Description

The cube_build step takes MIRI or NIRSpec IFU calibrated 2-D images and produces 3-D spectral cubes. The 2-D disjointed IFU slice spectra are corrected for distortion and assembled into a rectangular cube with three orthogonal axes: two spatial and one spectral.

The cube_build step can accept several different forms of input data, including:

  • a single file containing a 2-D slice image

  • a data model (IFUImageModel) containing a 2-D slice image

  • an association table (in json format) containing a list of input files

  • a model container with several 2-D slice data models

There are a number of arguments the user can provide either in a configuration file or on the command line that control the sampling size of the cube, as well as the type of data that is combined to create the cube. See the Step Arguments section for more details.

Assumptions

It is assumed that the assign_wcs step has been applied to the data, attaching the distortion and pointing information to the image(s). It is also assumed that the photom step has been applied to convert the pixel values from units of countrate to surface brightness. This step will only work with MIRI or NIRSpec IFU data.

Instrument Information

The JWST integral field unit (IFU) spectrographs obtain simultaneous spectral and spatial data on a relatively compact region of the sky. The MIRI Medium Resolution Spectrometer (MRS) consists of four IFU’s providing four simultaneous and overlapping fields of view ranging from 3.3” x 3.7” to ~7.2” x 7.7” and covering a wavelength range of 5-28 microns. The optics system for the four IFU’s is split into two paths. One path is dedicated to the two short wavelength IFU’s and the other one handles the two longer wavelength IFU’s. There is one 1024 x 1024 detector for each path. Light entering the MRS is spectrally separated into four channels by dichroic mirrors. Each of these channels has its own IFU that divides the image into several slices. Each slice is then dispersed using a grating spectrograph and imaged on one half of a detector. While four channels are observed simultaneously, each exposure only records the spectral coverage of approximately one third of the full wavelength range of each channel. The full 5-28 micron spectrum is obtained by making three exposures using three different gratings and three different dichroic sets. We refer to a sub-channel as one of the three possible configurations (A/B/C) of the channel where each sub-channel covers ~1/3 of the full wavelength range for the channel. Each of the four channels has a different sampling of the field, so the FOV, slice width, number of slices, and plate scales are different for each channel.

The NIRSpec IFU has a 3 x 3 arcsecond field of view that is sliced into thirty 0.1 arcsecond bands. Each slice is dispersed by a prism or one of six diffraction gratings. When using diffraction gratings as dispersive elements, three separate gratings are employed in combination with specific filters in order to avoid the overlapping of spectra caused by different grating orders. The three gratings span four partially overlapping bands (1.0 - 1.8 microns; 1.7 - 3.0 microns; 2.9 - 5 microns) covering the total spectral range in four separate exposures. Six gratings provide high-resolution (R = 1400-3600) and medium resolution (R = 500-1300) spectroscopy over the wavelength range 0.7-5 microns, while the prism yields lower-resolution (R = 30-300) spectroscopy over the range 0.6-5 microns.

The NIRSpec detector focal plane consists of two HgCdTe sensor chip assemblies (SCAs). Each SCA is a 2-D array of 2048 x 2048 pixels. The light-sensitive portions of the two SCAs are separated by a physical gap of 3.144 mm, which corresponds to 17.8 arcseconds on the sky. For low or medium resolution IFU data the 30 slices are imaged on a single NIRSpec SCA. In high resolution mode the 30 slices are imaged on the two NIRSpec SCAs. The physical gap between the SCAs causes a loss of spectral information over a range in wavelength that depends on the location of the target and dispersive element used. The lost information can be recovered by dithering the targets.

Terminology

MIRI Spectral Range Divisions

We use the following terminology to define the spectral range divisions of MIRI:

Channel

The spectral range covered by each MIRI IFU. The channels are labeled as 1, 2, 3 and 4.

Sub-Channel

The 3 sub-ranges that a channel is divided into. These are designated as Short (A), Medium (B), and Long (C).

Band

For MIRI, “band” is one of the 12 contiguous wavelength intervals (four channels times three sub-channels each) into which the spectral range of the MRS is divided. Each band has a unique channel/sub-channel combination. For example, the shortest wavelength range on MIRI is covered by Band 1-SHORT (aka 1A) and the longest is covered by Band 4-LONG (aka 4C).

NIRSpec IFU Disperser and Filter Combinations

Grating

Filter

Wavelength (microns)

Prism

Clear

0.6 -5.3

G140M

F070LP

0.7 - 1.2

G140M

F100LP

1 - 1.8

G235M

F170LP

1.7 - 3.1

G395M

F290LP

2.9 - 5.2

G140H

F070LP

0.7 - 1.2

G140H

F100LP

1 - 1.8

G235H

F170LP

1.7 - 3.1

G395H

F290LP

2.9 - 5.2

For NIRSpec we define a band as a single grating-filter combination, e.g. G140M-F070LP.

Coordinate Systems

An IFU spectrograph measures the intensity of a region of the sky as a function of wavelength. There are a number of different coordinate systems used in the cube building process. Here is an overview of these coordinate systems:

Detector System

Defined by the hardware and presents raw detector pixel values. Each detector or SCA will have its own pixel-based coordinate system. In the case of MIRI we have two detector systems because the MIRI IFUs disperse data onto two detectors.

Telescope (V2,V3)

The V2,V3 coordinates locate points on a spherical coordinate system. The frame is tied to the JWST focal plane and applies to the whole field of view, encompassing all the instruments. The V2,V3 coordinates are Euler angles in a spherical frame rather than Cartesian coordinates.

XAN,YAN

Similar to V2,V3, but flipped and shifted so the origin lies between the NIRCam detectors instead of at the telescope boresight. Note that what OSIM and OTE call ‘V2,V3’ are actually XAN,YAN.

Absolute

The standard astronomical equatorial RA/Dec system.

Cube

A three-dimensional system with two spatial axes and one spectral axis.

MRS-FOV

A MIRI-specific system that is the angular coordinate system attached to the FOV of each MRS band. There are twelve MRS-FOV systems for MIRI, because there are twelve bands (1A, 1B, 1C,… 4C). Each system has two orthogonal axes, one parallel (alpha) and the other perpendicular (beta) to the projection of the long axes of the slices in the FOV.

Types of Output Cubes

As mentioned above, the input data to cube_build can take a variety of forms, including a single file, a data model passed from another pipeline step, a list of files in an association table, or a collection of exposures in a data model container (ModelContainer) passed in by the user or from a preceding pipeline step. Because the MIRI IFUs project data from two channels onto a single detector, choices can or must be made as to which parts of the input data to use when constructing the output cube even in the simplest case of a single input image. The default behavior varies according to the context in which cube_build is being run.

In the case of the calwebb_spec2 pipeline, for example, where the input is a single MIRI or NIRSpec IFU exposure, the default output cube will be built from all the data in that single exposure. For MIRI this means using the data from both channels (e.g. 1A and 2A) that are recorded in a single exposure. For NIRSpec this means using data from the single grating+filter combination contained in the exposure.

In the calwebb_spec3 pipeline, on the other hand, where the input can be a collection of data from multiple exposures covering multiple bands, the default behavior is to create a set of single-band cubes. For MIRI, for example, this can mean separate cubes for bands 1A, 2A, 3A, 4A, 1B, 2B, …, 3C, 4C, depending on what’s included in the input. For NIRSpec this may mean multiple cubes, one for each grating+filter combination contained in the input collection.

Several cube_build step arguments are available to allow the user to control exactly what combinations of input data are used to construct the output cubes. See the Step Arguments section for details.

Output Cube Format

The output spectral cubes are stored in FITS files that contain 4 IMAGE extensions. The primary data array is empty and the primary header holds the basic parameters of the observations that went into making the cube. The 4 IMAGE extensions have the following characteristics:

EXTNAME

NAXIS

Dimensions

Data type

SCI

3

2 spatial and 1 spectral

float

ERR

3

2 spatial and 1 spectral

float

DQ

3

2 spatial and 1 spectral

integer

WMAP

3

2 spatial and 1 spectral

integer

The SCI image contains the surface brightness of cube spaxels in units of mJy/arcsecond^2. The ERR image contains the uncertainty on the SCI values, the DQ image contains the data quality flags for each spaxel, and the WMAP image contains the number of point cloud elements contained in the region of interest of the spaxel.

Output Product Name

If the input data is passed in as an ImageModel, then the IFU cube will be passed back as an IFUCubeModel. The cube model will be written to disk at the end of processing. The file name of the output cube is based on a rootname plus a string defining the type of IFU cube, along with the suffix ‘s3d.fits’. If the input data is a single exposure, then the rootname is taken from the input filename. If the input is an association table, the rootname is defined in the association table. The string defining the type of IFU is created according to the following rules:

  • For MIRI the output string name is determined from the channels and sub-channels used. The IFU string for MIRI is ‘ch’+ channel numbers used plus a string for the subchannel. For example if the IFU cube contains channel 1 and 2 data for the short subchannel, the output name would be, rootname_ch1-2_SHORT_s3d.fits. If all the sub-channels were used then the output name would be rootname_ch-1-2_ALL_s3d.fits.

  • For NIRSpec the output string is determined from the gratings and filters used. The gratings are grouped together in a dash (-) separated string and likewise for the filters. For example if the IFU cube contains data from grating G140M and G235M and from filter F070LP and F100LP, the output name would be, rootname_G140M-G225_F070LP-F100LP_s3d.fits

Algorithm

The default IFU Cubes contain data from a single band (channel/sub-channel or grating/filter). There are several options which control the type of cubes to create (see description given above). Based on the arguments defining the type of cubes to create, the program selects the data from each exposure that should be included in the spectral cube. The output cube is defined using the WCS information of all the included input data. This output cube WCS defines a field-of-view that encompasses the undistorted footprints on the sky of all the input images. The output sampling scale in all three dimensions for the cube is defined by a ‘cubepars’ reference file as a function of wavelength, and can also be changed by the user. The cubepars reference file contains a predefined scale to use for each dimension for each band. If the output IFU cube contains more than one band, then for MIRI the output scale corresponds to the channel with the smallest scale. In the case of NIRSpec only gratings of the same resolution are combined together in an IFU cube. The output spatial coordinate system is right ascension-declination.

All the pixels on each exposure that are included are mapped to the cube coordinate system. This input-to-output pixel mapping is determined via a mapping function derived from the WCS of each input image and the WCS of output cube. The mapping process corrects for the optical distortions and uses the spacecraft telemetry information to map each pixel location to its projected location in the cube coordinate system. The mapping is actually a series of chained transformations (detector -> alpha-beta-lambda), (alpha-beta-lambda -> v2-v3-lambda), (v2-v3-lambda - > right ascension-declination-lambda), and (right ascension-declination-lambda -> Cube coordinate1-Cube Coordinate2-lambda). The reverse of each transformation is also possible.

The mapping process results in an irregular spaced “cloud of points” that sample the specific intensity distribution at a series of locations on the sky. A schematic of this process is shown in Figure 1.

../../_images/pointcloud.png

Figure 1: Schematic of two dithered exposures mapped to the IFU output coordinate system (black regular grid). The plus symbols represent the point cloud mapping of detector pixels to effective sampling locations relative to the output coordinate system at a given wavelength. The black points are from exposure one and the red points are from exposure two.

Each point in the cloud represents a measurement of the specific intensity (with corresponding uncertainty) of the astronomical scene at a particular location. The final data cube is constructed by combining each of the irregularly-distributed samples of the scene into a regularly-sampled grid in three dimensions for which each spaxel (i.e., a spatial pixel in the cube) has a spectrum composed of many spectral elements.

The best algorithm with which to combine the irregularly-distributed samples of the point cloud to a rectilinear data cube is the subject of ongoing study, and depends on both the optical characteristics of the IFU and the science goals of a particular observing program. At present, the default method uses a flux-conserving variant of Shepards method in which the value of a given element of the cube is a distance-weighted average of all point-cloud members within a given region of influence. In order to explain this method we will introduce the follow definitions:

  • xdistance = distance between point in the cloud and spaxel center in units of arc seconds along the x axis

  • ydistance = distance between point in the cloud and spaxel center in units of arc seconds along the y axis

  • zdistance = distance between point cloud and spaxel center in the lambda dimension in units of microns along the wavelength axis

These distances are then normalized by the IFU cube sample size for the appropriate axis:

  • xnormalized = xdistance/(cube sample size in x dimension [cdelt1])

  • ynormalized = ydistance/(cube sample size in y dimension [cdelt2])

  • znormalized = zdistance/(cube sample size in z dimension [cdelt3])

The final spaxel value at a given wavelength is determined as the weighted sum of the point cloud members with a spatial and spectral region of influence centered on the spaxel. The default size of the region of influence is defined in the cubepar reference file, but can be changed by the user with the options: rois and roiw.

If n point cloud members are located within the ROI of a spaxel, the spaxel flux K = \(\frac{ \sum_{i=1}^n Flux_i w_i}{\sum_{i=1}^n w_i}\)

where

\(w_i =\frac{1.0} {\sqrt{({xnormalized}_i^2 + {ynormalized}_i^2 + {znormalized}_i^2)^{p} }}\)

The default value for p is 2, although the optimal choice for this value (along with the size of the region of influence and the cube sampling scale) is still under study. Similarly, other algorithms such as a 3d generalization of the drizzle algorithm are also being studied and may provide better performance for some science applications.

Additional constraints for weighting=MIRIPSF

For MIRI the weighting function can be adapted to use the width of the PSF and LSF in weighting the point cloud members within the ROI centered on the spaxel. The width of the MIRI PSF varies with wavelength, broader for longer wavelengths. The resolving power of the MRS varies with wavelength and band. Adjacent point-cloud elements may in fact originate from different exposures rotated from one another and even from different spectral bands. In order to properly weight the MIRI data the distances between the point cloud element and spaxel the distances are determined in the alpha-beta coordinate system and then normalized by the width of the PSF and the LSF. To weight in the alpha-beta coordinates system each cube spaxel center must be mapped to the alpha-beta system corresponding to the channel-band of the point cloud member. The xdistance and ydistances are redefined to mean:

  • xdistance = distance between point in the cloud and spaxel center along the alpha dimension in units of arc seconds

  • ydistance = distance between point in the cloud and spaxel center along the beta dimension in units of arc seconds

  • zdistance = distance between point cloud and spaxel center in the lambda dimension in units of microns along the wavelength axis

The spatial distances are then normalized by PSF width and the spectral distance is normalized by the LSF:

  • xnormalized = xdistance/(width of the PSF in the alpha dimension in units of arc seconds)

  • ynormalized = ydistance/(width of the PSF in the beta dimension in units of arc seconds)

  • znormalized = zdistance/( width of LSF in lambda dimension in units of microns)