The Tunable Magnetograph (TuMag), is a visible-light imaging spectropolarimeter designed to investigate the magnetic and dynamic structure of the photosphere and lower chromosphere. TuMag operates in the wavelength range around 517–525 nm, targeting magnetically sensitive spectral lines of Fe I (525.02 nm, 525.06 nm) and Mg I b₂ (517.3 nm). These lines sample different atmospheric heights and allow detailed studies of the coupling between plasma flows and magnetic fields in the lower solar atmosphere.

from del Toro Iniesta et al. (2025)

TuMag provides two-dimensional, diffraction-limited spectropolarimetric observations with a field of view of 63" X 63", measuring the full Stokes vector (I, Q, U, V) in one or two (sequentially) of the three target spectral lines. The spectral analysis is performed using a solid LiNbO₃ Fabry–Pérot etalon, which samples each spectral line at multiple wavelength positions. Polarization modulation is achieved with a system based on liquid-crystal variable retarders (LCVRs). TuMag records the two orthogonal polarization states simultaneously using two synchronized, in-house-developed scientific cameras in a dual-beam configuration, to significantly improve polarimetric sensitivity.

TuMag observing modes are defined by the number of wavelength sampling points, the polarimetric content (intensity only, longitudinal, or full vector), and the resulting temporal cadence. This allows observations to be optimized for magnetic field diagnostics, plasma velocities, or detailed spectral line analysis. The available observing modes are:

Mode	CWL (nm)	Element	Name	Description
Obs 0-s	517.3	Mg I	Spectroscopic mode	Records Stokes I only at 15 wavelength positions across the spectral line.
Obs 0-p	517.3	Mg I	Vector mode	Identical to Obs 0‑s, but recording the full Stokes vector (I, Q, U, V).
Obs 1	517.3	Mg I	Vector mode	Full Stokes measurements at 10 wavelength positions.
Obs 2	525.02, 525.06	Fe I	Vector mode	Full Stokes measurements at 8 wavelength positions across the Fe I lines.
Obs 3	525.02, 525.06	Fe I	Fast longitudinal mode	Measures Stokes I and V at 5 wavelength positions.
Obs 4	517.3	Mg I	Deep magnetograph mode	Records the full Stokes vector at 3 wavelength positions close to line center, optimized for high signal‑to‑noise magnetic field observations.
Obs 5	525.02, 525.06	Fe I	Deep magnetograph mode	Same as Obs 4, applied to the Fe I spectral lines.
Obs 5HC6	???	???	High cadence linear mode	Same as Obs-3, but with much shorter integration time.
Obs 1ss	???	???	High cadence snapshot mode	Same as Obs-0s, but with much shorter integration time.
Obs 12r10	???	???	Deep Mode	Recording the full Stokes vector (I, Q, U, V) at 12 positions and with much longer integration time.
Obs 10r3	???	???	Deep Mode II	Recording the full Stokes vector (I, Q, U, V) at 10 positions and with much longer integration time.

The execution times of TuMag observing modes span a well‑defined range depending on spectral sampling, polarimetric mode, and the number of accumulations used to reach the desired signal‑to‑noise ratio. The values given below should be regarded as typical total times per data set, including internal tuning overheads.
As an order of magnitude, fast modes, such as intensity‑only spectroscopic observations (Obs 0‑s) or the fast longitudinal mode (Obs 3), typically require ≈ 15 s per data set. Vector spectropolarimetric modes usually require 25-40 s per data set, and deep modes represent the slowest configurations and can take up to 55-60 s per data set.

In addition, TuMag uses a filter wheel to select the appropriate prefilter and calibration elements. Movements of the filter wheel introduce an additional overhead, which must be taken into account when two lines are observed sequentially. The overhead can reach up to 10 s in the worst case, which must be added to the nominal execution times quoted above.

Finally, TuMag includes a double filter wheel with prefilters and calibration targets. One of these targets is a phase‑diversity (PD) plate, which allows all observing modes to be post‑facto wavefront reconstructed, improving image quality and spatial resolution.