MIRI Detector Readout FASTR1

FASTR1 mode provides short MULTIACCUM exposures to maximize dynamic range and minimize noise in a background-dominated regime. It is the default readout mode for imaging, medium-resolution spectroscopy, low-resolution slit spectroscopy, and wide field slitless spectroscopy, and it is the only available readout mode for coronagraphy, low-resolution slitless prism spectroscopy and subarray imaging.

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Following MIRI readout terminology, FASTR1 mode obtains one sample of a pixel (Nsample = 1; t1 = 1 × 2.775 s = 2.775 s for the FULL array) and returns it to the ICDH (ISIM Command and Data Handling [ICDH]; ISIM is Integrated Science Instrument Module).  

Nsample = 1 is a fixed parameter in the FASTR1 mode and cannot be altered by the observer. Recommendations are provided in MIRI Cross-Mode Recommended Strategies for the minimum number of FASTR1 groups per integration to optimize the slope-fitting algorithm. If data volume issues arise or the desired SNR from the ETC results in more than 180 groups in FASTR1 mode, it is recommended that the user switch to the SLOWR1 or FASTGRPAVG8 mode instead. The user can alter the number of groups (Ngroups) and integrations (Nint). All groups will be stored and downloaded.

Compared to the MIRI traditional FAST mode (see Ressler et al. 2015), FASTR1  introduces an additional reset between integrations, reducing the severity of reset effects in the MIRI detectors and thus improving the flux calibration accuracy (see Morrison et al. 2023).


Pixel perspective

While not integrating, the MIRI detectors continue to be clocked but the pixels in the MIRI detectors are reset every frame. Once the exposure starts the FASTR1 readout pattern executes Ngroups − 1 non-destructive frames. The  focal plane electronics (FPE) dwells at each pixel for Nsample = 1 (10 µs total). The time between sampling an individual pixel is t1 = 2.775 s because the entire sensor chip assembly (SCA) is read before returning to the pixel. The final group of the integration is a read-reset, yielding the total of Ngroups reads. In multiple integration data the read-reset is followed by an additional reset frame that takes 2.775 s. This integration timing pattern may be repeated Nint times for an exposure, with the last integration ending in a read-reset. The duration of a single exposure in FASTR1 will therefore be:

Exposure_duration = (Ngroups × 2.775 s × Nints) + (2.775 s × (Nints-1))

Figure 1. Sampling up the ramp FASTR1 mode readout scheme seen from a single pixel’s point of view

Click on the figure for a larger view. 

The sampling up-the-ramp FASTR1 mode readout scheme, as seen from a single pixel’s point of view for Nsample = 1, Ngroups = 4, and Nint = 2. The x-axis is time and the y-axis is voltage or signal strength. Each blue vertical tick marks a read of the pixel and each red vertical tick marks a reset of the pixel. In this scheme, each reading of a pixel will have only one sample at each pixel. The time to read an entire frame is 2.775 s and there is only one frame per group.

References

Ressler, M. E. et al. 2015, PASP, 127, 675
The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System

Morrison, J. E. et al. 2023, PASP, 135, 075004
JWST MIRI Flight Performance: Detector Effects and Data Reduction Algorithms



Notable updates

  •  
    Added the FASTGRPAVG8 readout pattern as an available option.

  •  
    Change FAST by FASTR1; figure updated.


  • Figure updated 
Originally published