Astrocut Documentation¶
Introduction¶
Astrocut contains tools for creating image cutouts from sets images with shared footprints. This package is under active development, and will ultimately grow to encompass a range of cutout activities relevant to images from many missions, however at this time it is focused on the specific problem of creating image cutouts from sectors of TESS full frame images (FFIs).
There are two parts to this package, the CubeFactory class
allows you to create a large image cube from a list of FFI files.
This is what allows the cutout operation to be performed efficiently.
The CutoutFactory class performs the actual cutout and builds
a target pixel file (TPF) that is compatible with TESS pipeline TPFs.
Getting Started¶
The basic workflow is to first create an image cube from individual FFI files (this is one-time work), and then make individual cutout TPFs from this large cube file. If you are doing a small number of cutouts, it may make sense for you to use our tesscut web service: mast.stsci.edu/tesscut
Making image cubes¶
Making an image cube is a simple operation, but comes with a vert important limitation:
Warning
Memory Requirements
The entire cube file must be able to fit in your computer’s memory!
For a sector of TESS FFI images from a single camera/chip combination this is ~50 GB.
This operation can also take some time to run. For the 1348 FFI images of the TESS ete-6 simulated sector, it takes about 12 minutes to run on a computer with 65 GB of memory.
By default make_cube runs in verbose mode and prints out it’s progress, however setting verbose to false will silence all output.
>>> from astrocut import CubeFactory
>>> from glob import glob
>>> from astropy.io import fits
>>>
>>> my_cuber = CubeFactory()
>>> input_files = glob("data/*ffic.fits")
>>>
>>> cube_file = my_cuber.make_cube(input_files)
Completed file 0
Completed file 1
Completed file 2
.
.
.
Completed file 142
Completed file 143
Total time elapsed: 46.42 sec
File write time: 8.82 sec
>>> print(cube_file)
img-cube.fits
>>> cube_hdu = fits.open(cube_file)
>>> cube_hdu.info()
Filename: img-cube.fits
No. Name Ver Type Cards Dimensions Format
0 PRIMARY 1 PrimaryHDU 28 ()
1 1 ImageHDU 9 (2, 144, 2136, 2078) float32
2 1 BinTableHDU 302 144R x 147C [24A, J, J, J, J, J, J, D, 24A, J, 24A, 24A, J, J, D, 24A, 24A, 24A, J, D, 24A, D, D, D, D, 24A, 24A, D, D, D, D, D, 24A, D, D, D, D, J, D, D, D, D, D, D, D, D, D, D, D, D, J, J, D, J, J, J, J, J, J, J, J, J, J, D, J, J, J, J, J, J, D, J, J, J, J, J, J, D, J, J, J, J, J, J, D, J, J, J, J, J, J, J, J, 24A, D, J, 24A, 24A, D, D, D, D, D, D, D, D, J, J, D, D, D, D, D, D, J, J, D, D, D, D, D, D, D, D, D, D, D, D, 24A, J, 24A, 24A, J, J, D, 24A, 24A, J, J, D, D, D, D, J, 24A, 24A, 24A]
Making cutout target pixel files¶
To make a cutout, you must already have an image cube to cut out from.
Assuming that that step has been completed, you simply give the central
coordinate and cutout size (in either pixels or angular Quanitity)
to the cube_cut function.
You can either specify a target pixel file name, or it will be built as: “<cube_file_base>_<ra>_<dec>_<cutout_size>_astrocut.fits”. You can optionally also specify a output path, the directory in which the target pixel file will be saved, if unspecified it defaults to the current directory.
>>> from astrocut import CutoutFactory
>>> from astropy.io import fits
>>>
>>> my_cutter = CutoutFactory()
>>> cube_file = "img-cube.fits"
>>>
>>> cutout_file = my_cutter.cube_cut(cube_file, "251.51 32.36", 5, verbose=True)
Cutout center coordinate: 251.51,32.36
xmin,xmax: [26 31]
ymin,ymax: [149 154]
Image cutout cube shape: (144, 5, 5)
Uncertainty cutout cube shape: (144, 5, 5)
Target pixel file: img_251.51_32.36_5x5_astrocut.fits
Write time: 0.016 sec
Total time: 0.18 sec
>>> cutout_hdu = fits.open(cutout_file)
>>> cutout_hdu.info()
Filename: img_251.51_32.36_5x5_astrocut.fits
No. Name Ver Type Cards Dimensions Format
0 PRIMARY 1 PrimaryHDU 42 ()
1 PIXELS 1 BinTableHDU 222 144R x 12C [D, E, J, 25J, 25E, 25E, 25E, 25E, J, E, E, 38A]
2 APERTURE 1 ImageHDU 45 (5, 5) float64
A note about the cutout WCS object¶
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. Three header keywords have been added to the PIXELS extension to give additional information about the cutout WCS:
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.
