Operation modes

There are four distinct operation modes.

Venus observation

These observations will be performed in a pushbroom geometry: both slits (for LR and HR) will have their spatial axis perpendicular to the orbital swath (cross-track), which will provide the second (along-track) spatial dimension.

A typical observation of Venus (during a half-orbit, on day side) by VenSpec-U will therefore consist first in a preparation phase which will occur on night side preceding the observation (instrument switch, configuration, acquisiton of dark frames, open of the wheel mechanism). The acquisition parameters (binning scheme, stacking, etc.) can be updated several times during the observation to adapt to the observing conditions and resolution requirements. Data are acquired and processed on the fly then sent to the VenSpec-CCU for compression. After the observation is complete, the instrument is reconfigured (detector is warmed up, wheel is closed) and switched OFF until the next observation opportunity.



Pushbroom geometry
Illustration of the pushbroom geometry from Lustrement et al. (2024)



Solar calibration

VenSpec-U will have some opportunities during the mission to perform radiometric calibration on the Sun. This will be achieved by off-pointing the spacecraft every 112 days such that the Sun crosses the instrument FoV. The radiometric calibration will rely on the combination of two Sun scans. The first scan will use a set of pinholes providing the radiometric calibration for a small portion of the FoV. The pinholes are not affected by molecular contamination so the quality of this calibration method is expected to be very stable during the mission. The second scan will use a set of diffusers; these ones may be affected by molecular contamination, but will enable calibration of the full FoV.

Occasionally, a third scan will be requested to perform a calibration with another set of diffusers ("reference diffusers"). These diffusers are expected to contaminate much slowly than the nominal ones, thus enabling a relative tracking of the contamination.

Dark & internal calibration

The calibration plan also foresees dedicated sessions to characterize and track any evolution of the dark current of the CMOS sensor. This will be done periodically for different sets of parameters such that the collected data will feed a dark current model used to support the inversion pipeline.

The internal calibration, using the UV sources, will be used to perform in-flight PTC (Photon Transfer Curve), PRNU (Pixel Response Non Uniformity) characterization and diagnosis of the CMOS sensor. sensor.