# CMAG: A Mission to Study and Monitor the Inner Corona Magnetic Field
## 2 Mission Concept
### Payload Concept Design  
The proposed CMAG instrument is an imaging spectropolarimeter that measures all four Stokes parameters at given wavelength samples of selected emission spectral lines from the corona: Fe XIV 530 nm, Fe XI 789 nm, and Fe XIII 1074 nm. These spectral lines are among the ones showing the largest intensity above the K-corona. No other mission aims at carrying out vector magnetography and mapping of the LOS and plane-of-the-sky velocities of the highly dynamic inner corona. The baseline design (see Figure 5), with an entrance pupil diameter of 10 cm, combines the first stage of the full disk telescope from the Polarimetric and Helioseismic Imager (PHI) [46] instrument aboard Solar Orbiter with the conceptual design of the TuMag magnetograph [47] for the SUNRISE III mission [48]. Two athermalized doublets focus the Sun on an intermediate focal plane. A field lens placed before that focal plane re-images the entrance pupil to infinity to make the image telecentric. The doublets are optimized to produce the best image of the external occulter  
Figure 4: CMAG concept of operations. Autonomous operations (**left**) and non-operational orbit (**right**).  
where an internal occulter is placed. A polarization modulation package (PMP) based on Liquid Crystal Variable Retarders (LCVRs) carries out the polarization modulation right after a filter wheel containing three prefilters corresponding to the three target spectral lines. A collimator, consisting of a telephoto system to reduce the total track of the instrument, re-images the Sun onto infinity and focuses the pupil on its image focal plane, where the etalon and the Lyot stop are placed. Finally, a camera telephoto system refocuses the image onto the detector with a fixed spatial sampling of 1.25\({}^{\prime\prime}\) (2.5\({}^{\prime\prime}\) resolution). If the size limitations of the instrument allow it, the camera system will provide a telecentric image.  
The instrument electronics include: a Data Processing Unit (DPU) for the overall control of the instrument and data processing, communications with the S/C, to carry out the main processing functionalities, and with sufficient storage to keep 2.5 h worth of scientific data; an Analog, Mechanism, and Heater Drivers board (AMHD); a High-Voltage Power Supply (HVPS) for tuning the etalon; a Power Converter Module (PCM); and an Electrical Distribution System (EDS). The DPU is the core of the instrument electronics and has to be tailored to perform data acquisition and accumulation, demodulation, retrieval of the different subproducts (see Section 2.4), and compression. The AMHD will be in charge of synchronizing the PMPs and the etalon, taking care of the thermal stabilization of the more critical elements, and managing housekeeping data for the overall control of the instrument, among other analog tasks. The PCM is the direct interface to the platform and should hence fulfill the necessary requirements and redundancy. The spacecraft will have image stabilization capabilities to guarantee the required 0.125\({}^{\prime\prime}\) PDE stability in one minute. The E-unit will be equipped with the necessary firmware (heritage of the Solar Orbiter PHI instrument complex on-board data processing) to provide the required data products. On-board preprocessing of payload data will be key for the reduction in the downlink telemetry.