Astrocut File Formats
FITS Cutout Files
FITS files output by fits_cut consist of a PrimaryHDU extension
and one or more ImageHDU extensions, each containing a single cutout.
PRIMARY PrimaryHDU (Extension 0)
Keyword |
Value |
|---|---|
SIMPLE |
T (conforms to FITS standard) |
BITPIX |
8 (array data type) |
NAXIS |
0 (number of array dimensions) |
EXTEND |
Number of standard extensions |
ORIGIN |
STScI/MAST |
DATE |
File creation date |
PROCVER |
Software version |
RA_OBJ |
Center coordinate right ascension (deg) |
DEC_OBJ |
Center coordinat declination (deg) |
CHECKSUM |
HDU checksum |
DATASUM |
Data unit checksum |
CUTOUT ImageHDU (Subsequent extension(s))
The data in each CUTOUT extension is the cutout image. The header includes all of the
keywords from the extension that the cutout image was drawn from, with WCS keywords
updated to match the cutout image. Additionally the keyword ORIG_FLE has been added,
it contains the name of the file the cutout comes from.
Cube Files
See the TESS Science Data Products Description Document for detailed information on the TESS full-frame image file format.
PrimaryHDU (Extension 0)
The Primary Header of the TESS cube fits file is the same as that from an individual FFI with the following exceptions:
Keyword |
Value |
|---|---|
ORIGIN |
STScI/MAST |
DATE |
Date the cube was created |
CAMERA |
From the ImageHDU (EXT 1) of an FFI |
CCD |
From the ImageHDU (EXT 1) of an FFI |
Sector |
The TESS observing sector, passed by the user |
DATE-OBS |
From the ImageHDU (EXT 1) of the sector’s first FFI |
DATE-END |
From the ImageHDU (EXT 1) of the sector’s last FFI |
TSTART |
From the ImageHDU (EXT 1) of the sector’s first FFI |
TSTOP |
From the ImageHDU (EXT 1) of the sector’s last FFI |
ImageHDU (Extension 1)
The ImageHDU extension contains the TESS FFI datacube. It is 4 diminsional, the two spacial dimensions, time, and data vs. error flux values. Pixel valules are 32 bit floats. The cube dimensions are ordered in the FITS format as follows:
Keyword |
Value |
|---|---|
NAXIS |
4 (number of array dimensions) |
NAXIS1 |
2 (data value, error value) |
NAXIS2 |
Total umber of FFis |
NAXIS3 |
Length of first array dimension (NAXIS1 from FFIs) |
NAXIS4 |
Length of second array dimension (NAXIS2 from FFIs) |
BinTableHDU (Extension 2)
The BinTableHDU extension contains a table that holds all of the Image extension header keywords from the individual FFIs. There is one column for each keyword plus one additional column called “FFI_FILE” that contains FFI filename for each row. Each column name keyword also has an entry in the extension header, with the value being the keyword comment from the FFI header. This last allows the FFI Image extension headers to be reacreated completely if desired.
Target Pixel Files
The Astrocut target pixel file (TPF) format conforms as closely as possible to the TESS mission TPFs. See the TESS Science Data Products Description Document for detailed information on the TESS mission target pixel file format, here I describe how Astrocut TPFs differ from mission pipeline TPFs.
PRIMARY PrimaryHDU (Extension 0)
The Primary Header of an Astrocut TPF is the same as that from a mission TPF with the following exceptions:
Keyword |
Value |
|---|---|
ORIGIN |
STScI/MAST |
CREATOR |
astrocut |
PROCVER |
Astrocut version |
SECTOR |
Depends on this value having been filled in the cube |
Mission pipeline header values Astrocut cannot populate |
|
OBJECT |
“” |
TCID |
0 |
PXTABLE |
0 |
PMRA |
0.0 |
PMDEC |
0.0 |
PMTOTAL |
0.0 |
TESSMAG |
0.0 |
TEFF |
0.0 |
LOGG |
0.0 |
MH |
0.0 |
RADIUS |
0.0 |
TICVER |
0 |
TICID |
None |
PIXELS BinTableHDU (Extension 1)
The Astrocut PIXELS BinTableHDU extension comprises the same columns as that from
the mission pipeline, with one addition. The extra column is FFI_FILE and contains
the name of the FFI file that that row’s pixels come from.
While all of the columns present in mission pipeline TPFs are present, they do not all contain data. The columns that are empty in Astrocut TPFs are:
Column |
Value |
|---|---|
CADENCENO |
0 filled array in cutout shape |
RAW_CNTS |
-1 filles array in cutout shape |
FLUX_BKG |
0 filled array in cutout shape |
FLUX_BKG_ERR |
0 filled array in cutout shape |
POS_CORR1 |
0 |
POS_CORR2 |
0 |
The TIME column is formed by taking the average of the TSTART and TSTOP values
from the corresponding FFI for each row. The QUALITY column is taken from the DQUALITY
image keyword in the individual FFI files.
Three keywords have also been added to the PIXELS extension header to give additional information about the cutout world coordinate system (WCS). TESS FFIs are large and therefore are described by WCS objects that have many non-linear terms. Astrocut creates a new simpler (linear) WCS object from the matched set of cutout pixel coordinates and sky coordinates (from the FFI WCS). This linear WCS object will generally work very well, however at larger cutout sizes (100-200 pixels per side and above) the linear WCS fit will start to be noticeably incorrect at the edges of the cutout. The extra keywords allow the user to determine if the linear WCS is accurate enough for their purpose, and to retrieve the original WCS with distortion coefficients if it is needed.
Keyword |
Value |
|---|---|
WCS_FFI |
The name of the FFI file used to build the original WCS
from which the cutout and cutout WCS were calculated.
|
WCS_MSEP |
The maximum separation in degrees between the cutout’s
linear WCS and the FFI’s full WCS.
|
WCS_SIG |
The error in the cutout’s linear WCS, calculated as
sqrt((dist(Po_ij, Pl_ij)^2) where dist(Po_ij, Pl_ij)is the angular distance in degrees between the sky position
of of pixel i,j in the original full WCS and the new linear
WCS.
|
APERTURE ImageHDU (Extension 2)
The APERTURE ImageHDU extension is similar to that of mission pipeline TPFs, but contains slightly different data. For mission pipeline files, the aperture image gives information about each pixel, whether it was collected and what calculations it was used in. Because astrocut does not do any of the more complex claculations used in the mission pipeline, each pixel in the aperture image will either be 1 (pixel was collected and contains data in the cutout) or 0 (pixel is off the edge of the detector and contains no data in the cutout).
Cosmic Ray Binary Table Extension
This extension is not present in Astrocut TPFs, although it is a part of mission pipeline TPFs.
Path Focused Target Pixel Files
When the center_on_path function is used to create cutout target pixel files (TPFs)
where the individualimage cutouts move along a path in time and space, the TPF format has to be
adjusted accordingly. It still conformes as closely as possible to the TESS mission pipeline TPF
file format, but differs in several cruicial ways. The center_on_path function works
on astrocut TPFs, so that is the baseline file format. I will describe here only the differences
between path focussed astrocut TPFs and regular astrocut TPFs (see Target Pixel Files for
regular Astrocut TPF format).
PRIMARY PrimaryHDU (Extension 0)
Additional or updated keywords:
Keyword |
Value |
|---|---|
DATE |
Set the the time the path focussed cutout was performed |
OBJECT |
Moving target object name/identifier, only present if set by the user |
Removed keywords:
Keyword |
Reason |
|---|---|
RA_OBJ |
Cutout is no longer centered on a sky position |
DEC_OBJ |
Cutout is no longer centered on a sky position |
PIXELS BinTableHDU (Extension 1)
Additional columns:
Column |
Value |
|---|---|
TGT_X |
X position of the target in the cutout array at row time |
TGT_Y |
Y position of the target in the cutout array at row time |
TGT_RA |
Right ascension (deg) of the target at row time |
TGT_DEC |
Declination (deg) of the target at row time |
No world coordinate system (WCS) information is present, since it is no longer common across all cutout images.
APERTURE ImageHDU (Extension 2)
The APERTURE extension may or may not be present in a path focussed TPF, to be present
the user must have passed an FFI WCS object into the center_on_path function.
The APERTURE ImageHDU extension of path focussed TPFs is very different from other TESS TPFs. The aperture image, instead of being the size and shape of an individeual cutout, is the size of the full FFI image the cutouts were drawn from. All pixels used in any individual cutout are marked with 1, while the rest of the pixels are 0, so the entire trajectory of the cutout path is captured. Additionally the WCS information in the header is the WCS for the original FFI, including all distortion coefficients. This can be used in combination with the TGT_RA/DEC and TGT_X/Y columns to trace the path of the target across the FFI footprint and calculate the WCS object for individual cutout images if necessary.