Stuart M. Jefferies

DIMMI-2h a MOF-based instrument for Solar Satellite ADAHELI

The Doppler-Intensity-Magnetograms with a Magneto-optical filter Instrument at two heights (DIMMI-2h) is a double channel imager using Magneto Optical Filters (MOF) in the potassium 770 nm and sodium 589 nm lines. The instrument will provide simultaneous dopplergrams (velocity fields), continuum intensity and longitudinal magnetic flux images at two heights in the solar atmosphere corresponding to low and high photosphere. Dimmi- 2h is the possible piggy-back payload on ADAHELI satellite. The spatial resolution (approximately 4 arcsec) and the high temporal cadence (15 s) will permit to investigate low and medium oscillating modes (from 0 to below 1000) up to approximately 32 mHz in the frequency spectrum. The acquisition of long-term simultaneous velocity, intensity and magnetic information up to these high frequencies will permit also the study of the propagation and excitation of the waves with a frequency resolution never obtained before.

Daylight operation of a sodium laser guide star

We report photometric measurements of a sodium resonance guide star against the daylight sky when observed through a tuned magneto-optical filter (MOF). The MOF comprises a sodium vapor cell in a kilogauss-level magnetic field between crossed polarizers and has a very narrow transmission profile at the sodium D2 resonance of approximately 0.008 nm. Our observations were made with the 1.5 m Kuiper telescope on Mt. Bigelow, AZ, which has a separately mounted guide star laser projecting a circularly polarized single-frequency beam of approximately 6.5 W at 589.16 nm. Both the beam projector and the 1.5 m telescope were pointed close to zenith; the baseline between them is approximately 5 m. Measurements of the guide star were made on the morning of 2016 March 24 using an imaging camera focused on the beacon and looking through the full aperture of the telescope. The guide star flux was estimated at 1.20×106 photon/m2/s while at approximately 45 minutes after sunrise, the sky background through the MOF was 1100 photon/m2/s/arcsec2. We interpret our results in terms of thermal infrared observations with adaptive optics on the next generation of extremely large telescopes now being built.

Tomographic wave-front sensing with a single guide star

Adaptive optics or numerical restoration algorithms that restore high resolution imaging through atmospheric turbulence are subject to isoplanatic wave-front errors. Mitigating those errors requires that the wave-front aberrations be estimated within the 3D volume of the atmosphere. Present techniques rely on multiple beacons, either natural stars or laser guide stars, to probe the atmospheric aberration along different lines of sight, followed by tomographic projection of the measurements onto layers at defined ranges. In this paper we show that a three-dimensional estimate of the wave-front aberration can be recovered from measurements by a single guide star in the case where the aberration is stratified, provided that the telescope tracks across the sky with non-uniform angular velocity. This is generally the case for observations of artificial earth-orbiting satellites, and the new method is likely to find application in ground-based telescopes used for space situational awareness.

Atmospheric tomography for artificial satellite observations with a single guide star.

Estimation of wavefront errors in three dimensions is required to mitigate isoplanatic errors when using adaptive optics or numerical restoration algorithms to recover high-resolution images from blurred data taken through atmospheric turbulence. Present techniques rely on multiple beacons, either natural stars or laser guide stars, to probe the atmospheric aberration along different lines of sight, followed by tomographic projection of the measurements. In this Letter, we show that a three-dimensional estimate of the wavefront aberration can be recovered from measurements by a single guide star in the case where the aberration is stratified, provided that the telescope tracks across the sky with nonuniform angular velocity. This is generally the case for observations of artificial Earth-orbiting satellites, and the new method is likely to find application in ground-based telescopes used for space situational awareness.

The intensity effect in magneto-optical filters

We used a laser system for determining the bandpasses of the two vapour cells, the Magneto-Optical Filter (MOF) and the Wing Selector (WS), which are the core of solar narrow-band filters based on the MOF technology. A new result, which we called the Intensity Effect, was found: the MOF and WS bandpasses depend not only on the temperature at which the cell is heated and the external magnetic field in which the cell is embedded, but also on the radiation intensity entering the cell. A theoretical interpretation of the Intensity Effect is proposed in terms of the kinetic equilibrium of the potassium atomic populations inside the vapour cell. We need to take the Intensity Effect into account for setting-up MOF based instruments for solar and stellar observations as well as for modelling the MOF and WS spectral transmissions.