Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. Aims. The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods. Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results. (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4–7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.

Abstract. In order to complete characterization of the Oukaïmeden site and hoping to prospect other sites in the Moroccan High Atlas, we constructed a seeing monitor. We named the instrument DIMMAR for Differential Image Motion Monitor of Marrakesh. After more than one year of operation, we can conclude that the DIMMAR is quite reliable and that it can be a prototype for other similar low-cost instruments. In this work, the seeing measured at Oukaïmeden site using the DIMMAR on top of a 5 meters high tower is presented. Statistics from fifteen months seeing's are treated (from July 19, 2003 up to September 26, 2004. The median and mean zenith seeing for the entire period of observations are 0.84 and 0.92 , respectively, at 0.5 µm (or 0.96 and 1.05 , if corrected for exposure-time bias). The best seeing value measured is 0.32 .

No abstract

Aims. We seek to produce apodized apertures for application in stellar coronagraphy to help in direct detections of exoplanets. We show that chromatic apodized apertures of any shape in transmission can be obtained with a specific MZI and we demonstrate this capability in two cases. Methods. The method takes advantage of the capabilities of the MZI, in which the two outputs correspond to the addition and subtraction of the two wave amplitudes in both arms. The result is obtained by re-imaging the entrance aperture of the telescope in the arms of the MZI where two complementary phase masks ± ϕ(x, y) are set. At the two outputs of the MZI, the re-imaged apertures interfere, and their transmissions are multiplied respectively by a factor of the form cos[ϕ(x, y)] and i sin[ϕ(x, y)]. They correspond to the apodized and anti-apodized complementary outputs. Results. We present the results obtained for two types of apodization. A 1D cosine apodization for a square aperture is obtained by introducing a thin wedge-shaped air film, slightly tilting one of the mirrors of the MZI. A 2D circular symmetric apodization of the form cos[x 2 + y 2 ] is obtained for a circular aperture using two complementary convergent and divergent lenses as phase masks. Aperture transmissions (in intensity) and corresponding point spread functions (PSFs) are given in each case and compared to the theoretical expectations. Conclusions. We have demonstrated the capability of the MZI to produce an apodized aperture. This result is obtained with no loss of photons, considering the fact that there are two complementary outputs. Considerations are given on the wavelength dependence of this technique.

No abstract