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 (N_sample = 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).  

N_sample = 1 is a fixed parameter in FAST mode and cannot be altered by the observer. FAST mode currently requires a minimum of four groups (N_groups = 4) per integration to optimize the slope-fitting algorithm. The user can alter the number of groups (N_groups) and integrations (N_int). All groups will be stored and downloaded.

¹  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.

Pixel perspective

From a pixel's point of view, FAST mode patterns will start with a read-reset frame, followed by N_groups − 1 non-destructive frames while integrating. The  focal plane electronics (FPE) dwells at each pixel for N_sample = 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 N_groups and this integration timing pattern may be repeated N_int times for an exposure.