MIRISim 2.2.0
These are the release notes for MIRISim 2.2.0. Visit the
MIRISim Releases page to find information on the latest release available.
About MIRISim
MIRISim is a Python 3 based simulator for the MIRI instrument on JWST. It can be used to simulate the Imager, the Low Resolution Spectrograph (LRS, in slit and slitless mode), and the Medium Resolution Spectrometer (MRS). It does not simulate coronographic observations.
The simulator can be run at the command line (with the aid of input files specifying the properties of the simulation), or within a Python shell (either with the same input files, or with parameters set in-line by the user). Please see the
MIRISim Overview Website for a user guide and more information on MIRISim.
MIRISim produces output as FITS files consistent with the 'level 1b' data to be produced by JWST, and which can be read into the JWST Pipeline. These data products are consistent with the MIRI sensitivity model, and are therefore representative of what users can expect from MIRI observations. Note that MIRISim cannot be used as a sensitivity calculator, and users planning observations should use exposure times derived from the JWST Exposure Time Calculator (ETC) for their proposals.
Installing MIRISim 2.2.0
To install this version of MIRISim, use
mirisim_install.bash --version 10
If you are upgrading from a previous MIRISim-MIRICLE installation, please follow the warnings on the installation page.
See
MIRISim - MIRICLE Installation page for more information on installation.
Change log
Changes since version 2.1.0:
- General:
- Due to discontinued software channels at Anaconda and STScI?, the previous MIRISim releases (2.1.0 and earlier) can no longer be installed with the MIRICLE install script.
- A new command line script "mirisim_from_apt" is available; this allows one to run a series of MIRISim simulations for a set of observations defined in an APT file and for a provided scene. The same scene will be used for all observations regardless of the APT specifications concerning targets.
- This release uses datamodels from the "jwst" package v0.13.8 (build 7.3.1), upgraded from v0.9.6 (build 7.1.3)
- Experimental: added support to allow custom dither pattern files, as specified in simulation config file; this feature is not yet supported, but made available for expert testers.
- Scene generation
- The parameter "tconstant" in the scene configuration file was unused and is now deprecated; it will be removed in a future release.
- Added support for "PlanetDisk" object.
- Imager:
- Bug fix: the Imager now correctly reads out the sub-array set by the "ReadDetect" parameter; before the fix, it was re-using the "ConfigPath" parameter (used to select which field-of-view to center the scene on).
- LRS:
- Updated to use the latest version of LRS Pixel FLAT CDP files (07.01.01), that required a cosmetic update to metadata to be compatible with JWST/MIRI data models.
- MRS:
- Updated to use the latest version of the MRS Distortion CDP files (8B.05.01)
- Updated to use the latest version of the MRS Pixel Flat CDP files (8B.01.00)
- Output
- RA_V1 and DEC_V1 are now correctly set to the RA and Dec at (V2, V3) = (0, 0).
- The dither pattern file used in the simulation is now stored in the output directory.
User Guide
The MIRISim user guide (v.2.2.0) can be found here:
MIRISim.pdf
Jupyter Notebooks
The Jupyter notebooks linked below are copies of the ones appended to the User Guide, which help the user understand how to create scenes, and give example (e.g. walk through) simulations through each of the different light paths.
Known Issues
MIRISim compatibility with JWST pipeline
There can often be updates to either JWST pipeline or MIRISim that make them incompatible with specific versions of each other. This version of MIRISim has been tested with build 7.2.1 of the JWST pipeline. There are likely to be incompatibilities with version 7.3, 7.4 and beyond as different versions of various CDPs are updated.
Output fluxes in MRS channel 4 are not consistent with the JWST ETC
The example simulation below shows two examples of point source simulations where the target had a flat spectral energy distribution across the MRS wavelength range (of 10 mJy in the top panel, 50 in the bottom), which were observed for 10 and 50 frames (respectively), both that the faint sources that are observed for less time will have higher noise levels in general, and that in both cases, that there is both a signal drop, and an increase in noise in channel 4 (the three spectra spanning roughly 17.6 to 28.5 microns, see the
MRS overview page from the JWST website). This phenomenon is under investigation by the MIRI team, and once understood, will be incorporated into the simulator.
MRS PSF is oversized
Cubes reconstructed with the STScI Pipeline from MIRISIM simulations (4-pt dither) of a point source show a significantly broader (~25% in RA/alpha, ~35% in Dec/Beta) PSF core than expected from the PSF CDP, as measured by the fit of a 2D-Gaussian aligned in cube space to account for the rotation between RA,Dec and alpha, beta. The attached PSF size plot (below) shows 2Dgaussian fits to the CDP v7 PSF candidate files (blue), seq1 skycubes of the simulator (these are undersampled, orange), and the reconstructed cubes from the STScI pipeline for simulated data using both the CDP v5 (red) and CDPv7 (green).
MRS Spectral Response Function off by a known factor
The MRS Spectral Response Function (SRF) is quantified in MIRISim through the SRF Calibration Data Product (CDP). There is a known issue with the current CDP that its values are consistently off by a factor of 0.55. Because the same CDP is used in the simulator and in the 7.2.1 build of the
JWST Pipeline, the effects are not noticeable in the final, reduced MIRISim data.
There are flux discrepancies increasing at longer wavelengths when input flux is compared to the pipeline results or ETC results. This mis-match is being investigated.
LRS subarray: offsets in reference position
The simulator is not properly simulating the 4 left-most (dark) reference columns at the left edge of subarrays which touch the left edge of the detector. As a result,LRS simulations in slitless mode show a 4 column shift of the reference (center) position to the left.
Simulation of some detector effects is approximate
The simulation of detector latency and drift uses a charge trapping and release model derived from ground-based measurements made in FAST mode. The model is more accurate at a timescale of around 300s, but at much longer or much shorter timescales our model is an approximation. Latent and drift effects are turned off in SLOW mode, as we don't yet have reliable measurements in that mode. The latent and drift effects seen in flight will depend on the complete history of the illumination incident on the MIRI detectors, whereas the MIRI simulator begins with a "zero trapped charge" initial condition. The latent decay during wait periods between observations and dither manoeuvres is currently not simulated correctly, as the detector is assumed not to be illuminated, and the wait time interval is not the same as in FAST mode.
Detector nonlinearity is simulated by applying an inverse of the function defined in the nonlinearity CDP. This effect should cancel out when the same CDP is used by the
JWST Pipeline to correct the nonlinearity. Our tests show the two algorithms mostly cancel out, but there are two residual effects we are still investigating:
- In MRS simulations, there is a slight difference remaining in 3B, 4A, 4B and 4C aperture spectra but these are within 5% up to 4A and less than 10% in 4B and half-way into 4C.
- The continuum level in these MRS channels is higher than expected by about 10%. This may be related to the other flux issues mentioned above.
Simulation of galaxies much smaller than a pixel size may lead to memory overflow
Simulating of a high number of extremely tiny extended sources (galaxies) much smaller than a pixel size 0.11” may lead to memory overflow. The fix is on the way under testing and will occur in a patch soon. Until then a workaround is to replace the very tiny galaxies by point sources with the same SED.
Highly redshifted spectra
When placing a highly red-shifted (z > 0.5) object into a scene, the user should not include any velocity component in the definition of the source. Set v or z, but not both.
Imager coronagraphic fields
The coronagraphic fields included in the imager field of view have been provided primarily to identify any (bright) sources which may fall in the fields but cause problems for the LRS or imaging. The fields do not simulate any coronagraphic functionality and so do not show the 4QPM nulling pattern. Future versions may include the small (>= 0.95) transmission factor for the 4QPM optical elements.