JWST ETC Instrument Throughputs
Throughputs for all 4 JWST science instruments can be calculated with the Pandeia Engine using files found in the "pandeia_data" directory.
An important component of the Pandeia Engine download is the data directory, referred to as "pandeia_data". This directory contains all the data files necessary for running calculations in Pandeia engine calculations. These data are also identical to those used in calculations with the JWST Exposure Time Calculator (ETC) web application. These data may also be used to compute throughputs for the various instruments and observing modes.
Instructions for downloading and installing the Pandeia engine and the "pandeia_data" ($pandeia_refdata) directory are found in the Installing Pandeia article.
All files relevant for calculating throughputs for the 4 JWST science instruments are found in the "jwst" folder of "pandeia_data". The subdirectories in this folder are "background", "detector", "miri", "nircam", "niriss", "nirspec", "source", and "telescope". Relevant subdirectories in each of the instrument folders are "filter", "optical", and "qe".
Users interested in throughputs for the various instrument modes should use the
get_total_eff() function included in the Pandeia Engine. This function considers all files for relevant instrument elements and properties (listed in the sections below) and outputs the throughput across a desired wavelength range. Below is a code snippet showing the use of the
- Getting the correct total throughput for the SOSS cross-disperser gr700xd requires setting
instrument_factory.order = 1(or
2) before running
The total throughput calculated using the procedure above does not include pupil losses, which can be significant for coronagraphic imaging and the NIRISS AMI mode. Pupil losses can be extracted from the PSF files using the FITS keyword
PUP_THRU. These pupil losses are achromatic, so can be ignored for color calculations, but should be considered for absolute photometry. The code snippet below, which is a continuation of the code above, shows how to account for pupil losses:
Example configuration dictionaries
For a more comprehensive list of instrument mode, disperser, and filter keys, see Pandeia Quickstart.
- The quantum efficiency (QE) files in this directory are not used by the ETC or the Pandeia engine. The relevant QE files for the science instruments are in the "/miri/qe", "/nircam/qe", "/niriss/qe", and "/nirspec/qe" subdirectories.
- As shown in the schematic in the MIRI Optics and Focal Plane article, the light path to the 4 different MRS channels is complex. Two separate wheels contain 3 dichroics for each of the 3 wavelength ranges (Short (A), Medium (B), and Long (C)). The blaze files in "/miri/blaze" have the transmission and reflection effects of the various dichroics incorporated into the files, in addition to the actual blaze functions of the dispersers themselves.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
- The "/miri/filters" subdirectory files for the F1065C, F1140C, and F1550C coronagraphic filters also include the transmission effects of the germanium coatings on the 4-quadrant phase masks (4QPMs) for coronagraphic imaging.
- The QE file for the MIRI imaging detector, "jwst_miri_imager_qe", is used for calculating throughputs for MIRI imaging, MIRI imaging time series, MIRI low resolution spectroscopy (LRS) slit, MIRI low resolution spectroscopy (LRS) slitless, and MIRI coronagraphic imaging.
- The MIRI MRS is composed of 2 separate detectors. The "jwst_miri_mrs-sw_qe" file is the relevant QE file for Channel 1 and Channel 2 of the MRS ("sw" stands for short wavelength). Similarly, "jwst_miri_mrs-lw_qe" is the relevant QE file for Channel 3 and Channel 4 of the MRS ("lw" stands for long wavelength).
The "jwst_nircam_sw-lyot_trans_modmean" and "jwst_nircam_lw-lyot_trans_modmean" files in the "/nircam/optical" subdirectory are the average transmissions of module A and module B for coronagraphic imaging with the Lyot spots.
The "jwst_nircam_wlp4" and "jwst_nircam_wlp8" optical transmission files correspond to the weak lenses, used in the NIRCam SW time-series mode.
- The "jwst_nircam_sw_dbs" and "jwst_nircam_lw_dbs" files are the dichroic beam splitter (DBS) for each channel which directs incoming illumination to the shortwave (SW) or longwave (LW) channel.
- The "jwst_nircam_moda_com_substrate_trans" is for the coronagraphic optical substrate for Module A which is used for coronagraphic imaging mode.
- The "jwst_nircam_sw_qe" and "jwst_nircam_lw_qe" files apply to any observations made from 0.6 to 2.3 μm (detectors NRCA1-NRCA4, and NRCB1-NRCB4) and 2.4 to 5.0 μm (detector NRCA5 and NRCB5), respectively.
- Two options are available in the "/niriss/optical" subdirectory:
- "jwst_niriss_internaloptics_throughput": Used when neither the CLEAR nor CLEARP slots are selected in the filter and pupil wheels, respectively.
- "jwst_niriss_internaloptics-clear_throughput": Used when the CLEAR slot is selected in the filter wheel and a filter or grating is selected in the pupil wheel.
Under "/nirspec/optical", the "jwst_nirspec_mos_internaloptics_throughput" file is applicable to the fixed slits and the bright object time-series modes. "jwst_nirspec_ifu_internaloptics_throughput" is only applicable to the IFU mode.
- The files in "/nirspec/blaze" and "/nirspec/filter" contain pre-launch measurements of the transmission of those optical elements. The "/nirspec/cf" directory contains correction files to be applied to filter+disperser combinations, as they cannot be measured separately on-orbit.
- The "telescope" subdirectory contains the throughput of the optical telescope element (OTE), "jwst_telescope_ote_thruput". This file is relevant for calculating throughputs for all observing modes since all light first passes through the OTE before entering the optical paths of the science instruments.