits
and for C16 they are kplr2017344214411_ffi-orig.fits and 
kplr2018030100110_ffi-orig.fits These are the only FFIs now online 
for C15 and C16. More documentation should be available soon.

Because the intial calibrated K2 FFIs did not contain the
World Coordinate System (WCS) information, the early K2 campaigns
also have a fourth file type with file names beginning with "wcs-"
(e.g., wcs-ktwo2014070234206-c00_ffi-cal.fits). These files were
produced at MAST by submitting the existing FFI cal files to 
astrometry.net (see http://astrometry.net/summary.html). By 
detecting recognized stars, astrometry.net is able to produce
the necessary WCS FITS keywords and add them to each FITS extension. 
(The WCS values describe the transformation between image coordinates 
and sky coordinates and allows one to determine the coordinates of any 
object in the image.)  These have been moved to the old_release_files/
directory, now that the mission has provided FFIs with WCS for these
older Campaigns.

Starting in July, 2018 the project began reprocessing data from earlier campaigns
starting with Campaign 13. This meant new 'cal' and 'uncert' FFIs. The previous 
versions however are saved online in the old_release_files subdirectory.