In the definition of METIS observing modes the following naming conventions are applied:
Size of raw frames (sets of samples representing the digitized signal given by each pixel of the relevant detector):
VLD: (2048x2048) @ 14 bits (analogue mode; spatial scale per pixel: 10”);
UVD: (1024x1024) @ 14 bits (analogue mode; spatial scale per pixel: 20”).
The UVD can be operated in photon counting mode also; the photon counting mode is mainly required to overcome possible degradation of the UV detector during a long-lived mission such as Solar Orbiter and to ensure the capability of observing also in such a case.
DIT: Detector Integration Time, i.e., the time interval among which all detector pixels are collecting photons. It represents the actual exposure of a single read out of the detector (frame).
NDIT: number of detector integrations, i.e., the number of frames to be averaged in order to obtain an “Acquisition”; this is the image obtained as result of averaging, pixel by pixel, NDIT frames.
TACQ: Acquisition Time, the overall integration time corresponding to a single acquisition. The values are set in order to get a limited number of events (cosmic rays) to be removed.
Cosmic ray (CR) removal: software procedure to clean the acquired images from spurious signal given by the cosmic rays and SEP.
CME flag: software procedure to automatically trigger the rising of a CME event.
NACQ: number of acquisitions to be averaged in order to obtain an “Exposure”, i.e., the image obtained as result of averaging, pixel by pixel, NACQ acquisitions.
TEXP: Exposure Time, i.e., the overall time corresponding to a single Exposure.
Single acquisitions will be averaged over the exposure time interval, unless a CME flag occurs. Exposure times are determined considering the countrates estimated for the Sun at its minimum of activity (Tables 6-8), as expected for a launch date of July 2017. The values are set:
in order to get the best performance of the detector for the brightest features in the expected image and, contemporarily,
get still a significant signal-to-noise ratio for the faintest features in the same expected image;
on the basis of the typical life-times of the coronal structures considered;
taking into account the need of limiting the data volume of the final science images downloaded to the ground.
NPOL: number of polarization (LCVR) angles used (cycled) during the scientific measurement.
TOH: Overhead Time, given by the instrument in order to perform the scientific measurement (i.e., synchronization, LCVR commanding, etc.).
TW: Waiting Time, defined by the operator within the observation timeline.
TCAD: Cadence Time, needed to get NPOL VL images or 1 UV image (within the same scientific measurement, the time between two consecutive Exposures).
13. The counts per pixel in the final science images (to be downloaded by TM) are average values calculated over the summed-up acquisitions and the pixels binned together. In this case the depth of each final science image is 14 bits.
The acquisition time relationships are
|TACQ = NDIT * DIT = DIT , NDIT = 1||TACQ = NDIT * DIT|
|TEXP = NACQ * TACQ||TEXP = NACQ * TACQ|
|TCAD = NPOL * TEXP + TOH + TW||TCAD = TEXP + TOH + TW|