MIRI Spectro-Photometric Performance

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The MIRI Imager (MIRIM) spectrophotometric performance has been studied using data from the flight model (FM) testing campaign at the Rutherford Appleton Laboratory (RAL) in the UK. The relative flux calibration factors were estimated using the MIRI Telescope Simulator (MTS) extended source; the use of point-source data was ruled out due to discrepancies between measured and expected signals as predicted by MTS. All extended source data were taken using the MTS black body at 200K, the FULL array in "FAST" readout mode, and the variable aperture system (VAS) at 100% (i.e., fully open). Exposure times were chosen for the detector to reach half-well (about 2.5 × 104 DN). Complementary background data were taken for each integration by placing the MTS filter in the blank position. Raw data were processed with the MIRI DHAS, including cosmic ray removal, background subtraction, and filter-dependent flat-field correction. 

Calibration factors were estimated by comparing the measured slopes with the simulated flux density from MTS converted to an object on the sky in Jy arcsecond-2  (see table below). For this particular case in which the source SED is known, color corrections were calculated and applied. 

The MIRIM absolute calibration will not be fully determined until the telescope is on orbit. At that time a set of stars (A0V stars, solar-type stars, and white dwarfs) preselected to have accurately predicted fluxes in all bands will be measured and will determine the final conversion. Currently, results are limited by the accuracy of the knowledge of the MIRI Telescope Simulator and of the transmissions of the MIRI subsystems. However, the results are in good agreement with expectations from the properties of the instrument components, giving confidence that the instrument optics is working at high efficiency. 

The conversion between number of incident photons at the MIRI entrance focal plane and number of measured electrons (photo conversion efficiency, PCE) for each MIRIM filter has also been derived. The estimated PCE is a relative one, as it has been based on the MTS emission predicted by MTS. The measured slope values at the focal plane array (FPA) were converted to electron flux at the MIRI entrance plane, using a gain value of 5.5 e- DN-1, the pixel area and the filter widths. These measurements were afterward compared to the MTS predicted emission at the entrance focal plane in photons allowing us thus to estimate the relative PCE for each MIRIM filter (see table below). 

FilterPCD (e- photon-1)

Calibration Factor (per pixel)
(Jy arcsec-2 per DN s-1)


0.3157 ± 0.120 

2.41 × 10-


0.3210 ± 0.0835 

1.32 × 10-


0.3685 ± 0.0923 

1.76 × 10-5 


0.2010 ± 0.0405 

1.37 × 10-


0.3857 ± 0.0801 

5.76 × 10-


0.1880 ± 0.0371 

1.43 × 10-


0.3443 ± 0.0769 

2.11 × 10-


0.3498 ± 0.0715 

1.84 × 10-


0.1488 ± 0.0283 

1.81 × 10-


0.3115 ± 0.0617 

2.68 × 10-


0.2885 ± 0.0517 

2.04 × 10-


0.1810 ± 0.0323 

3.65 × 10-


0.2112 ± 0.0380 

4.42 × 10-


0.1894 ± 0.0340 

4.60 × 10-

Spatial photometric repeatability 

Consistency between point-source measurements throughout the detector plane has also been verified using RAL FM data. The 100 and 25 μm MTS pinholes were used to scan the imager part of the detector using nine locations with each point source and in all filters. All integrations used the MTS BB at 800 K. Exposure times and VAS positions were chosen to reach detector half-well and prevent saturation (the 800 K emission is very strong at short wavelengths). 

Point-source positions distributed over the detector used to verify the photometric repeatability. Circles and crosses indicate the locations of the 100 and 25 μm point sources, respectively.  

Data reduction included cosmic ray removal, background subtraction and filter-dependent flat field and linearity correction. Aperture photometry was performed on all 100 μm and 25 μm point-source data, using an annulus to measure and subtract the background. The estimated noise was based on the rms sky annulus. This may not be optimal for absolute calibration purposes, but since only relative differences between the point sources in the same filter are of interest here, this choice is justified. An annulus with inner and outer radius of 45 and 50 pixels, respectively, was used in all cases to subtract the background. 

The measured standard deviation over the point-source fluxes at the different positions ranges from 9.4 to 24.8% for filters F560W to F2550W, respectively. We explain these large deviations being partially due to vignetting caused by the mechanical structure of the MTS pinhole (monochromatic model by M. Wells, private communication), but mostly because of wavelength dependent effects resulting from the individual structures of the pinholes themselves. This hypothesis is confirmed by analyzing the differences between the fitted surfaces to the 25 μm and 100 μm point-source observations over the entire detector. After applying the monochromatic model and correcting with a surface model, the 100 μm pinhole shows a standard deviation of less than 2%. This is not the case for the 25 μm pinhole, where the nonideal effects are largest, as a result of differences between the fitted surfaces. These results indicate that the measured (uncorrected) standard deviation is related to the construction of the two pinholes as part of the MTS, rather than being intrinsic to the MIRI imager detector. This conclusion indicates that we can expect good inflight stability and spatial uniformity. 

Cross-calibration with the medium-resolution spectrometer 

One important aspect of the spectrophometric performance of MIRI at RAL has been the verification of the cross-calibration between the MIRIM and the MRS. We based the cross-calibration on 100 μm point-source data in both imager and spectrometer. For MIRIM all filters were used with the MTS BB temperature set at 800 K and a fully open VAS. To prevent detector saturation at the shortest wavelengths (filters F560W to F1500W), we used the AXIS64 subarray, that was specifically defined at RAL for testing purposes. The AXIS64 is centered at [518, 514] in the FULL frame, and presents an effective illuminated area of 64 × 64 pixels2. Longer wavelength filters were used with FULL frame readout. 

After standard data reduction, aperture photometry was performed on the images, and the relative calibration factors derived as explained above were used to calibrate the point-source observations. Because of the change in observed flux level depending on the position of the point source on the imager detector (as discussed above), a single achromatic correction factor was applied to account for the vignetting of the 100 μm pinhole (77.41%, based on the geometric model provided by M. Wells, private communication) and thus to correct the results for this effect before comparing them with the MRS ones. 

The short-wavelength (SW) and long-wavelength (LW) MRS detectors were also used to observe the 100 μm point source with the exact MTS configuration used for the imager.  The spectral cube was derived and afterward calibrated relatively with respect to the MTS following Wells et al. (2014). As before, MRS spectra were also corrected for vignetting using the achromatic model. The calibrated spectrum was extracted from the spectral cube by applying aperture photometry on each spectral element in the cube. Allowance was made for the different spaxel sizes in the MRS bands to extract equivalent regions over the full spectral range.

At the shortest wavelengths, the match between the MRS and the imager calibration differs by only 8.4% at 5.6 μm, and 1.5% at 7.7 μm. Toward longer wavelengths, we see an increasing discrepancy between the imager and the MRS, which we attribute to the fact that no wavelength dependent corrections were applied, e.g., for the reflectance inside the vignetting tunnel obscuring the 100 μm pinhole. We thus conclude that the discrepancy at longer wavelengths can be attributed to the test setup and not to the MIRI. 



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