One of the main advantage of space observatories is the stability of the point spread function th... more One of the main advantage of space observatories is the stability of the point spread function that allows programs not feasible from the ground. However, when pushed to the limits, even an observatory like the Hubble Space Telescope may exhibit variations in the PSF that can be problematic for studies like weak lensing or identification of the host halos of
We report observations of dusty clouds in Saturn&... more We report observations of dusty clouds in Saturn's rings, which we interpret as resulting from impacts onto the rings that occurred between 1 and 50 hours before the clouds were observed. The largest of these clouds was observed twice; its brightness and cant angle evolved in a manner consistent with this hypothesis. Several arguments suggest that these clouds cannot be due to the primary impact of one solid meteoroid onto the rings, but rather are due to the impact of a compact stream of Saturn-orbiting material derived from previous breakup of a meteoroid. The responsible interplanetary meteoroids were initially between 1 centimeter and several meters in size, and their influx rate is consistent with the sparse prior knowledge of smaller meteoroids in the outer solar system.
We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow-and wide-angle... more We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow-and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10 7 , gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency. Hot pixel behavior is more complicated than can be understood by a process of activation by cosmic ray damage and deactivation by annealing. Point spread function (PSF) analysis revealed a ghost feature associated with the narrow-angle camera Green filter. More generally, the observed PSFs do not fall off with distance as rapidly as expected if diffraction were the primary contributor. Stray light produces significant signal far from the center of the PSF. Our photometric analysis made use of calibrated spectra from eighteen stars and the spectral shape of the satellite Enceladus. The analysis revealed a shutter offset that differed from pre-launch calibration. It affects the shortest exposures. Star photometry results are reproducible to a few percent in most filters. No degradation in charge transfer efficiency has been detected although uncertainties are large. The results of this work have been digitally archived and incorporated into our calibration software CISSCAL available online.
One of the main advantages of space observatories is the quality and stability of the point sprea... more One of the main advantages of space observatories is the quality and stability of the point spread function that allows programs not feasible from the ground. However, when pushed to the limits, even the Hubble Space Telescope exhibits variations in the PSF that can be problematic for studies like weak lensing or identification of the host halos of bright quasars at high redshift. These variations are primarily due to small displacements in the focus of thetelescope, which to a first approximation can be ascribed to temperature variations. The aim of this report is to characterize the variation of the focus position for HST in terms of the average temperature sensor values of the telescope. We propose a comprehensive temperature-focus model able to predict the position of the focus at the micron level over a dynamic range that extends from sub-orbital variations (< 1 hour) to seasonal and yearly variations. This allows us to predict the focus position significantly more accuratel...
We present a study of the properties of the star-cluster systems around pseudo-bulges of late-typ... more We present a study of the properties of the star-cluster systems around pseudo-bulges of late-type spiral galaxies using a sample of 11 galaxies with distances from 17 to 37 Mpc. Star clusters are identified from multiband HST ACS and WFPC2 imaging data by combining detections in 3 bands (F435W and F814W with ACS and F606W with WFPC2). The photometric data are then compared to population synthesis models to infer the masses and ages of the star clusters. Photometric errors and completeness are estimated by means of artificial source Monte Carlo simulations. Dust extinction is estimated by considering F160W NICMOS observations of the central regions of the galaxies, augmenting our wavelength coverage. In all galaxies we identify star clusters with a wide range of ages, from young (age 8 Myr) blue clusters, with typical mass of 10 3 M ⊙ to older (age > 100-250 Myr), more massive, red clusters. Some of the latter might likely evolve into objects similar to the Milky Way's globular clusters. We compute the specific frequencies for the older clusters with respect to the galaxy -2and bulge luminosities. Specific frequencies relative to the galaxy light appear consistent with the globular cluster specific frequencies of early-type spirals. We compare the specific frequencies relative to the bulge light with the globular cluster specific frequencies of dwarf galaxies, which have a surface-brightness profile that is similar to that of the pseudo-bulges in our sample. The specific frequencies we derive for our sample galaxies are higher than those of the dwarf galaxies, supporting an evolutionary scenario in which some of the dwarf galaxies might be the remnants of harassed late-type spiral galaxies which hosted a pseudo-bulge.
One of the main advantage of space observatories is the stability of the point spread function th... more One of the main advantage of space observatories is the stability of the point spread function that allows programs not feasible from the ground. However, when pushed to the limits, even an observatory like the Hubble Space Telescope may exhibit variations in the PSF that can be problematic for studies like weak lensing or identification of the host halos of
We report observations of dusty clouds in Saturn&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;... more We report observations of dusty clouds in Saturn&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s rings, which we interpret as resulting from impacts onto the rings that occurred between 1 and 50 hours before the clouds were observed. The largest of these clouds was observed twice; its brightness and cant angle evolved in a manner consistent with this hypothesis. Several arguments suggest that these clouds cannot be due to the primary impact of one solid meteoroid onto the rings, but rather are due to the impact of a compact stream of Saturn-orbiting material derived from previous breakup of a meteoroid. The responsible interplanetary meteoroids were initially between 1 centimeter and several meters in size, and their influx rate is consistent with the sparse prior knowledge of smaller meteoroids in the outer solar system.
We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow-and wide-angle... more We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow-and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10 7 , gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency. Hot pixel behavior is more complicated than can be understood by a process of activation by cosmic ray damage and deactivation by annealing. Point spread function (PSF) analysis revealed a ghost feature associated with the narrow-angle camera Green filter. More generally, the observed PSFs do not fall off with distance as rapidly as expected if diffraction were the primary contributor. Stray light produces significant signal far from the center of the PSF. Our photometric analysis made use of calibrated spectra from eighteen stars and the spectral shape of the satellite Enceladus. The analysis revealed a shutter offset that differed from pre-launch calibration. It affects the shortest exposures. Star photometry results are reproducible to a few percent in most filters. No degradation in charge transfer efficiency has been detected although uncertainties are large. The results of this work have been digitally archived and incorporated into our calibration software CISSCAL available online.
One of the main advantages of space observatories is the quality and stability of the point sprea... more One of the main advantages of space observatories is the quality and stability of the point spread function that allows programs not feasible from the ground. However, when pushed to the limits, even the Hubble Space Telescope exhibits variations in the PSF that can be problematic for studies like weak lensing or identification of the host halos of bright quasars at high redshift. These variations are primarily due to small displacements in the focus of thetelescope, which to a first approximation can be ascribed to temperature variations. The aim of this report is to characterize the variation of the focus position for HST in terms of the average temperature sensor values of the telescope. We propose a comprehensive temperature-focus model able to predict the position of the focus at the micron level over a dynamic range that extends from sub-orbital variations (< 1 hour) to seasonal and yearly variations. This allows us to predict the focus position significantly more accuratel...
We present a study of the properties of the star-cluster systems around pseudo-bulges of late-typ... more We present a study of the properties of the star-cluster systems around pseudo-bulges of late-type spiral galaxies using a sample of 11 galaxies with distances from 17 to 37 Mpc. Star clusters are identified from multiband HST ACS and WFPC2 imaging data by combining detections in 3 bands (F435W and F814W with ACS and F606W with WFPC2). The photometric data are then compared to population synthesis models to infer the masses and ages of the star clusters. Photometric errors and completeness are estimated by means of artificial source Monte Carlo simulations. Dust extinction is estimated by considering F160W NICMOS observations of the central regions of the galaxies, augmenting our wavelength coverage. In all galaxies we identify star clusters with a wide range of ages, from young (age 8 Myr) blue clusters, with typical mass of 10 3 M ⊙ to older (age > 100-250 Myr), more massive, red clusters. Some of the latter might likely evolve into objects similar to the Milky Way's globular clusters. We compute the specific frequencies for the older clusters with respect to the galaxy -2and bulge luminosities. Specific frequencies relative to the galaxy light appear consistent with the globular cluster specific frequencies of early-type spirals. We compare the specific frequencies relative to the bulge light with the globular cluster specific frequencies of dwarf galaxies, which have a surface-brightness profile that is similar to that of the pseudo-bulges in our sample. The specific frequencies we derive for our sample galaxies are higher than those of the dwarf galaxies, supporting an evolutionary scenario in which some of the dwarf galaxies might be the remnants of harassed late-type spiral galaxies which hosted a pseudo-bulge.
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Papers by Daiana Di Nino