MIRI Detector Readout Fast
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 and low-resolution slit spectroscopy, and it is the only available readout mode for coronagraphy, low resolution slitlessprism spectroscopy and 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.
Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.
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.