When optical measurements are acquired from the ground, photons from a source like a star or the Sun are scattered by molecules in the atmosphere and affected by turbulence. These effects cause the solar edge to blur and can alter its value and variations. The MISOLFA instrument’s initial goal was therefore to specify and quantify the effects of atmospheric turbulence on ground measurements of the Sun’s diameter by the DORaySOL and SODISM II instruments, and subsequently to validate methods to correct for these perturbing effects.
For this purpose, MISOLFA measured all optical parameters required to qualify observing conditions. These are the Fried parameter r 0 , the spatial coherence outer scale L 0 , the isoplanatic angle 0 , the characteristic timescale(s) for wavefront evolution 0 and the vertical turbulence profile C n 2 (h).
How MISOLFA works
MISOLFA’s design is based on statistical analysis of fluctuations in defined angles of arrival like the slope at each point of the degraded incident wavefront entering the pupil. In daytime conditions, these fluctuations are highlighted by observing the solar edge. For this purpose, two measurement channels (see optical diagram) were used:
- A direct channel where the solar image is formed on a CCD camera with the requisite magnification; this channel enables the wavefront’s spatial coherence parameters (Fried parameter, outer scale and isoplanatic angle) and the turbulence profiles to be estimated.
- A second channel (pupil channel) that forms the pupil image through a diaphragm on the solar edge uses photodiodes to estimate the wavefront’s temporal parameters.
The instrument was located on the Calern plateau in France.
MISOLFA programme execution
A numerical simulation of the behaviour of the inflexion point of the solar edge (position, precision, etc.) in relation to observing conditions was performed. This study factored in the effect of optical parameters (r 0 and L 0 ) measured by MISOLFA and the effect induced by each turbulent layer present during observations (turbulence profile C n 2 (h)). To determine the diameter, we need to measure the brightness at points diametrically opposite the solar image, which have likely been affected by different atmospheric perturbations. To confirm or disprove this hypothesis, a specific study analysed low-frequency correlations (related to L 0 and 0 ) that might persist at these points, as well as their effects on diameter measurements.
The results of these studies are feeding into the next goal, which is to establish models capable of explaining the effects induced by turbulence on ground-based measurements. Such models are needed to gauge effects induced by atmospheric turbulence and any corrections to be applied to measurements. Corrections deduced from numerical simulations were then applied to data from DORaySOL.
The last step came with the measurements of the Sun’s diameter from outside the atmosphere obtained by the Picard satellite. It consisted in applying the envisioned corrections to ground-based measurements, comparing them with measurements from space to validate them and then assessing their effectiveness.