Description

This routine takes MIRI or NIRSPEC IFU calibrated 2D slope images and produces 3-D spectral cubes. In this cube_build routine the IFU slice distorted and disjointed 2D spectra are corrected for distortion and put back together into a rectangular cube with three orthogonal axes, two spatial and one spectral with regular sampling in the three axes. The disortion information should have been incorporated into the calibrated images using the latest assign_wcs pipeline step.

The cube_build package can take as input either:

  • a single 2D input image updated by assign_wcs
  • a model passed in containing 2D slope image
  • an association table (in json format) containing the list of exposure to combine

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 a cube. See the Arguments section in the documentation for more details.

Assumption

It is assumed the assign_wcs step has been run on the data, attaching the distortion and pointing information to the image. It is also assumed that the input data is from the MIRI or NIRSPEC IFU.

Background Information

The JWST integral field spectrographs obtain simultaneous spectral and spatial data on a relatively compact region of the sky. The MIRI Medium Resolution Spectrograph (MRS) consists of four integral field units providing four simultaneous and overlapping fields of view ranging from 3.7” X 3.7” to ~7.7” X 7.7” covering a wavelength range from 5 to 28 microns. The optics system for the four IFUs is split into two paths. One path is dedicated to the two short wavelength IFUs and the other one handles the two longer wavelength IFUs. There is one 1024 X 1024 SCA 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 SCA. 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 to 28 micron spectrum is then 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 of the channel where each sub-channel covers ~1/3 of the full spectrum for the channel. Each of the four channels have 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 seperate gratings are employed in combination with specific filters in order to avoid the overlapping of spectra caused by different grating orders. The three grating 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 microns to 5 microns, while a prism yields lower-reolution (R = 30-300) spectroscopy over the range 0.6 microns to 5 microns.

The NIRSPEC detector focal plan consists of two HgCdTe sensor chip assemblies (SCAs). Each chip is a 2D array of 2048 X 2048 pixels. The light-sensitive poritions of the two SCAS are separated by a physical gap of 3.144 mm which corresponds to 17.8 arc seconds 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

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 or 4.

  • Sub-Channel each of the 3 sub-ranges that a channel is divided into. We will designate these as Short, Medium, or Long.

  • Band

    For MIRI band is one of the 12 sub-ranges the spectral range of the MRS can be divided, each band has unique channel/sub-channel combination, i.e., the shortest wavelength range on MIRI is covered by Band 1-SHORT and the longest is covered by Band 4-LONG.

    For NIRSPEC is defined by a single grating-filter combination, i.e. G140M-F070LP

NIRSPEC Optical Element and Filter possibilities for IFU mode:

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

Coordinate Systems:

An integral field spectrograph measures the intensity of in 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: is 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 the MIRI IFUs disperse data onto two SCAs.
  • Telescope (V2,V3): the V2,V3 coordinates locate points on a spherical coordinate system. The frame is tied to JWST and applies to the whole field of view, encompassing all the instruments. The coordinate (V2,V3) are Euler angles in a spherical frame rather than Cartesian. The transformation between the V2-V3 and MRS-FOV system is fixed mission and is determined during ground testing.
  • XAN,YAN: like V2,V3 but flipped and shifted so the origin lies between the NIRCAM detectors. Note what OSIM and
    OTE call ‘V2,V3’ are actually XAN,YAN.
  • Absolute is the standard astronomical equatorial system of Ra-Dec.
  • Cube is a three dimensional system with two spatial axes and one spectral axis.
  • MRS-FOV this a MIRI specific system which is the angular coordinate system attached to the FOV of each MRS band. There are twelve MRS-FOV systems for MIRI, since 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.

Output

The input to cube build can be a single exposure or a set of exposures. There are a number of user options that control the type of IFU Cube to create. If no options are provided then all the data provided in the input will be used to create the final cube. In the case of MIRI that means that if the input is a single exposure both channels will be used to construct the IFU cube. If the input file is an association containing twelve MIRI exposures covering the four channels and three sub-channels and no user options are used then the final cube will be an uber cube containing all the data. In the case of NIRSPEC only exposures from the same resolution will be combined in an IFU Cube, therefore, association tables will contain NIRSPEC IFU exposures of the same resolution.

Below is a list of the user options that can be used to select the type of data to be used to create the IFU Cube:

  • --channel #

This is a MIRI only option and the only valid values for # are 1,2,3,4, or ALL. If the channel argument is given, then only data corresponding to that channel will be used in constructing the cube. If the user wants more than one channel to make cube, then all the values are contained in a comma separated string string. For example, to create a cube with channel 1 and 2 the argument list is --channel='1, 2'. If this value is not specified then all the channels contained in the input will be used in constructing the cube.

  • --band [string]

This is a MIRI option and the only valid values are SHORT,MEDIUM,LONG, or ALL. If the band argument is given, then only data corresponding to that subchannel will be used in constructing the cube. Only one option is possible, so IFU cubes are created either per subchannel or using all the subchannels the input data cover. If this value is not specified then all the subchannels contained in the input list of files will be used in constructing the cube. Note we used band instead of subchannel, because the keyword band in the science fits is used to denote which MIRI subchannel the data covers.

  • --weighting ['string]

This is for MIRI data and the only valid values are STANDARD and MIRPSF. This option defines how the distances between the point cloud members and spaxel centers are determined. The default value is STANDARD and the distances are determined in the cube output coordinate system. If this paramter is set to MIRIPSF then the distances are determined in the alpha-beta coordinate system of the point cloud member and are normalized by the PSF and LSF.

  • --grating [string]

This is a NIRSPEC option and only valid values are PRISM, G140M, G140H, G235M, G235H, G395M, G395H, or ALL. If the option ALL is used then all the gratings in the assocation are used. Since association tables will only contain exposures of the same resolution, the use of ALL, will at most combine data from grating G140M, G235M & G395M or G140H, G235H & G395H together. The user can supply a comma separated string containing the gratings to use.

  • --filter [string]

This is a NIRSPEC option and the only valid options are Clear, F100LP, F070LP, F170LP, F290LP, or ALL. To cover the full wavelength range of NIRSPEC the option ALL can be used (provided the exposures in the association table contain all the filters). The user can supply a comma separated string containing the filters to use.

Output Product

If the input is passed as an Image Model then the IFU cube will be passed back as an IFU cube model. If the input is passed as a filename or association table then an output IFU cube will be written to disk. In these cases the output name is based on a rootname plus a string defining the type of IFU cube created plus the string ‘s3d.fits’. If the input data is a single exposure then the rootname is formed from the input filename; while if the input is an association table the rootname is defined in the assocation table. The string defining the type of IFU is created according to the following rules:

  • for MIRI the string is determined from the channels and subchannels 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 subchannels were used then the output name would be rootname_ch-1-2_ALL_s3d.fits.
  • for NIRSPEC the string is determined from the gratings and filters used. The gratings are grouped together in a dash (-) separted string and likewize for the gratings. 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

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 cube. The output cube is defined using the WCS information of all included the 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 cube sample size for the three dimensions is either determined from defaults or set by the user. Each MIRI channel or NIRSPEC grating setting has a predefined scale to use for each dimension. In the case of MIRI - if the data consists of more than one channel of data - 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 in the output cube 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 in one rebinning step to map a pixel from the the detector to 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 irregulary spaced “cloud of points” in the cube coordinate system. A schematic of this process is shown in Figure 1. Two dithered exposures are mapped the output coordinate system. The detector pixels from the first exposure are shown in black, while the detector pixels from the second exposure are shown in red.

../../_images/pointcloud.png

Schematic of two exposures mapped to the IFU output coordinate system. The point cloud shown by the plus symbols are the detector pixels mapped to the output coordinate system. The black points are from exposure one and the red points are from exposure two.

Each point in the cloud contains information of the flux of the original detector pixel and error of this flux. The final flux that is derived for each cube pixel (spaxel) is a combination of all the point cloud values with a specified region of interest from the center of the spaxel. How to best combine the point cloud values into a final flux is an on-going process. The current method uses a weighting function based on the distance between the center of spaxel center and point cloud member. For MIRI the weighting function also depends on the width of the PSF and LSF. 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. For NIRSPEC the distances between the spaxel center and point cloud member are determined in the final cube coordinate system.

  • 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

Additional constraints for MIRI (if the –weighting=MIRIPSF) If the These distances are determined in the alpha - beta system from where the point cloud value orginated. We want to combine many points -possibly coming from a variety of bands- together. To apply the correct weighting to these points we normalize the distance between the cube spaxel and point cloud value by the PSF and the LSF which where defined in the alpha-beta coordinate system. We therefore, transform the cube spaxel coordinates to each alpha-beta system that is found within the region of interset. The xdistance is the distance between the point cloud and spaxel center in the alpha dimension and the ydistance is determined in the beta dimension.

  • xnorm width of the PSF in the alpha dimension in units of arc seconds
  • ynorm width of the PSF in the beta dimension in units of arc seconds
  • znorm width of LSF in lambda dimension in units of microns
  • xn = xdistance/xnorm
  • yn = ydistance/ynorm
  • zn = zdistance/znorm

For NIRSPEC (and for MIRI data when –weight=’STANDARD’ the distances are determined in the output cube coordinate system)

  • xn = xdistance/spaxel size along axis 1
  • yn = ydistance/spaxel size along axis 2
  • zn = zdistance/spaxel size along the wavelenght axis.

Define n to be the number of point cloud members within the region of interest of a given spaxel.

For each spaxel find the n points in the cloud what fall within the region of interest. The size of the region of interesting is set by Radius_X, Radius_Y and Radius_Z and determining the best set of radi is an on-going stufy. Using these n points calculated the

The spaxel flux K = \frac{ \sum_{i=1}^n Flux_i w_i}{\sum_{i=1}^n w_i}

Where

w_i = \sqrt{({xn}^2 + {yn}^2 + {zn}^2)}

w_i = {w_i}^{-p}

The default value for the p is 2. The optiminal choice of this value is still TBD, but one should consider the degree of smoothing desired in the interpolation, the density of the point cloud elements, and the region of interest when chosing the value.