A. Righini

Mean vertical profile of atmospheric turbulence relevant for astronomical seeing

With measurements of the atmospheric microthermal field obtained from 67 independent radiosondes (balloon) scans up to 20–25 km above sea level, from three European sites, we have calculated an average vertical profile of the atmospheric refractive index structure coefficient CN2.

Optical remote sensing of atmospheric turbulence: a comparison with simultaneous thermal measurements

In this paper we describe a two-part experiment: measuring atmospheric turbulence both optically and thermally from 2 km up to the stratosphere. The consistency of the results shows that the optical technique may be a powerful tool for studying atmospheric turbulence from ground-based equipment.

The JOSO site testing campaigns in the Canary Islands

Abstract In 1968 an informal cooperation of European solar research institutions was established under the name of JOSO (Joint Organization for Solar Observations). A brief account is given of the site testing carried out by this organization to find an “ideal” solar observatory site. After having tested nearly 40 sites in the Mediterranean and Atlantic coastal areas, it was found that, notwithstanding the great homogeneity of the maritime airmasses, coastal sites were not suitable for high resolution solar observations. Consequently, in the search for mountain sites imbedded in highly homogeneous air, two sites were chosen in the Canary Islands: Izana (Tenerife) and Roque de los Muchachos (La Palma). Aircraft and radiosonde tests of the microthermal vertical profile in the atmosphere have shown the superior homogeneity of the Atlantic airmass above the Canary Islands. An extended test of 160 days (in 1979) of meteorological and solar observations performed simultaneously with two telescopes, both photographically and with photoelectric seeing monitors, has led to the conclusion that the Izana site is somewhat superior to the site at Roque de los Muchachos as far as daytime seeing is concerned. Measurements during and after the campaign have also shown that for certain periods during daytime the telescopes installed at both sites appear to be inside the large-scale airmass whose residual inhomogeneities limit the night-time seeing.

Daytime r(0) evaluated from vertical microthermal measurements.

The usefulness of parameter r(0) for describing the effect of atmospheric turbulence on wavefront distortion sensitive optical systems has been widely discussed. In this paper we present in situ daytime r(0) measurements (5500 A, Zenith propagation) obtained from balloon-borne microthermal radiosondes. We find that the parameter r(0) is log normal distributed at various levels aloft and that r(0) follows a power law in the 100-10,000-m range. Comparing our results with nighttime ground based observations we conclude that local turbulence prevails in determining image quality.

Imaging performance of multi-etalon bidimensional spectrometers

Aims. In recent years, several new solar and nighttime panoramic spectrometers based on Fabry-Perot interferometers have been successfully developed. In this paper we evaluate the imaging performance of the two types of mountings that have been adopted, telecentric and classic, in particular trying to understand which one might be more suitable for future large-aperture solar telescopes. Methods. Numerical code was written to simulate the behavior of such spectrometers, on the basis of the theory of Fourier optics. This code was used to simulate different instrument configurations and was tested on previous results obtained either analytically or numerically by other authors. Results. Calculations of the system MTF and Strehl ratios show that both mountings may perform very close to theoretical expectations. However, gap irregularities in the interferometers may alter the optical quality of the monochromatic images. In the case of the classical mounting in a collimated beam, it is possible to partially compensate for the resulting errors in the wavefront emerging from the interferometers with a suitable phase plate. We also performed an observational test of the optical quality delivered by the IBIS interferometer installed at the Dunn Solar Telescope of the National Solar Observatory, with the results substantially confirming the calculations. Conclusions. It follows from our results that both mountings may be efficiently used for solar bidimensional spectroscopy. The final choice depends on the tradeoff between factors such as image quality, field of view, and acceptable wavelength shift.

Thermal effects in the Solar Disk Sextant telescope

The Solar Disk Sextant (SDS) is an instrument conceived to monitor the diameter of the Sun and its oscillations. A key component of the SDS is the Beam Splitting Wedge (BSW), whose function is to provide calibration to the geometry of the focal plane. The thermal behavior of the BSW is critical, as it affects the overall performance of the instrument. Modeling the elements of the BSW and the basic thermal processes is shown to account for experimental evidences of defocusing observed in early measurements with a balloon borne prototype. Basic requirements for accurate thermal stabilization on board of the final instrument are derived.

Interferometric test of optical systems: effects of probe beam collimation errors

The effects of probe beam residual power in interferometry are investigated. It is seen that residual power produces an asymmetry between the forward and backward beams that is scaled with f 2 # , i.e., the effect can be easily seen in very slow optical systems. Common methods to detect residual power are discussed, showing the effectiveness of lateral shear interferometry to carefully set the collimation of the probe beam. A device approaching the order of magnitude established by Rayleighs quarter wavelength rule is demonstrated.

High-resolution spectroscopic imaging of the Sun with a universal birefringent filter and a Fabry-Perot Interferometer

SummaryWe describe an optical device consisting of a Universal Birefringent Filter and a Fabry-Perot interferometer used in tandem to obtain monochromatic images of the Sun in the visible spectrum (between 4200 and 7000 Å). The spectral resolving power is about 300000 and the wavelength stability might reach 0.02 mÅ at 5000 Å (≈1.2 m/s) with an integration time of 1 s. The image is recorded by a CCD camera based on a Thomson THX31159 device with 512×512 square pixels 19 μm in size. A prototype of this instrument was tested at the G. B. Donati Solar Tower of the Arcetri Astrophysical Observatory by obtaining, at fixed time intervals, several series of full disk monochromatic images of the Sun at different wavelengths inside the Fe I 5905.7 Å photospheric line. In this paper we describe the instrument and its characteristics and we give an cccount of the preliminary results obtained by measuring the global velocity field on the solar photosphere. These results show that the instrument is well suited for global oscillation measurements.

A Shack interferometer setup for optical testing in undergraduate courses

The Shack interferometer is a simple and effective device to test optical surfaces in reflection and optical systems in transmission. An essential setup on a reduced scale with a minimum number of components is presented, suited to gain familiarity and practice with optical testing in a laboratory course for undergraduate students. The basic layout and the optics principles are discussed, fringe processing is detailed and the calculation of the point spread function in the presence of typical aberrations is demonstrated.

Line Bisectors in and out Magnetic Regions

Some photospheric lines have been recently observed in active regions at different distances from the solar disk center and the asymmetry and shift dependence on the magnetic field has been evaluated. In particular, near the disk center, we find that the red-shift of the line bisectors increases with the field and decreases towards the core, while, at the bottom of a strong line, a small blue-shift is observed. Measurements performed between 0.5 and 0.8 solar radii from the disk center show that the line bisectors are red-shifted near the continuum and blue-shifted at the bottom. These results are qualitatively interpreted as due to the magnetic inhibition of the vertical and horizontal component of the convective motions.