MIRI Detector Readout Fast

JWST's MIRI FAST mode detector readout pattern is the default readout mode for imaging, low-resolution spectroscopy, and coronagraphy. For subarray imaging and target acquisition, FAST mode is the only available readout pattern.

FAST 1 mode provides short MULTIACCUM exposures to maximize dynamic range and minimize noise in a background-dominated regime. FAST mode is the default readout mode for imaging, low-resolution spectroscopy, and coronagraphy, and is the only mode available for subarray imaging.

Following MIRI readout terminology, FAST mode obtains one sample of a pixel (Nsample = 1; t1 = 1 × 2.775 s = 2.775 s) 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 FAST mode and cannot be altered by the observer. FAST mode currently requires a minimum of four groups (Ngroups = 4) per integration to optimize the slope-fitting algorithm. The user can alter the number of groups (Ngroups) and integrations (Nint). All groups will be stored and downloaded.

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Pixel perspective

From a pixel's point of view, FAST mode patterns will start with a read-reset frame, followed by Ngroups − 1 non-destructive frames while integrating. The  focal plane electronics (FPE) dwells at each pixel for Nsample = 1 (10 µs total). The time between sampling the pixel is t1 = 2.775 s because the entire sensor chip assembly (SCA) is read before returning to the pixel. The integration ends with a read-reset after the specified number of Ngroups and this integration timing pattern may be repeated Nint times for an exposure.

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

The sampling up-the-ramp FAST 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 black vertical tick mark notes a read of the pixel and each red vertical tick mark notes 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 ~3 s and there is only one frame per integration.


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